1.1 Getting Started

The only way to learn a new programming language is by writing programs in it. The first program to write is the same for all languages:

Print the words
hello, world

This is the big hurdle; to leap over it you have to be able to create the program text somewhere, compile it successfully, load it, run it, and find out where your output went. With these mechanical details mastered, everything else is comparatively easy.

In C, the program to print “hello, world” is

\#include \<stdio.h\>  
  
main()  
{  
  printf("hello, world\\n");  
}

Just how to run this program depends on the system you are using. As a specific example, on the UNIX operating system you must create the program in a file whose name ends in “.c,” such as hello.c, then compile it with the command

cc hello.c

If you haven’t botched anything, such as omitting a character or misspelling something, the compilation will proceed silently, and make an executable file called a.out. If you run a.out by typing the command

a.out

it will print

hello, world

On other systems, the rules will be different; check with a local expert.

Now for some explanations about the program itself. A C program, whatever its size, consists of functions and variables. A function contains statements that specify the computing operations to be done, and variables store values used during the computation. C functions are like the subroutines and functions of Fortran or the procedures and functions of Pascal. Our example is a function named main. Normally you are at liberty to give functions whatever names you like, but “main” is special—your program begins executing at the beginning of main. This means that every program must have a main somewhere.

main will usually call other functions to help perform its job, some that you wrote, and others from libraries that are provided for you. The first line of the program,

\#include \<stdio.h\>

tells the compiler to include information about the standard input/output library; this line appears at the beginning of many C source files. The standard library is described in Chapter 7 and Appendix B.

One method of communicating data between functions is for the calling function to provide a list of values, called arguments, to the function it calls. The parentheses after the function name surround the argument list. In this example, main is defined to be a function that expects no arguments, which is indicated by the empty list ().

#include <stdio.h>

include information about standard library

main()

define a function named main
that receives no argument values

{

statements of main are enclosed in braces

<span class="code">printf (</span><span class="code">"hello,

world\n");
}

main calls library function printf
to print this sequence of characters;
\n represents the newline character

The first C program.

The statements of a function are enclosed in braces {}. The function main contains only one statement,

printf("hello, world\\n");

A function is called by naming it, followed by a parenthesized list of arguments, so this calls the function printf with the argument “hello, world\n”. printf is a library function that prints output, in this case the string of characters between the quotes.

A sequence of characters in double quotes, like “hello, world\n”, is called a character string or string constant. For the moment our only use of character strings will be as arguments for printf and other functions.

The sequence \n in the string is C notation for the newline character, which when printed advances the output to the left margin on the next line. If you leave out the \n (a worthwhile experiment), you will find that there is no line advance after the output is printed. You must use \n to include a newline character in the printf argument; if you try something like

printf("hello, world  
");

the C compiler will produce an error message.

printf never supplies a newline automatically, so several calls may be used to build up an output line in stages. Our first program could just as well have been written

\#include \<stdio.h\>  
  
main()  
{  
  printf("hello, ");  
  printf("world");  
  printf("\\n");  
}

to produce identical output.

Notice that \n represents only a single character. An escape sequence like \n provides a general and extensible mechanism for representing hard-to-type or invisible characters. Among the others that C provides are \t for tab, \b for backspace, \" for the double quote, and \\ for the backslash itself. There is a complete list in Section 2.3

! cat Chapter1/Exercise\ 1-01/helloworld.c

#include <stdio.h>

int main(void)
{
    printf("hello world!\n");
    return 0;
}

! gcc Chapter1/Exercise\ 1-01/helloworld.c -o Chapter1/Exercise\ 1-01/helloworld

! Chapter1/Exercise\ 1-01/helloworld

hello world!

! cat Chapter1/Exercise\ 1-02/escape_sequence.c

#include <stdio.h>

int main(void)
{
    printf("tab: \t\n");
    printf("backslash: \\\n");
    printf("single quote: \'\n");
    printf("double quote: \"\n");
    return 0;
}

! gcc Chapter1/Exercise\ 1-02/escape_sequence.c -o Chapter1/Exercise\ 1-02/escape

! Chapter1/Exercise\ 1-02/escape

tab:    
backslash: \
single quote: '
double quote: "

1.2 Variables and Arithmetic Expressions

The next program uses the formula °C = (5/9) (°F−32) to print the following table of Fahrenheit temperatures and their centigrade or Celsius equivalents:

0 −17
20 −6
40 4
60 15
80 26
100 37
120 48
140 60
160 71
180 82
200 93
220 104
240 115
260 126
280 137
300 148

The program itself still consists of the definition of a single function named main. It is longer than the one that printed “hello, world,” but not complicated. It introduces several new ideas, including comments, declarations, variables, arithmetic expressions, loops, and formatted output.

#include <stdio.h>

/* print Fahrenheit-Celsius table
for fahr = 0, 20, …, 300 */
main()
{
int fahr, celsius;
int lower, upper, step;

lower = 0; /* lower limit of temperature table */
upper = 300; /* upper limit */
step = 20; /* step size */

fahr = lower;
while (fahr <= upper) {
celsius = 5 * (fahr-32) / 9;
printf(“%d\t%d\n,” fahr, celsius);
fahr = fahr + step;
}
}

The two lines

/* print Fahrenheit-Celsius table
for fahr = 0, 20, …, 300 */

are a comment, which in this case explains briefly what the program does. Any characters between /* and */ are ignored by the compiler; they may be used freely to make a program easier to understand. Comments may appear anywhere a blank or tab or newline can.

In C, all variables must be declared before they are used, usually at the beginning of the function before any executable statements. A declaration announces the properties of variables; it consists of a type name and a list of variables, such as

int fahr, celsius;
int lower, upper, step;

The type int means that the variables listed are integers, by contrast with float, which means floating point, i.e., numbers that may have a fractional part. The range of both int and float depends on the machine you are using; 16-bit ints, which lie between −32768 and +32767, are common, as are 32-bit ints. A float number is typically a 32-bit quantity, with at least six significant digits and magnitude generally between about 10⁻³⁸ and 10⁺³⁸.

C provides several other basic data types besides int and float, including:

char character—a single byte
short short integer
long long integer
double double-precision floating point

The sizes of these objects are also machine-dependent. There are also arrays, structures and unions of these basic types, pointers to them, and functions that return them, all of which we will meet in due course.

Computation in the temperature conversion program begins with the assignment statements

lower = 0;
upper = 300;
step = 20;
fahr = lower;

which set the variables to their initial values. Individual statements are terminated by semicolons.

Each line of the table is computed the same way, so we use a loop that repeats once per output line; this is the purpose of the while loop

while (fahr <= upper) {
…
}

The while loop operates as follows: The condition in parentheses is tested. If it is true (fahr is less than or equal to upper), the body of the loop (the three statements enclosed in braces) is executed. Then the condition is re-tested, and if true, the body is executed again. When the test becomes false (fahr exceeds upper) the loop ends, and execution continues at the statement that follows the loop. There are no further statements in this program, so it terminates.

The body of a while can be one or more statements enclosed in braces, as in the temperature converter, or a single statement without braces, as in

while (i < j)
i = 2 * i;

In either case, we will always indent the statements controlled by the while by one tab stop (which we have shown as four spaces) so you can see at a glance which statements are inside the loop. The indentation emphasizes the logical structure of the program. Although C compilers do not care about how a program looks, proper indentation and spacing are critical in making programs easy for people to read. We recommend writing only one statement per line, and using blanks around operators to clarify grouping. The position of braces is less important, although people hold passionate beliefs. We have chosen one of several popular styles. Pick a style that suits you, then use it consistently.

Most of the work gets done in the body of the loop. The Celsius temperature is computed and assigned to the variable celsius by the statement

celsius = 5 * (fahr-32) / 9;

The reason for multiplying by 5 and then dividing by 9 instead of just multiplying by 5/9 is that in C, as in many other languages, integer division truncates: any fractional part is discarded. Since 5 and 9 are integers, 5/9 would be truncated to zero and so all the Celsius temperatures would be reported as zero.

This example also shows a bit more of how printf works. printf is a general-purpose output formatting function, which we will describe in detail in Chapter 7.

Its first argument is a string of characters to be printed, with each % indicating where one of the other (second, third, …) arguments is to be substituted, and in what form it is to be printed. For instance, %d specifies an integer argument, so the statement

printf(“%d\t%d\n,” fahr, celsius);

causes the values of the two integers fahr and celsius to be printed, with a tab (\t) between them.

Each % construction in the first argument of printf is paired with the corresponding second argument, third argument, etc.; they must match up properly by number and type, or you’ll get wrong answers.

By the way, printf is not part of the C language; there is no input or output defined in C itself. printf is just a useful function from the standard library of functions that are normally accessible to C programs. The behavior of printf is defined in the ANSI standard, however, so its properties should be the same with any compiler and library that conforms to the standard.

In order to concentrate on C itself, we won’t talk much about input and output until Chapter 7. In particular, we will defer formatted input until then. If you have to input numbers, read the discussion of the function scanf in Section 7.4. scanf is like printf, except that it reads input instead of writing output.

There are a couple of problems with the temperature conversion program. The simpler one is that the output isn’t very pretty because the numbers are not right-justified. That’s easy to fix; if we augment each %d in the printf statement with a width, the numbers printed will be right-justified in their fields. For instance, we might say

printf(“%3d %6d\n,” fahr, celsius);

to print the first number of each line in a field three digits wide, and the second in a field six digits wide, like this:

0 −17
20 −6
40 4
60 15
80 26
100 37
…

The more serious problem is that because we have used integer arithmetic, the Celsius temperatures are not very accurate; for instance, 0°F is actually about −17.8°C, not −17. To get more accurate answers, we should use floating-point arithmetic instead of integer. This requires some changes in the program. Here is a second version:

#include <stdio.h>

/* print Fahrenheit-Celsius table
for fahr = 0, 20, …, 300; floating-point version */
main()
{
float fahr, celsius;
int lower, upper, step;

lower = 0; /* lower limit of temperature table */
upper = 300; /* upper limit */
step = 20; /* step size */

fahr = lower;
while (fahr <= upper) {
celsius = (5.0/9.0) * (fahr−32.0);
printf(“%3.0f %6.1f\n,” fahr, celsius);
fahr = fahr + step;
}
}

This is much the same as before, except that fahr and celsius are declared to be float, and the formula for conversion is written in a more natural way. We were unable to use 5/9 in the previous version because integer division would truncate it to zero. A decimal point in a constant indicates that it is floating point, however, so 5.0/9.0 is not truncated because it is the ratio of two floating-point values.

If an arithmetic operator has integer operands, an integer operation is performed. If an arithmetic operator has one floating-point operand and one integer operand, however, the integer will be converted to floating point before the operation is done. If we had written fahr−32, the 32 would be automatically converted to floating point. Nevertheless, writing floating-point constants with explicit decimal points even when they have integral values emphasizes their floating-point nature for human readers.

The detailed rules for when integers are converted to floating point are in Chapter 2. For now, notice that the assignment

fahr = lower;

and the test

while (fahr <= upper)

also work in the natural way—the int is converted to float before the operation is done.

The printf conversion specification %3.0f says that a floating-point number (here fahr) is to be printed at least three characters wide, with no decimal point and no fraction digits. %6.1f describes another number (celsius) that is to be printed at least six characters wide, with 1 digit after the decimal point. The output looks like this:

0 −17.8
20 −6.7
40 4.4
…

Width and precision may be omitted from a specification: %6f says that the number is to be at least six characters wide; %.2f specifies two characters after the decimal point, but the width is not constrained; and %f merely says to print the number as floating point.

Among others, printf also recognizes %o for octal, %x for hexadecimal, %c for character, %s for character string, and %% for % itself.

! cat Chapter1/Exercise\ 1-03/temperature.c

#include <stdio.h>

int main(void)
{
    float fahr, celsius;
    float lower, upper, step;

    lower = 0; /* lower limit of temperatuire scale */
    upper = 300; /* upper limit */
    step = 20; /* step size */

    printf("  Fahr      Celsius\n");

    fahr = lower;
    while (fahr <= upper) {
        celsius = (5.0 / 9.0) * (fahr - 32.0);
        printf("%3.0f %6.1f\n", fahr, celsius);
        fahr = fahr + step;
    }
    return 0;
}

! gcc Chapter1/Exercise\ 1-03/temperature.c -o Chapter1/Exercise\ 1-03/temperature

! Chapter1/Exercise\ 1-03/temperature

  Fahr      Celsius
  0  -17.8
 20   -6.7
 40    4.4
 60   15.6
 80   26.7
100   37.8
120   48.9
140   60.0
160   71.1
180   82.2
200   93.3
220  104.4
240  115.6
260  126.7
280  137.8
300  148.9

! cat Chapter1/Exercise\ 1-04/ctof.c

#include <stdio.h>

int main(void)
{
    float fahr, celsius;
    float lower, upper, step;

    lower = -20; /* lower limit of temperatuire scale */
    upper = 150; /* upper limit */
    step = 10; /* step size */

    printf("  celsius      fahr\n");

    celsius = lower;
    while (celsius <= upper) {
        fahr = celsius * (9.0 / 5.0) + 32.0;
        printf("%3.0f %6.1f\n", celsius, fahr);
        celsius = celsius + step;
    }
    return 0;
}

! gcc Chapter1/Exercise\ 1-04/ctof.c -o Chapter1/Exercise\ 1-04/ctof

! Chapter1/Exercise\ 1-04/ctof

  celsius      fahr
-20   -4.0
-10   14.0
  0   32.0
 10   50.0
 20   68.0
 30   86.0
 40  104.0
 50  122.0
 60  140.0
 70  158.0
 80  176.0
 90  194.0
100  212.0
110  230.0
120  248.0
130  266.0
140  284.0
150  302.0

1.3 The For Statement

There are plenty of different ways to write a program for a particular task. Let’s try a variation on the temperature converter.

#include <stdio.h>

/* print Fahrenheit-Celsius table */
main()
{
int fahr;

for (fahr = 0; fahr <= 300; fahr = fahr + 20)
printf(“%3d %6.1f\n,” fahr, (5.0/9.0)*(fahr-32));
}

This produces the same answers, but it certainly looks different. One major change is the elimination of most of the variables; only fahr remains, and we have made it an int. The lower and upper limits and the step size appear only as constants in the for statement, itself a new construction, and the expression that computes the Celsius temperature now appears as the third argument of printf instead of as a separate assignment statement.

This last change is an instance of a general rule—in any context where it is permissible to use the value of a variable of some type, you can use a more complicated expression of that type. Since the third argument of printf must be a floating-point value to match the %6.1f, any floating-point expression can occur there.

The for statement is a loop, a generalization of the while. If you compare it to the earlier while, its operation should be clear. Within the parentheses, there are three parts, separated by semicolons. The first part, the initialization

fahr = 0

is done once, before the loop proper is entered. The second part is the test or condition that controls the loop:

fahr <= 300

This condition is evaluated; if it is true, the body of the loop (here a single printf) is executed. Then the increment step

fahr = fahr + 20

is executed, and the condition re-evaluated. The loop terminates if the condition has become false. As with the while, the body of the loop can be a single statement, or a group of statements enclosed in braces. The initialization, condition, and increment can be any expressions.

The choice between while and for is arbitrary, based on which seems clearer. The for is usually appropriate for loops in which the initialization and increment are single statements and logically related, since it is more compact than while and it keeps the loop control statements together in one place.

! cat Chapter1/Exercise\ 1-05/reverse_temperature.c

#include <stdio.h>

int main(void)
{
    float fahr, celsius;
    float lower, upper, step;

    lower = 0; /* lower limit of temperatuire scale */
    upper = 300; /* upper limit */
    step = 20; /* step size */

    printf("  fahr      celsius\n");

    fahr = upper;
    while (fahr >= lower) {
        celsius = (5.0 / 9.0) * (fahr - 32.0);
        printf("%3.0f %6.1f\n", fahr, celsius);
        fahr = fahr - step;
    }
    return 0;
}

! gcc Chapter1/Exercise\ 1-05/reverse_temperature.c -o Chapter1/Exercise\ 1-05/reverse_temperature

! Chapter1/Exercise\ 1-05/reverse_temperature

  fahr      celsius
300  148.9
280  137.8
260  126.7
240  115.6
220  104.4
200   93.3
180   82.2
160   71.1
140   60.0
120   48.9
100   37.8
 80   26.7
 60   15.6
 40    4.4
 20   -6.7
  0  -17.8

1.4 Symbolic Constants

A final observation before we leave temperature conversion forever. It’s bad practice to bury “magic numbers” like 300 and 20 in a program; they convey little information to someone who might have to read the program later, and they are hard to change in a systematic way. One way to deal with magic numbers is to give them meaningful names. A #define line defines a symbolic name or symbolic constant to be a particular string of characters:

#define name replacement text

Thereafter, any occurrence of name (not in quotes and not part of another name) will be replaced by the corresponding replacement text. The name has the same form as a variable name: a sequence of letters and digits that begins with a letter. The replacement text can be any sequence of characters; it is not limited to numbers.

#include <stdio.h>

#define LOWER 0 /* lower limit of table */
#define UPPER 300 /* upper limit */
#define STEP 20 /* step size */

/* print Fahrenheit-Celsius table */
main()
{
int fahr;

for (fahr = LOWER; fahr <= UPPER; fahr = fahr + STEP)
printf(“%3d %6.1f\n,” fahr, (5.0/9.0)*(fahr-32));
}

The quantities LOWER, UPPER and STEP are symbolic constants, not variables, so they do not appear in declarations. Symbolic constant names are conventionally written in upper case so they can be readily distinguished from lower case variable names. Notice that there is no semicolon at the end of a #define line.

1.5 Character Input and Output

We are now going to consider a family of related programs for processing character data. You will find that many programs are just expanded versions of the prototypes that we discuss here.

The model of input and output supported by the standard library is very simple. Text input or output, regardless of where it originates or where it goes to, is dealt with as streams of characters. A text stream is a sequence of characters divided into lines; each line consists of zero or more characters followed by a newline character. It is the responsibility of the library to make each input or output stream conform to this model; the C programmer using the library need not worry about how lines are represented outside the program.

The standard library provides several functions for reading or writing one character at a time, of which getchar and putchar are the simplest. Each time it is called, getchar reads the next input character from a text stream and returns that as its value. That is, after

c = getchar()

the variable c contains the next character of input. The characters normally come from the keyboard; input from files is discussed in Chapter 7.

The function putchar prints a character each time it is called:

putchar(c)

prints the contents of the integer variable c as a character, usually on the screen. Calls to putchar and printf may be interleaved; the output will appear in the order in which the calls are made.

! cat Chapter1/Exercise\ 1-06/eof.c

#include <stdio.h>

int main(void)
{
    printf("Please enter a character:\n");
    printf("The expression \"getchar() != EOF\" is %d.\n", getchar() != EOF);
    return 0;
}

! gcc Chapter1/Exercise\ 1-06/eof.c -o Chapter1/Exercise\ 1-06/eof

! Chapter1/Exercise\ 1-06/eof

Please enter a character:

! cat Chapter1/Exercise\ 1-07/eof.c

#include <stdio.h>

int main(void)
{
    printf("The value of EOF is %d.\n", EOF);
    return 0;
}

! gcc Chapter1/Exercise\ 1-07/eof.c -o Chapter1/Exercise\ 1-07/eof

! Chapter1/Exercise\ 1-07/eof

The value of EOF is -1.

! cat Chapter1/Exercise\ 1-08/count.c

#include <stdio.h>

int main(void)
{
    int c, bl, tl, nl;
    bl = 0;
    tl = 0;
    nl = 0;

    printf("Input some characters, then press Ctrl+D.\n");
    while ((c = getchar()) != EOF)
        if (c == ' ')
            ++bl;
        else if (c == '\t')
            ++tl;
        else if (c == '\n')
            ++nl;

    printf("Count of blanks is %d, count of tabs is %d, count of newlines is %d.\n", bl, tl, nl);

    return 0;
}

! gcc Chapter1/Exercise\ 1-08/count.c -o Chapter1/Exercise\ 1-08/count

! Chapter1/Exercise\ 1-08/count

Input some characters, then press Ctrl+D.

! cat Chapter1/Exercise\ 1-09/blank.c

#include <stdio.h>

int main(void)
{
    int c, blank_recieved;
    blank_recieved = 0;
    printf("Input some characters, when you finishes, press Ctrl+D.\n");
    while ((c = getchar()) != EOF)
        if (c == ' ') {
            if (!blank_recieved) {
                blank_recieved = 1;
                putchar(c);
            }
        } else {
            blank_recieved = 0;
            putchar(c);
        }

    return 0;
}

! gcc Chapter1/Exercise\ 1-08/count.c -o Chapter1/Exercise\ 1-08/count

! Chapter1/Exercise\ 1-08/count

Input some characters, then press Ctrl+D.

! cat Chapter1/Exercise\ 1-10/replace.c

#include <stdio.h>

int main(void)
{
    int c;

    printf("Input some characters, then press Ctrl+D.\n");
    while ((c = getchar()) != EOF)
        if (c == '\t')
            printf("\\t");
        else if (c == '\b')
            printf("\\b");
        else if (c == '\\')
            printf("\\\\");
        else
            printf("%c", c);

    return 0;
}

! gcc Chapter1/Exercise\ 1-10/replace.c -o Chapter1/Exercise\ 1-10/replace

! Chapter1/Exercise\ 1-10/replace

Input some characters, then press Ctrl+D.

! cat Chapter1/Exercise\ 1-12/one_word_per_line.c

#include <stdio.h>

int main(void)
{
    int c, linestarted;

    linestarted = 0;
    printf("Input some characters, then press Ctrl+D.\n");
    while ((c = getchar()) != EOF)
        if (c == ' ' || c == '\t' || c == '\n') {
            if (!linestarted) {
                putchar('\n');
                linestarted = 1;
            }
        } else {
            putchar(c);
            linestarted = 0;
        }

    return 0;
}

! gcc Chapter1/Exercise\ 1-12/one_word_per_line.c -o Chapter1/Exercise\ 1-12/one_word_per_line

! Chapter1/Exercise\ 1-12/one_word_per_line

Input some characters, then press Ctrl+D.

1.6 Arrays

Let us write a program to count the number of occurrences of each digit, of white space characters (blank, tab, newline), and of all other characters. This is artificial, but it permits us to illustrate several aspects of C in one program.

There are twelve categories of input, so it is convenient to use an array to hold the number of occurrences of each digit, rather than ten individual variables. Here is one version of the program:

#include <stdio.h>

/* count digits, white space, others */
main()
{
int c, i, nwhite, nother;
int ndigit[10];

nwhite = nother = 0;
for (i = 0; i < 10; ++i)
ndigit[i] = 0;

while ((c = getchar()) != EOF)
if (c >= ′0′ && c <= ′9′)
++ndigit[c-′0′];
else if (c == ′ ′ || c == ′\n′ || c == ′\t′)
++nwhite;
else
++nother;

printf(“digits =”);
for (i = 0; i < 10; ++i)
printf(" %d“, ndigit[i]);
printf(,” white space = %d, other = %d\n",
nwhite, nother);
}

The output of this program on itself is

digits = 9 3 0 0 0 0 0 0 0 1, white space = 123, other = 345

The declaration

int ndigit[10];

declares ndigit to be an array of 10 integers. Array subscripts always start at zero in C, so the elements are ndigit[0], ndigit[1], …, ndigit[9]. This is reflected in the for loops that initialize and print the array.

A subscript can be any integer expression, which includes integer variables like i, and integer constants.

This particular program relies on the properties of the character representation of the digits. For example, the test

if (c >= ′0′ && c <= ′9′) …

determines whether the character in c is a digit. If it is, the numeric value of that digit is

c - ′0′

This works only if ′0′, ′1′, …, ′9′ have consecutive increasing values. Fortunately, this is true for all character sets.

By definition, chars are just small integers, so char variables and constants are identical to ints in arithmetic expressions. This is natural and convenient; for example, c-′0′ is an integer expression with a value between 0 and 9 corresponding to the character ′0′ to ′9′ stored in c, and is thus a valid subscript for the array ndigit.

The decision as to whether a character is a digit, white space, or something else is made with the sequence

if (c >= ′0′ && c <= ′9′)
++ndigit[c-′0′];
else if (c == ′ ′ || c == ′\n′ || c == ′\t′)
++nwhite;
else
++nother;

The pattern

if (condition₁)
statement₁
else if (condition₂)
statement₂
…
…
else
statement_n

occurs frequently in programs as a way to express a multi-way decision. The conditions are evaluated in order from the top until some condition is satisfied; at that point the corresponding statement part is executed, and the entire construction is finished. (Any statement can be several statements enclosed in braces.) If none of the conditions is satisfied, the statement after the final else is executed if it is present. If the final else and statement are omitted, as in the word count program, no action takes place. There can be any number of

else if (condition)
statement

groups between the initial if and the final else.

As a matter of style, it is advisable to format this construction as we have shown; if each if were indented past the previous else, a long sequence of decisions would march off the right side of the page.

The switch statement, to be discussed in Chapter 3, provides another way to write a multi-way branch that is particularly suitable when the condition is whether some integer or character expression matches one of a set of constants. For contrast, we will present a switch version of this program in Section 3.4.

! cat Chapter1/Exercise\ 1-13/word_length.c

#include <stdio.h>

#define MAX_LENGTH 15
#define IN_WORD 1
#define OUT_WORD 0

int main(void)
{
    int c, word_in_out;
    int word_length[MAX_LENGTH + 1];
    int l;
    int i, j;
    unsigned int max_count;

    for (i = 0; i <= MAX_LENGTH; ++i)
        word_length[i] = 0;

    word_in_out = OUT_WORD;
    printf("Input some characters, then press Ctrl+D.\n");
    while (1) {
        c = getchar();
        if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')) {
            if (word_in_out == OUT_WORD) {
                l = 0;
                word_in_out = IN_WORD;
            }
            ++l;
        } else {
            if (word_in_out == IN_WORD) {
                if (l <= MAX_LENGTH)
                    ++word_length[l - 1];
                else
                    ++word_length[MAX_LENGTH];
                word_in_out = OUT_WORD;
            }
            if (c == EOF)
                break;
        }
    }

    printf("\nHorizontal histogram:\n\n");
    printf(" length | graph\n");
    for (i = 0; i <= MAX_LENGTH; ++i) {
        if (i != MAX_LENGTH)
            printf("     %2d | ", i + 1);
        else
            printf("    >%2d | ", MAX_LENGTH);
        for (j = 0; j < word_length[i]; ++j)
            putchar('+');
        putchar('\n');
    }

    printf("\nVertical histogram:\n\n");
    max_count = 0;
    for (i = 0; i <= MAX_LENGTH; ++i)
        if (word_length[i] > max_count)
            max_count = word_length[i];

    for (i = 0; i < max_count; ++i) {
        printf("  %2u | ", max_count - i);
        for (j = 0; j <= MAX_LENGTH; ++j)
            if (word_length[j] >= max_count - i)
                printf("  +");
            else
                printf("   ");
        printf("\n");
    }

    printf(" ------");
    for (i = 0; i <= MAX_LENGTH; ++i)
        printf("---");
    printf("--\n");

    printf("       ");
    for (i = 0; i < MAX_LENGTH;)
        printf(" %2u", ++i);
    printf(" >%2u", MAX_LENGTH);
    printf("\n");

    return 0;
}

! gcc Chapter1/Exercise\ 1-13/word_length.c -o Chapter1/Exercise\ 1-13/word_length

! Chapter1/Exercise\ 1-13/word_length

Input some characters, then press Ctrl+D.

! cat Chapter1/Exercise\ 1-14/frequency.c

#include <stdio.h>

#define NUM_CHARS 128
#define IN_WORD 1
#define OUT_WORD 0

int main(void)
{
    int c;
    int char_frequencies[NUM_CHARS + 1];
    int i;

    for (i = 0; i <= NUM_CHARS; ++i)
        char_frequencies[i] = 0;

    printf("Input some characters, then press Ctrl+D.\n");
    while ((c = getchar()) != EOF)
        ++char_frequencies[c];

    printf("\n ASCII | Count\n");
    for (i = 0; i <= NUM_CHARS; ++i) 
        if (char_frequencies[i] > 0)
            printf(" %5d : %5d\n", i, char_frequencies[i]);

    return 0;
}

! gcc Chapter1/Exercise\ 1-14/frequency.c -o Chapter1/Exercise\ 1-14/frequency

! Chapter1/Exercise\ 1-14/frequency

Input some characters, then press Ctrl+D.

1.7 Functions

In C, a function is equivalent to a subroutine or function in Fortran, or a procedure or function in Pascal. A function provides a convenient way to encapsulate some computation, which can then be used without worrying about its implementation. With properly designed functions, it is possible to ignore how a job is done; knowing what is done is sufficient. C makes the use of functions easy, convenient and efficient; you will often see a short function defined and called only once, just because it clarifies some piece of code.

So far we have used only functions like printf, getchar, and putchar that have been provided for us; now it’s time to write a few of our own. Since C has no exponentiation operator like the ** of Fortran, let us illustrate the mechanics of function definition by writing a function power(m,n) to raise an integer m to a positive integer power n. That is, the value of power(2,5) is 32. This function is not a practical exponentiation routine, since it handles only positive powers of small integers, but it’s good enough for illustration. (The standard library contains a function pow(x,y) that computes x^y.)

Here is the function power and a main program to### Exercise it, so you can see the whole structure at once.

#include <stdio.h>

int power(int m, int n);

/* test power function */
main()
{
int i;

for (i = 0; i < 10; ++i)
printf(“%d %d %d\n,” i, power(2,i), power(-3,i));
return 0;
}

/* power: raise base to n-th power; n >= 0 */
int power(int base, int n)
{
int i, p;

p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}

A function definition has this form:

return-type function-name(parameter declarations, if any)
{
declarations
statements
}

Function definitions can appear in any order, and in one source file or several, although no function can be split between files. If the source program appears in several files, you may have to say more to compile and load it than if it all appears in one, but that is an operating system matter, not a language attribute. For the moment, we will assume that both functions are in the same file, so whatever you have learned about running C programs will still work.

The function power is called twice by main, in the line

printf(“%d %d %d\n,” i, power(2,i), power(-3,i));

Each call passes two arguments to power, which each time returns an integer to be formatted and printed. In an expression, power(2,i) is an integer just as 2 and i are. (Not all functions produce an integer value; we will take this up in Chapter 4)

The first line of power itself,

int power(int base, int n)

declares the parameter types and names, and the type of the result that the function returns. The names used by power for its parameters are local to power, and are not visible to any other function: other routines can use the same names without conflict. This is also true of the variables i and p: the i in power is unrelated to the i in main.

We will generally use parameter for a variable named in the parenthesized list in a function definition, and argument for the value used in a call of the function. The terms formal argument and actual argument are sometimes used for the same distinction.

The value that power computes is returned to main by the return statement. Any expression may follow return:

return expression;

A function need not return a value; a return statement with no expression causes control, but no useful value, to be returned to the caller, as does “falling off the end” of a function by reaching the terminating right brace. And the calling function can ignore a value returned by a function.

You may have noticed that there is a return statement at the end of main. Since main is a function like any other, it may return a value to its caller, which is in effect the environment in which the program was executed. Typically, a return value of zero implies normal termination; non-zero values signal unusual or erroneous termination conditions. In the interests of simplicity, we have omitted return statements from our main functions up to this point, but we will include them hereafter, as a reminder that programs should return status to their environment.

The declaration

int power(int m, int n);

just before main says that power is a function that expects two int arguments and returns an int. This declaration, which is called a function prototype, has to agree with the definition and uses of power. It is an error if the definition of a function or any uses of it do not agree with its prototype.

Parameter names need not agree. Indeed, parameter names are optional in a function prototype, so for the prototype we could have written

int power(int, int);

Well-chosen names are good documentation, however, so we will often use them.

A note of history: The biggest change between ANSI C and earlier versions is how functions are declared and defined. In the original definition of C, the power function would have been written like this:

/* power: raise base to n-th power; n >= 0 */
/* (old-style version) */
power(base, n)
int base, n;
{
int i, p;

p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}

The parameters are named between the parentheses, and their types are declared before the opening left brace; undeclared parameters are taken as int. (The body of the function is the same as before.)

The declaration of power at the beginning of the program would have looked like this:

int power();

No parameter list was permitted, so the compiler could not readily check that power was being called correctly. Indeed, since by default power would have been assumed to return an int, the entire declaration might well have been omitted.

The new syntax of function prototypes makes it much easier for a compiler to detect errors in the number of arguments or their types. The old style of declaration and definition still works in ANSI C, at least for a transition period, but we strongly recommend that you use the new form when you have a compiler that supports it.

! cat Chapter1/Exercise\ 1-15/temperature.c

#include <stdio.h>

float ftoc(float fahr);

int main(void)
{
    float fahr, celsius;
    float lower, upper, step;
    lower = 0; /* lower limit of temperatuire scale */
    upper = 300; /* upper limit */
    step = 20; /* step size */
    fahr = lower;

    while (fahr <= upper) {
        celsius = ftoc(fahr);
        printf("%3.0f %6.1f\n", fahr, celsius);
        fahr = fahr + step;
    }

    return 0;
}

float ftoc(float fahr)
{
    float celsius;
    celsius = (5.0 / 9.0) * (fahr - 32.0);
    return celsius;
}

! gcc Chapter1/Exercise\ 1-15/temperature.c -o Chapter1/Exercise\ 1-15/temperature

! Chapter1/Exercise\ 1-15/temperature

  0  -17.8
 20   -6.7
 40    4.4
 60   15.6
 80   26.7
100   37.8
120   48.9
140   60.0
160   71.1
180   82.2
200   93.3
220  104.4
240  115.6
260  126.7
280  137.8
300  148.9

1.8 Arguments—Call by Value

One aspect of C functions may be unfamiliar to programmers who are used to some other languages, particularly Fortran. In C, all function arguments are passed “by value.” This means that the called function is given the values of its arguments in temporary variables rather than the originals. This leads to some different properties than are seen with “call by reference” languages like Fortran or with var parameters in Pascal, in which the called routine has access to the original argument, not a local copy.

The main distinction is that in C the called function cannot directly alter a variable in the calling function; it can only alter its private, temporary copy.

Call by value is an asset, however, not a liability. It usually leads to more compact programs with fewer extraneous variables, because parameters can be treated as conveniently initialized local variables in the called routine. For example, here is a version of power that makes use of this property.

/* power: raise base to n-th power; n>=0; version 2 */
int power(int base, int n)
{
int p;

for (p = 1; n > 0; –n)
p = p * base;
return p;
}

The parameter n is used as a temporary variable, and is counted down (a for loop that runs backwards) until it becomes zero; there is no longer a need for the variable i. Whatever is done to n inside power has no effect on the argument that power was originally called with.

When necessary, it is possible to arrange for a function to modify a variable in a calling routine. The caller must provide the address of the variable to be set (technically a pointer to the variable), and the called function must declare the parameter to be a pointer and access the variable indirectly through it. We will cover pointers in Chapter 5.

The story is different for arrays. When the name of an array is used as an argument, the value passed to the function is the location or address of the beginning of the array—there is no copying of array elements. By subscripting this value, the function can access and alter any element of the array. This is the topic of the next section.

1.9 Character Arrays

The most common type of array in C is the array of characters. To illustrate the use of character arrays and functions to manipulate them, let’s write a program that reads a set of text lines and prints the longest. The outline is simple enough:

while (there’s another line)
if (it’s longer than the previous longest)
save it
save its length
print longest line

This outline makes it clear that the program divides naturally into pieces. One piece gets a new line, another tests it, another saves it, and the rest controls the process.

Since things divide so nicely, it would be well to write them that way too. Accordingly, let us first write a separate function getline to fetch the next line of input. We will try to make the function useful in other contexts. At the minimum, getline has to return a signal about possible end of file; a more useful design would be to return the length of the line, or zero if end of file is encountered. Zero is an acceptable end-of-file return because it is never a valid line length. Every text line has at least one character; even a line containing only a newline has length 1.

When we find a line that is longer than the previous longest line, it must be saved somewhere. This suggests a second function, copy, to copy the new line to a safe place.

Finally, we need a main program to control getline and copy. Here is the result.

#include <stdio.h>
#define MAXLINE 1000 /* maximum input line size */

int getline(char line[], int maxline);
void copy(char to[], char from[]);

/* print longest input line */
main()
{
int len; /* current line length */
int max; /* maximum length seen so far */
char line[MAXLINE]; /* current input line */
char longest[MAXLINE]; /* longest line saved here */

max = 0;
while ((len = getline(line, MAXLINE)) > 0)
if (len > max) {
max = len;
copy(longest, line);
}
if (max > 0) /* there was a line */
printf(“%s,” longest);
return 0;
}

/* getline: read a line into s, return length */
int getline(char s[], int lim)
{
int c, i;

for (i=0; i<lim-1 && (c=getchar())!=EOF && c!=′\n′; ++i)
s[i] = c;
if (c == ′\n′) {
s[i] = c;
++i;
}
s[i] = ′\0′;
return i;
}

/* copy: copy ′from′ into ′to′; assume to is big enough */
void copy(char to[], char from[])
{
int i;

i = 0;
while ((to[i] = from[i]) != ′\0′)
++i;
}

The functions getline and copy are declared at the beginning of the program, which we assume is contained in one file.

main and getline communicate through a pair of arguments and a returned value. In getline, the arguments are declared by the line

int getline(char s[], int lim)

which specifies that the first argument, s, is an array, and the second, lim, is an integer. The purpose of supplying the size of an array in a declaration is to set aside storage. The length of the array s is not necessary in getline since its size is set in main. getline uses return to send a value back to the caller, just as the function power did. This line also declares that getline returns an int; since int is the default return type, it could be omitted.

Some functions return a useful value; others, like copy, are used only for their effect and return no value. The return type of copy is void, which states explicitly that no value is returned.

getline puts the character ′\0′ (the null character, whose value is zero) at the end of the array it is creating, to mark the end of the string of characters. This convention is also used by the C language: when a string constant like

“hello\n”

appears in a C program, it is stored as an array of characters containing the characters of the string and terminated with a ′\0′ to mark the end.

The %s format specification in printf expects the corresponding argument to be a string represented in this form. copy also relies on the fact that its input argument is terminated by ′\0′, and it copies this character into the output argument. (All of this implies that ′\0′ is not a part of normal text.)

It is worth mentioning in passing that even a program as small as this one presents some sticky design problems. For example, what should main do if it encounters a line which is bigger than its limit? getline works safely, in that it stops collecting when the array is full, even if no newline has been seen. By testing the length and the last character returned, main can determine whether the line was too long, and then cope as it wishes. In the interests of brevity, we have ignored the issue.

There is no way for a user of getline to know in advance how long an input line might be, so getline checks for overflow. On the other hand, the user of copy already knows (or can find out) how big the strings are, so we have chosen not to add error checking to it.

! cat Chapter1/Exercise\ 1-16/longest_line.c

#include <stdio.h>

#define MAXLINE 1000 /* maximum input line length */

int get_line(char line[], int maxline);
void copy(char to[], char from[]);

/* print the longest input line */
int main(void)
{
    int len; /* current line length */
    int max; /* maximum length seen so far */
    char line[MAXLINE]; /* current input line */
    char longest[MAXLINE]; /* longest line saved here */

    max = 0;
    while ((len = get_line(line, MAXLINE)) > 0)
        if (len > max) {
            max = len;
            copy(longest, line);
        }
    if (max > 0) {
        printf("The longest line is:\n");
        printf("%s\n", longest);
        printf("The length of it is %d.\n", max);
    }
        

    return 0;
}

/* get_line: read a line into s, return length */
int get_line(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i) {
        if (i < lim - 1)
            s[l++] = c;
    }
    if (c == '\n') {
        if (l < lim - 1)
            s[l++] = c;
        ++i;
    }   
    s[l] = '\0';

    return i;
}

/* copy: copy 'from' into 'to'; assume to is big enough */
void copy(char to[], char from[])
{
    int i;

    i = 0;
    while ((to[i] = from[i]) != '\0')
        ++i;
}

! gcc Chapter1/Exercise\ 1-16/longest_line.c -o Chapter1/Exercise\ 1-16/longest_line

! Chapter1/Exercise\ 1-16/longest_line

! cat Chapter1/Exercise\ 1-17/lines.c

#include <stdio.h>

#define MAXLINE 1000
#define MAXLENGTH 10

int get_line(char line[], int maxline);

int main(void)
{
    int len; /* current line length */
    char line[MAXLINE]; /* current input line */

    while ((len = get_line(line, MAXLINE)) > 0)
        if (len > MAXLENGTH)
            printf("%s\n", line);

    return 0;
}

/* get_line: read a line into s, return length */
int get_line(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i) {
        if (i < lim - 1)
            s[l++] = c;
    }
    if (c == '\n')
        if (l < lim - 1)
            s[l++] = c;
    s[l] = '\0';

    return l;
}

! gcc Chapter1/Exercise\ 1-17/lines.c -o Chapter1/Exercise\ 1-17/lines

! Chapter1/Exercise\ 1-17/lines

! cat Chapter1/Exercise\ 1-18/lines.c

#include <stdio.h>

#define MAXLINE 1000

int get_line(char line[], int maxline);

int main(void)
{
    int len; /* current line length */
    char line[MAXLINE]; /* current input line */

    while ((len = get_line(line, MAXLINE)) > 0) {
        if (len == 1 && line[0] == '\n')
            continue;
        printf("%s\n", line);
    }

    return 0;
}

int get_line(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i)
        if (i < lim - 1)
            s[l++] = c;
    if (c == '\n' && l < lim - 1)
        s[l++] = c;
    s[l] = '\0';

    return l;
}

! gcc Chapter1/Exercise\ 1-18/lines.c -o Chapter1/Exercise\ 1-18/lines

! Chapter1/Exercise\ 1-18/lines

! cat Chapter1/Exercise\ 1-19/reverse.c

#include <stdio.h>

#define MAXLINE 1000

int get_line(char line[], int maxline);
void reverse(char s[]);

int main(void)
{
    char line[MAXLINE]; /* current input line */

    while (get_line(line, MAXLINE) > 0) {
        reverse(line);
        printf("%s\n", line);
    }

    return 0;
}

int get_line(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i)
        if (i < lim - 1)
            s[l++] = c;
    if (c == '\n' && l < lim - 1)
        s[l++] = c;
    s[l] = '\0';

    return l;
}

void reverse(char s[])
{
    int i, j;
    int tmp;

    for (j = 0; s[j] != '\0'; ++j)
        ;
    --j;
    if (s[j - 1] == '\n')
        --j;

    for (i = 0; i < j; ++i, --j) {
        tmp = s[i];
        s[i] = s[j];
        s[j] = tmp;
    }
}

! gcc Chapter1/Exercise\ 1-19/reverse.c -o Chapter1/Exercise\ 1-19/reverse

! Chapter1/Exercise\ 1-19/reverse

1.10 External Variables and Scope

The variables in main, such as line, longest, etc., are private or local to main. Because they are declared within main, no other function can have direct access to them. The same is true of the variables in other functions; for example, the variable i in getline is unrelated to the i in copy. Each local variable in a function comes into existence only when the function is called, and disappears when the function is exited. This is why such variables are usually known as automatic variables, following terminology in other languages. We will use the term automatic henceforth to refer to these local variables. (Chapter 4 discusses the static storage class, in which local variables do retain their values between calls.)

Because automatic variables come and go with function invocation, they do not retain their values from one call to the next, and must be explicitly set upon each entry. If they are not set, they will contain garbage.

As an alternative to automatic variables, it is possible to define variables that are external to all functions, that is, variables that can be accessed by name by any function. (This mechanism is rather like Fortran COMMON or Pascal variables declared in the outermost block.) Because external variables are globally accessible, they can be used instead of argument lists to communicate data between functions. Furthermore, because external variables remain in existence permanently, rather than appearing and disappearing as functions are called and exited, they retain their values even after the functions that set them have returned.

An external variable must be defined, exactly once, outside of any function; this sets aside storage for it. The variable must also be declared in each function that wants to access it; this states the type of the variable. The declaration may be an explicit extern statement or may be implicit from context. To make the discussion concrete, let us rewrite the longest-line program with line, longest, and max as external variables. This requires changing the calls, declarations, and bodies of all three functions.

#include <stdio.h>

#define MAXLINE 1000 /* maximum input line size */

int max; /* maximum length seen so far */
char line[MAXLINE]; /* current input line */
char longest[MAXLINE]; /* longest line saved here */

int getline(void);
void copy(void);

/* print longest input line; specialized version */
main()
{
int len;
extern int max;
extern char longest[];

max = 0;
while ((len = getline()) > 0)
if (len > max) {
max = len;
copy();
}
if (max > 0) /* there was a line */
printf(“%s,” longest);
return 0;
}

/* getline: specialized version */
int getline(void)
{
int c, i;
extern char line[];

for (i = 0; i < MAXLINE-1
&& (c=getchar()) != EOF && c != ′\n′; ++i)
line[i] = c;
if (c == ′\n′) {
line[i] = c;
++i;
}
line[i] = ′\0′;
return i;
}

/* copy: specialized version */
void copy(void)
{
int i;
extern char line[], longest[];

i = 0;
while ((longest[i] = line[i]) != ′\0′)
++i;
}

The external variables in main, getline, and copy are defined by the first lines of the example above, which state their type and cause storage to be allocated for them. Syntactically, external definitions are just like definitions of local variables, but since they occur outside of functions, the variables are external. Before a function can use an external variable, the name of the variable must be made known to the function. One way to do this is to write an extern declaration in the function; the declaration is the same as before except for the added keyword extern.

In certain circumstances, the extern declaration can be omitted. If the definition of an external variable occurs in the source file before its use in a particular function, then there is no need for an extern declaration in the function. The extern declarations in main, getline and copy are thus redundant. In fact, common practice is to place definitions of all external variables at the beginning of the source file, and then omit all extern declarations.

If the program is in several source files, and a variable is defined in file1 and used in file2 and file3, then extern declarations are needed in file2 and file3 to connect the occurrences of the variable. The usual practice is to collect extern declarations of variables and functions in a separate file, historically called a header, that is included by #include at the front of each source file. The suffix .h is conventional for header names. The functions of the standard library, for example, are declared in headers like <stdio.h>. This topic is discussed at length in Chapter 4, and the library itself in Chapter 7 and Appendix B.

Since the specialized versions of getline and copy have no arguments, logic would suggest that their prototypes at the beginning of the file should be getline() and copy(). But for compatibility with older C programs the standard takes an empty list as an old-style declaration, and turns off all argument list checking; the word void must be used for an explicitly empty list. We will discuss this further in Chapter 4.

You should note that we are using the words definition and declaration carefully when we refer to external variables in this section. “Definition” refers to the place where the variable is created or assigned storage; “declaration” refers to places where the nature of the variable is stated but no storage is allocated.

By the way, there is a tendency to make everything in sight an extern variable because it appears to simplify communications—argument lists are short and variables are always there when you want them. But external variables are always there even when you don’t want them. Relying too heavily on external variables is fraught with peril since it leads to programs whose data connections are not at all obvious—variables can be changed in unexpected and even inadvertent ways, and the program is hard to modify. The second version of the longest-line program is inferior to the first, partly for these reasons, and partly because it destroys the generality of two useful functions by wiring into them the names of the variables they manipulate.

At this point we have covered what might be called the conventional core of C. With this handful of building blocks, it’s possible to write useful programs of considerable size, and it would probably be a good idea if you paused long enough to do so. These### Exercises suggest programs of somewhat greater complexity than the ones earlier in this chapter.

! cat Chapter1/Exercise\ 1-20/detab.c

#include <stdio.h>

#define MAXLINE 1000
#define TAB_WIDTH 4

int getchars(char s[], int maxline);
void detab(char s1[], char s2[], int tabwidth);

int main(void)
{
    char s1[MAXLINE];
    char s2[MAXLINE];

    printf("Input some characters, then press enter:\n");
    while (getchars(s1, MAXLINE) == 0)
        ;

    detab(s1, s2, TAB_WIDTH);
    printf("detab result:\n%s\n", s2);

    return 0;
}

int getchars(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i)
        if (i < lim - 1)
          s[l++] = c;
    s[l] = '\0';

    return l;
}

// copy characters in s1 to s2 and replace tabs with proper number of blanks
void detab(char s1[], char s2[], int w)
{
    int i, j, l, c, blanks;

    i = 0;
    l = 0;
    while ((c = s1[i++]) != '\0') {
        if (c == '\t') {
            blanks = w - l % w;
            for (j = 0; j < blanks; ++j)
                s2[l++] = ' ';
        } else {
            s2[l++] = c;
        }
    }
    s2[l] = '\0';
}

! gcc Chapter1/Exercise\ 1-20/detab.c -o Chapter1/Exercise\ 1-20/detab

! Chapter1/Exercise\ 1-20/detab

! cat Chapter1/Exercise\ 1-21/entab.c

#include <stdio.h>

#define MAXLINE 1000
#define TAB_WIDTH 8

int getchars(char s[], int maxline);
void entab(char s1[], char s2[], int tabwidth);

int main(void)
{
    char s1[MAXLINE];
    char s2[MAXLINE];

    printf("Input some characters, then press enter:\n");
    while (getchars(s1, MAXLINE) == 0)
        ;

    entab(s1, s2, TAB_WIDTH);
    printf("entab result:\n%s\n", s2);

    return 0;
}

int getchars(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i)
        if (i < lim - 1)
          s[l++] = c;
    s[l] = '\0';

    return l;
}

// copy characters in s1 to s2 and replace blanks with tabs
void entab(char s1[], char s2[], int w)
{
    int i, j, l, c, blanks;
    int blanksenough;

    i = 0;
    l = 0;
    while ((c = s1[i]) != '\0') {
        if (c == ' ') {
            blanksenough = 1;
            blanks = w - i % w;
            for (j = 1; j < blanks; ++j)
                if (s1[i + j] != ' ') {
                    blanksenough = 0;
                    break;
                }
            if (blanksenough) {
                s2[l++] = '\t';
                i += blanks - 1;
            } else {
                s2[l++] = c;
            }
        } else {
            s2[l++] = c;
        }
        ++i;
    }
    s2[l] = '\0';
}

! gcc Chapter1/Exercise\ 1-21/entab.c -o Chapter1/Exercise\ 1-21/entab

! Chapter1/Exercise\ 1-21/entab

! cat Chapter1/Exercise\ 1-22/fold.c

#include <stdio.h>

#define MAXLINE 1000
#define TAB_WIDTH 8
#define LINE_WIDTH 10

int getchars(char s[], int maxline);
void fold(char s1[], char s2[], int linewidth, int tabwidth);

int main(void)
{
    char s1[MAXLINE];
    char s2[MAXLINE];

    printf("Input some characters, then press enter:\n");
    while (getchars(s1, MAXLINE) == 0)
        ;

    fold(s1, s2, LINE_WIDTH, TAB_WIDTH);
    printf("\nfold result:\n%s\n", s2);

    return 0;
}

int getchars(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i)
        if (i < lim - 1)
          s[l++] = c;
    s[l] = '\0';

    return l;
}

void fold(char s1[], char s2[], int lw, int tw)
{
    int i, l, c, k, w;

    w = 0;
    k = -1;
    i = l = 0;
    while ((c = s1[i]) != '\0') {
        if (c == ' ') {
            if (w + 1 == lw) {
                s2[l++] = '\n';
                w = 0;
                i = k;
            } else {
                s2[l++] = ' ';
                ++w;
            }
        } else if (c == '\t') {
            if (w + tw - w % tw >= lw) {
                s2[l++] = '\n';
                w = 0;
                i = k;
            } else {
                s2[l++] = '\t';
                w += tw;
            }
        } else {
            s2[l++] = c;
            if (w + 1 == lw) {
                s2[l++] = '\n';
                w = 0;
            } else {
                k = i;
                ++w;
            }
        }
        ++i;
    }
    s2[l++] = '\0';
}

! gcc Chapter1/Exercise\ 1-22/fold.c -o Chapter1/Exercise\ 1-22/fold

! Chapter1/Exercise\ 1-22/fold

! cat Chapter1/Exercise\ 1-23/comments.c

/ 
 * Test text, replace '@' with '/'

 @ 
  * Hello World
  *@
 #include <stdio.h> // for printf

 int main(void)
 {
     char s[] = "// hello \
         world!\n";
     printf("%s\n", s);
 
     return 0;
 }

 */


#include <stdio.h>

#define MAXLENGTH 5000

int _getline(char s[], int max);

int main(void)
{
    int len, i;
    char s[MAXLENGTH];

    
    printf("Input code, then press Ctrl+D:\n");
    while ((len = _getline(s, MAXLENGTH)) > 0) {
        printf("\nResult:\n=========================\n");
        i = 0;
        while (s[i] != '\0') {
            if (s[i] == '/' && s[i+1] == '/') {
                i += 2;
                while (s[i] != '\n' && s[i] != '\0')
                    ++i;
            } else if (s[i] == '/' && s[i+1] == '*') {
                i += 2;
                while (s[i] != '\0' && s[i+1] != '\0' && (s[i] != '*' || s[i+1] != '/'))
                    ++i;
                if (s[i] != '\0' && s[i+1] == '\0')
                    ++i;
                if (s[i] == '*')
                    i += 2;
            } else if (s[i] == '\"') {
                putchar('\"');
                ++i;
                while (s[i] != '\0' && (s[i-1] == '\\' || s[i] != '\"'))
                    putchar(s[i++]);
            } else {
                putchar(s[i++]);
            }
        }
    }
    
    return 0;
}

int _getline(char s[], int max)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF; ++i)
        if (i < max - 1)
            s[l++] = c;
    s[l] = '\0';

    return l;
}

! gcc Chapter1/Exercise\ 1-23/comments.c -o Chapter1/Exercise\ 1-23/comments

! Chapter1/Exercise\ 1-23/comments

2.1 Variable Names

Although we didn’t say so in Chapter 1, there are some restrictions on the names of variables and symbolic constants. Names are made up of letters and digits; the first character must be a letter. The underscore “_” counts as a letter; it is sometimes useful for improving the readability of long variable names. Don’t begin variable names with underscore, however, since library routines often use such names. Upper case and lower case letters are distinct, so x and X are two different names. Traditional C practice is to use lower case for variable names, and all upper case for symbolic constants.

At least the first 31 characters of an internal name are significant. For function names and external variables, the number may be less than 31, because external names may be used by assemblers and loaders over which the language has no control. For external names, the standard guarantees uniqueness only for 6 characters and a single case. Keywords like if, else, int, float, etc., are reserved: you can’t use them as variable names. They must be in lower case.

It’s wise to choose variable names that are related to the purpose of the variable, and that are unlikely to get mixed up typographically. We tend to use short names for local variables, especially loop indices, and longer names for external variables.

2.2 Data Types and Sizes

There are only a few basic data types in C:

In addition, there are a number of qualifiers that can be applied to these basic types. short and long apply to integers:

short int sh;
long int counter;

The word int can be omitted in such declarations, and typically is.

The intent is that short and long should provide different lengths of integers where practical; int will normally be the natural size for a particular machine. short is often 16 bits, long 32 bits, and int either 16 or 32 bits. Each compiler is free to choose appropriate sizes for its own hardware, subject only to the restriction that shorts and ints are at least 16 bits, longs are at least 32 bits, and short is no longer than int, which is no longer than long.

The qualifier signed or unsigned may be applied to char or any integer. unsigned numbers are always positive or zero, and obey the laws of arithmetic modulo 2ⁿ, where n is the number of bits in the type. So, for instance, if chars are 8 bits, unsigned char variables have values between 0 and 255, while signed chars have values between -128 and 127 (in a two’s complement machine). Whether plain chars are signed or unsigned is machine-dependent, but printable characters are always positive.

The type long double specifies extended-precision floating point. As with integers, the sizes of floating-point objects are implementation-defined; float, double and long double could represent one, two or three distinct sizes.

The standard headers <limits.h> and <float.h> contain symbolic constants for all of these sizes, along with other properties of the machine and compiler. These are discussed in Appendix B.

! cat Chapter2/Exercise\ 2-01/ranges.c

#include <stdio.h>
#include <limits.h>

int main(void)
{
    printf("Range of signed char is [%d, %d].\n", SCHAR_MIN, SCHAR_MAX);
    printf("Range of unsigned char is [%u, %u].\n", 0, UCHAR_MAX);
    printf("Range of char is [%d, %d].\n", CHAR_MIN, CHAR_MAX);

    printf("Range of signed short int is [%hd, %hd].\n", SHRT_MIN, SHRT_MAX);
    printf("Range of unsigned short int is [%hu, %hu].\n", 0, USHRT_MAX);

    printf("Range of signed int is [%d, %d].\n", INT_MIN, INT_MAX);
    printf("Range of unsigned int is [%u, %u].\n", 0, UINT_MAX);

    printf("Range of signed long int is [%ld, %ld].\n", LONG_MIN, LONG_MAX);
    printf("Range of unsigned long int is [%lu, %lu].\n", 0UL, ULONG_MAX);

    return 0;
}

! gcc Chapter2/Exercise\ 2-01/ranges.c -o Chapter2/Exercise\ 2-01/ranges

! Chapter2/Exercise\ 2-01/ranges

Range of signed char is [-128, 127].
Range of unsigned char is [0, 255].
Range of char is [-128, 127].
Range of signed short int is [-32768, 32767].
Range of unsigned short int is [0, 65535].
Range of signed int is [-2147483648, 2147483647].
Range of unsigned int is [0, 4294967295].
Range of signed long int is [-9223372036854775808, 9223372036854775807].
Range of unsigned long int is [0, 18446744073709551615].

2.3 Constants

An integer constant like 1234 is an int. A long constant is written with a terminal l (ell) or L, as in 123456789L; an integer too big to fit into an int will also be taken as a long. Unsigned constants are written with a terminal u or U, and the suffix ul or UL indicates unsigned long.

Floating-point constants contain a decimal point (123.4) or an exponent (1e-2) or both; their type is double, unless suffixed. The suffixes f or F indicate a float constant; l or L indicate a long double.

The value of an integer can be specified in octal or hexadecimal instead of decimal. A leading 0 (zero) on an integer constant means octal; a leading 0x or 0X means hexadecimal. For example, decimal 31 can be written as 037 in octal and 0x1f or 0X1F in hex. Octal and hexadecimal constants may also be followed by L to make them long and U to make them unsigned: 0XFUL is an unsigned long constant with value 15 decimal.

A character constant is an integer, written as one character within single quotes, such as ‘x’. The value of a character constant is the numeric value of the character in the machine’s character set. For example, in the ASCII character set the character constant ‘0’ has the value 48, which is unrelated to the numeric value 0. If we write ‘0’ instead of a numeric value like 48 that depends on character set, the program is independent of the particular value and easier to read. Character constants participate in numeric operations just as any other integers, although they are most often used in comparisons with other characters.

Certain characters can be represented in character and string constants by escape sequences like \n (newline); these sequences look like two characters, but represent only one. In addition, an arbitrary byte-sized bit pattern can be specified by

’\ooo ’

where ooo is one to three octal digits (0…7) or by

’\xhh ’

where hh is one or more hexadecimal digits (0…9, a…f, A…F). So we might write

#define VTAB ‘\013’ /* ASCII vertical tab */
#define BELL ‘\007’ /* ASCII bell character */

or, in hexadecimal,

#define VTAB ‘\xb’ /* ASCII vertical tab */
#define BELL ‘\x7’ /* ASCII bell character */

The complete set of escape sequences is

\a alert (bell) character
\b backspace
\f formfeed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\? question mark
\ ’ single quote
\" double quote
\ooo octal number
\xhh hexadecimal number

The character constant ‘\0’ represents the character with value zero, the null character. ‘\0’ is often written instead of 0 to emphasize the character nature of some expression, but the numeric value is just 0.

A constant expression is an expression that involves only constants. Such expressions may be evaluated during compilation rather than run-time, and accordingly may be used in any place that a constant can occur, as in

#define MAXLINE 1000
char line[MAXLINE+1];

or

#define LEAP 1 /* in leap years */
int days[31+28+LEAP+31+30+31+30+31+31+30+31+30+31];

A string constant, or string literal, is a sequence of zero or more characters surrounded by double quotes, as in

“I am a string”

or

"" /* the empty string */

The quotes are not part of the string, but serve only to delimit it. The same escape sequences used in character constants apply in strings; \" represents the double-quote character. String constants can be concatenated at compile time:

“hello,” " world"

is equivalent to

“hello, world”

This is useful for splitting long strings across several source lines.

Technically, a string constant is an array of characters. The internal representation of a string has a null character ‘\0’ at the end, so the physical storage required is one more than the number of characters written between the quotes. This representation means that there is no limit to how long a string can be, but programs must scan a string completely to determine its length. The standard library function strlen(s) returns the length of its character string argument s, excluding the terminal ‘\0’. Here is our version:

/* strlen: return length of s */
int strlen(char s[])
{
int i;

i = 0;  
while (s[i] !=  '\\0 ')  
    ++i;  
return i;  

}

strlen and other string functions are declared in the standard header <string.h>.

Be careful to distinguish between a character constant and a string that contains a single character: ‘x’ is not the same as “x”. The former is an integer, used to produce the numeric value of the letter x in the machine’s character set. The latter is an array of characters that contains one character (the letter x) and a ‘\0’.

There is one other kind of constant, the enumeration constant. An enumeration is a list of constant integer values, as in

enum boolean { NO, YES };

The first name in an enum has value 0, the next 1, and so on, unless explicit values are specified. If not all values are specified, unspecified values continue the progression from the last specified value, as in the second of these examples:

enum escapes { BELL = ‘\a,’ BACKSPACE = ‘\b,’ TAB = ‘\t,’
NEWLINE = ‘\n,’ VTAB = ‘\v,’ RETURN = ‘\r’ };

enum months { JAN = 1, FEB, MAR, APR, MAY, JUN,
JUL, AUG, SEP, OCT, NOV, DEC };
/* FEB is 2, MAR is 3, etc. */

Names in different enumerations must be distinct. Values need not be distinct in the same enumeration.

Enumerations provide a convenient way to associate constant values with names, an alternative to #define with the advantage that the values can be generated for you. Although variables of enum types may be declared, compilers need not check that what you store in such a variable is a valid value for the enumeration. Nevertheless, enumeration variables offer the chance of checking and so are often better than #defines. In addition, a debugger may be able to print values of enumeration variables in their symbolic form.

2.4 Declarations

All variables must be declared before use, although certain declarations can be made implicitly by context. A declaration specifies a type, and contains a list of one or more variables of that type, as in

int lower, upper, step;
char c, line[1000];

Variables can be distributed among declarations in any fashion; the lists above could equally well be written as

int lower;
int upper;
int step;
char c;
char line[1000];

This latter form takes more space, but is convenient for adding a comment to each declaration or for subsequent modifications.

A variable may also be initialized in its declaration. If the name is followed by an equals sign and an expression, the expression serves as an initializer, as in

char esc = ‘\\’;
int i = 0;
int limit = MAXLINE+1;
float eps = 1.0e-5;

If the variable in question is not automatic, the initialization is done once only, conceptually before the program starts executing, and the initializer must be a constant expression. An explicitly initialized automatic variable is initialized each time the function or block it is in is entered; the initializer may be any expression. External and static variables are initialized to zero by default. Automatic variables for which there is no explicit initializer have undefined (i.e., garbage) values.

The qualifier const can be applied to the declaration of any variable to specify that its value will not be changed. For an array, the const qualifier says that the elements will not be altered.

const double e = 2.71828182845905;
const char msg[] = “warning:”;

The const declaration can also be used with array arguments, to indicate that the function does not change that array:

int strlen(const char[]);

The result is implementation-defined if an attempt is made to change a const.

2.5 Arithmetic Operators

The binary arithmetic operators are +, -, *, /, and the modulus operator %. Integer division truncates any fractional part. The expression

x % y

produces the remainder when x is divided by y, and thus is zero when y divides x exactly. For example, a year is a leap year if it is divisible by 4 but not by 100, except that years divisible by 400 are leap years. Therefore

if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0)
printf(“%d is a leap year\n,” year);
else
printf(“%d is not a leap year\n,” year);

The % operator cannot be applied to float or double. The direction of truncation for / and the sign of the result for % are machine-dependent for negative operands, as is the action taken on overflow or underflow.

The binary + and - operators have the same precedence, which is lower than the precedence of *, /, and %, which is in turn lower than unary + and -. Arithmetic operators associate left to right.

Table 2-1 at the end of this chapter summarizes precedence and associativity for all operators.

2.6 Relational and Logical Operators

The relational operators are

> >= < <=

They all have the same precedence. Just below them in precedence are the equality operators:

== !=

Relational operators have lower precedence than arithmetic operators, so an expression like i < lim-1 is taken as i < (lim-1), as would be expected.

More interesting are the logical operators && and ||. Expressions connected by && or || are evaluated left to right, and evaluation stops as soon as the truth or falsehood of the result is known. Most C programs rely on these properties. For example, here is a loop from the input function getline that we wrote in Chapter 1:

for (i=0; i<lim-1 && (c=getchar()) != ‘\n’ && c != EOF; ++i)
s[i] = c;

Before reading a new character it is necessary to check that there is room to store it in the array s, so the test i < lim-1 must be made first. Moreover, if this test fails, we must not go on and read another character.

Similarly, it would be unfortunate if c were tested against EOF before getchar is called; therefore the call and assignment must occur before the character in c is tested.

The precedence of && is higher than that of ||, and both are lower than relational and equality operators, so expressions like

i<lim-1 && (c = getchar()) != ‘\n’ && c != EOF

need no extra parentheses. But since the precedence of != is higher than assignment, parentheses are needed in

(c = getchar()) != ‘\n’

to achieve the desired result of assignment to c and then comparison with ‘\n’.

By definition, the numeric value of a relational or logical expression is 1 if the relation is true, and 0 if the relation is false.

The unary negation operator ! converts a non-zero operand into 0, and a zero operand into 1. A common use of ! is in constructions like

if (!valid)

rather than

if (valid == 0)

It’s hard to generalize about which form is better. Constructions like !valid read nicely (“if not valid”), but more complicated ones can be hard to understand.

! cat Chapter2/Exercise\ 2-02/loop.c

#include <stdio.h>

#define MAXLINE 1000

int main(void)
{
    char line[MAXLINE];
    int c, i;

    i = 0;
    while (i < MAXLINE - 1) {
    if ((c=getchar()) == EOF)
        break;
    line[i++] = c;
    if (c == '\n')
        break;
    }
    line[i] = '\0';

    printf("%s\n", line);

    return 0;
}

! gcc Chapter2/Exercise\ 2-02/loop.c -o Chapter2/Exercise\ 2-02/loop

2.7 Type Conversions

When an operator has operands of different types, they are converted to a common type according to a small number of rules. In general, the only automatic conversions are those that convert a “narrower” operand into a “wider” one without losing information, such as converting an integer to floating point in an expression like f + i. Expressions that don’t make sense, like using a float as a subscript, are disallowed. Expressions that might lose information, like assigning a longer integer type to a shorter, or a floating-point type to an integer, may draw a warning, but they are not illegal.

A char is just a small integer, so chars may be freely used in arithmetic expressions. This permits considerable flexibility in certain kinds of character transformations. One is exemplified by this naive implementation of the function atoi, which converts a string of digits into its numeric equivalent.

/* atoi: convert s to integer */
int atoi(char s[])
{
int i, n;

n = 0;  
for (i = 0; s[i] \>=  '0 ' && s[i] \<=  '9 '; ++i)  
    n = 10 <span class="bottom">\*</span> n + (s[i] -  '0 ');  
return n;  

}

As we discussed in Chapter 1, the expression

s[i] - ‘0’

gives the numeric value of the character stored in s[i], because the values of ‘0’, ‘1’, etc., form a contiguous increasing sequence.

Another example of char to int conversion is the function lower, which maps a single character to lower case for the ASCII character set. If the character is not an upper case letter, lower returns it unchanged.

/* lower: convert c to lower case; ASCII only */
int lower(int c)
{
if (c >= ‘A’ && c <= ‘Z’)
return c + ‘a’ - ‘A’;
else
return c;
}

This works for ASCII because corresponding upper case and lower case letters are a fixed distance apart as numeric values and each alphabet is contiguous ’"there is nothing but letters between A and Z. This latter observation is not true of the EBCDIC character set, however, so this code would convert more than just letters in EBCDIC.

The standard header <ctype.h>, described in Appendix B, defines a family of functions that provide tests and conversions that are independent of character set. For example, the function tolower(c) returns the lower case value of c if c is upper case, so tolower is a portable replacement for the function lower shown above. Similarly, the test

c >= ‘0’ && c <= ‘9’

can be replaced by

isdigit(c)

We will use the <ctype.h> functions from now on.

There is one subtle point about the conversion of characters to integers. The language does not specify whether variables of type char are signed or unsigned quantities. When a char is converted to an int, can it ever produce a negative integer? The answer varies from machine to machine, reflecting differences in architecture. On some machines a char whose leftmost bit is 1 will be converted to a negative integer (“sign extension”). On others, a char is promoted to an int by adding zeros at the left end, and thus is always positive.

The definition of C guarantees that any character in the machine’s standard printing character set will never be negative, so these characters will always be positive quantities in expressions. But arbitrary bit patterns stored in character variables may appear to be negative on some machines, yet positive on others. For portability, specify signed or unsigned if non-character data is to be stored in char variables.

Relational expressions like i > j and logical expressions connected by && and || are defined to have value 1 if true, and 0 if false. Thus the assignment

d = c >= ‘0’ && c <= ‘9’

sets d to 1 if c is a digit, and 0 if not. However, functions like isdigit may return any non-zero value for true. In the test part of if, while, for, etc., “true” just means “non-zero,” so this makes no difference.

Implicit arithmetic conversions work much as expected. In general, if an operator like + or * that takes two operands (a binary operator) has operands of different types, the “lower” type is promoted to the “higher” type before the operation proceeds. The result is of the higher type. Section 6 of Appendix A states the conversion rules precisely. If there are no unsigned operands, however, the following informal set of rules will suffice:

If either operand is long double, convert the other to long double.

Otherwise, if either operand is double, convert the other to double.

Otherwise, if either operand is float, convert the other to float.

Otherwise, convert char and short to int.

Then, if either operand is long, convert the other to long.

Notice that floats in an expression are not automatically converted to double; this is a change from the original definition. In general, mathematical functions like those in <math.h> will use double precision. The main reason for using float is to save storage in large arrays, or, less often, to save time on machines where double-precision arithmetic is particularly expensive.

Conversion rules are more complicated when unsigned operands are involved. The problem is that comparisons between signed and unsigned values are machine-dependent, because they depend on the sizes of the various integer types. For example, suppose that int is 16 bits and long is 32 bits. Then -1L < 1U, because 1U, which is an int, is promoted to a signed long. But -1L > 1UL, because -1L is promoted to unsigned long and thus appears to be a large positive number.

Conversions take place across assignments; the value of the right side is converted to the type of the left, which is the type of the result.

A character is converted to an integer, either by sign extension or not, as described above.

Longer integers are converted to shorter ones or to chars by dropping the excess high-order bits. Thus in

int i;
char c;

i = c;
c = i;

the value of c is unchanged. This is true whether or not sign extension is involved. Reversing the order of assignments might lose information, however.

If x is float and i is int, then x = i and i = x both cause conversions; float to int causes truncation of any fractional part. When double is converted to float, whether the value is rounded or truncated is implementation-dependent.

Since an argument of a function call is an expression, type conversions also take place when arguments are passed to functions. In the absence of a function prototype, char and short become int, and float becomes double. This is why we have declared function arguments to be int and double even when the function is called with char and float.

Finally, explicit type conversions can be forced (“coerced”) in any expression, with a unary operator called a cast. In the construction

(type-name) expression

the expression is converted to the named type by the conversion rules above. The precise meaning of a cast is as if the expression were assigned to a variable of the specified type, which is then used in place of the whole construction. For example, the library routine sqrt expects a double argument, and will produce nonsense if inadvertently handed something else. (sqrt is declared in <math.h>.) So if n is an integer, we can use

sqrt((double) n)

to convert the value of n to double before passing it to sqrt. Note that the cast produces the value of n in the proper type; n itself is not altered. The cast operator has the same high precedence as other unary operators, as summarized in the table at the end of this chapter.

If arguments are declared by a function prototype, as they normally should be, the declaration causes automatic coercion of any arguments when the function is called. Thus, given a function prototype for sqrt:

double sqrt(double);

the call

root2 = sqrt(2);

coerces the integer 2 into the double value 2.0 without any need for a cast.

The standard library includes a portable implementation of a pseudo-random number generator and a function for initializing the seed; the former illustrates a cast:

unsigned long int next = 1;

/* rand: return pseudo-random integer on 0..32767 */
int rand(void)
{
next = next * 1103515245 + 12345;
return (unsigned int)(next/65536) % 32768;
}

/* srand: set seed for rand() */
void srand(unsigned int seed)
{
next = seed;
}

! cat Chapter2/Exercise\ 2-03/htoi.c

#include <stdio.h>

#define MAXLINE 1000

int get_line(char line[], int maxline);
unsigned long htoi(char s[]);

int main(void)
{
    int len;
    char line[MAXLINE];

    while ((len = get_line(line, MAXLINE)) > 0)
        printf("%lu\n", htoi(line));

    return 0;
}

int get_line(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i)
        if (i < lim - 1)
            s[l++] = c;
    // if (c == '\n' && l < lim - 1)
    //      s[l++] = c;
    s[l] = '\0';

    return l;
}

unsigned long htoi(char s[])
{
    unsigned long n;
    int i, c;

    n = 0;
    for (i = 0; s[i] != '\0'; ++i) {
        c = s[i];
        if (i == 0 && c == '0' && (s[1] == 'x' || s[1] == 'X')) {
            i = 1;
            continue;
    }
    n *= 16;
    if (c >= '0' && c <= '9')
        n += c - '0';
    else if (c >= 'a' && c <= 'f')
        n += c - 'a';
    else if (c >= 'A' && c <= 'F')
        n += c - 'A';
    }

    return n;
}

! gcc Chapter2/Exercise\ 2-03/htoi.c -o Chapter2/Exercise\ 2-03/htoi

2.8 Increment and Decrement Operators

C provides two unusual operators for incrementing and decrementing variables. The increment operator ++ adds 1 to its operand, while the decrement operator – subtracts 1. We have frequently used ++ to increment variables, as in

if (c == ‘\n’)
++nl;

The unusual aspect is that ++ and – may be used either as prefix operators (before the variable, as in ++n), or postfix (after the variable: n++). In both cases, the effect is to increment n. But the expression ++n increments n before its value is used, while n++ increments n after its value has been used. This means that in a context where the value is being used, not just the effect, ++n and n++ are different. If n is 5, then

x = n++;

sets x to 5, but

x = ++n;

sets x to 6. In both cases, n becomes 6. The increment and decrement operators can only be applied to variables; an expression like (i+j)++ is illegal.

In a context where no value is wanted, just the incrementing effect, as in

if (c == ‘\n’)
nl++;

prefix and postfix are the same. But there are situations where one or the other is specifically called for. For instance, consider the function squeeze(s,c), which removes all occurrences of the character c from the string s.

/* squeeze: delete all c from s */
void squeeze(char s[], int c)
{
int i, j;

for (i = j = 0; s[i] !=  '\\0 '; i++)  
    if (s[i] != c)  
        s[j++] = s[i];  
s[j] =  '\\0 ';  

}

Each time a non-c occurs, it is copied into the current j position and only then is j incremented to be ready for the next character. This is exactly equivalent to

if (s[i] != c) {
s[j] = s[i];
j++;
}

Another example of a similar construction comes from the getline function that we wrote in Chapter 1, where we can replace

if (c == ‘\n’) {
s[i] = c;
++i;
}

by the more compact

if (c == ‘\n’)
s[i++] = c;

As a third example, consider the standard function strcat(s,t), which concatenates the string t to the end of the string s. strcat assumes that there is enough space in s to hold the combination. As we have written it, strcat returns no value; the standard library version returns a pointer to the resulting string.

/* strcat: concatenate t to end of s; s must be big enough */
void strcat(char s[], char t[])
{
int i, j;

i = j = 0;  
while (s[i] !=  '\\0 ')    /<span

class=“bottom”>* find end of s */
i++;
while ((s[i++] = t[j++]) != ‘\0’) /* copy t */
;
}

As each character is copied from t to s, the postfix ++ is applied to both i and j to make sure that they are in position for the next pass through the loop.

! cat Chapter2/Exercise\ 2-04/squeeze.c

#include <stdio.h>

#define MAXLINE 1000

int get_line(char line[], int maxline);
void squeeze(char s1[], char s2[]);

int main(void)
{
    int len;
    char s1[MAXLINE];
    char s2[MAXLINE];

    printf("Input string s1:\n");
    while ((len = get_line(s1, MAXLINE)) == 0)
        ;

    printf("Input string s2:\n");
    while ((len = get_line(s2, MAXLINE)) == 0)
        ;

    squeeze(s1, s2);
    printf("Result is %s\n", s1);

    return 0;
}

int get_line(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i)
        if (i < lim - 1)
            s[l++] = c;
    s[l] = '\0';

    return l;
}

/* This implementation is a bit more complicated*/
void squeeze(char s1[], char s2[])
{
    int i, j, k;

    i = 0;
    while (s2[i] != '\0') {
        j = 0;
        while (s1[j] != '\0') {
            if (s1[j] == s2[i]) {
                k = j;
                while ((s1[k] = s1[++k]) != '\0')
                    ;
            } else
                ++j;
        }
        ++i;
    }
}

! gcc Chapter2/Exercise\ 2-04/squeeze.c -o Chapter2/Exercise\ 2-04/squeeze

! cat Chapter2/Exercise\ 2-05/any.c

#include <stdio.h>

#define MAXLINE 1000

int get_line(char line[], int maxline);
int any(char s1[], char s2[]);

int main(void)
{
    int len;
    char s1[MAXLINE];
    char s2[MAXLINE];

    printf("Input string s1:\n");
    while ((len = get_line(s1, MAXLINE)) == 0)
        ;

    printf("Input string s2:\n");
    while ((len = get_line(s2, MAXLINE)) == 0)
        ;

    printf("The first location is %d\n", any(s1, s2));

    return 0;
}

int get_line(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i)
        if (i < lim - 1)
            s[l++] = c;
    s[l] = '\0';

    return l;
}

int any(char s1[], char s2[])
{
    int i, j;

    i = 0;
    while (s2[i] != '\0') {
        j = 0;
        while (s1[j] != '\0') {
            if (s1[j] == s2[i])
                return j;
            ++j;
        }
        ++i;
    }

    return -1;
}

! gcc Chapter2/Exercise\ 2-05/any.c -o Chapter2/Exercise\ 2-05/any

2.9 Bitwise Operators

C provides six operators for bit manipulation; these may only be applied to integral operands, that is, char, short, int, and long, whether signed or unsigned.

& bitwise AND
| bitwise inclusive OR
^ bitwise exclusive OR
<< left shift
>> right shift
~ one’s complement (unary)

The bitwise AND operator & is often used to mask off some set of bits; for example,

n = n & 0177;

sets to zero all but the low-order 7 bits of n.

The bitwise OR operator | is used to turn bits on:

x = x | SET_ON;

sets to one in x the bits that are set to one in SET_ON.

The bitwise exclusive OR operator ^ sets a one in each bit position where its operands have different bits, and zero where they are the same.

One must distinguish the bitwise operators & and | from the logical operators && and ||, which imply left-to-right evaluation of a truth value. For example, if x is 1 and y is 2, then x & y is zero while x && y is one.

The shift operators << and >> perform left and right shifts of their left operand by the number of bit positions given by the right operand, which must be positive. Thus x << 2 shifts the value of x left by two positions, filling vacated bits with zero; this is equivalent to multiplication by 4. Right shifting an unsigned quantity always fills vacated bits with zero. Right shifting a signed quantity will fill with sign bits (“arithmetic shift”) on some machines and with 0-bits (“logical shift”) on others.

The unary operator ~ yields the one’s complement of an integer; that is, it converts each 1-bit into a 0-bit and vice versa. For example,

x = x & ~077

sets the last six bits of x to zero. Note that x & ~077 is independent of word length, and is thus preferable to, for example, x & 0177700, which assumes that x is a 16-bit quantity. The portable form involves no extra cost, since ~077 is a constant expression that can be evaluated at compile time.

As an illustration of some of the bit operators, consider the function getbits(x,p,n) that returns the (right adjusted) n-bit field of x that begins at position p. We assume that bit position 0 is at the right end and that n and p are sensible positive values. For example, getbits(x,4,3) returns the three bits in bit positions 4, 3 and 2, right adjusted.

/* getbits: get n bits from position p */
unsigned getbits(unsigned x, int p, int n)
{
return (x >> (p+1-n)) & ~(~0 << n);
}

The expression x >> (p+1-n) moves the desired field to the right end of the word. ~0 is all 1-bits; shifting it left n bit positions with ~0<<n places zeros in the rightmost n bits; complementing that with ~ makes a mask with ones in the rightmost n bits.

! cat Chapter2/Exercise\ 2-06/setbits.c

#include <stdio.h>

unsigned setbits(unsigned x, int p, int n, unsigned y);

int main(void)
{
    printf("%u\n", setbits(5732, 6, 3, 9823));
    return 0;
}

unsigned setbits(unsigned x, int p, int n, unsigned y)
{
    // xxxxx00000xxxxx
    unsigned a = x & ~(~(~0U << n) << (p + 1 - n));

    // 00000yyyyy00000
    unsigned b = (y & ~(~0U << n)) << (p + 1 - n);

    // xxxxxyyyyyxxxxx
    return a | b;
}

! gcc Chapter2/Exercise\ 2-06/setbits.c -o Chapter2/Exercise\ 2-06/setbits

! cat Chapter2/Exercise\ 2-07/invert.c

#include <stdio.h>

unsigned invert(unsigned x, int p, int n);

int main(void)
{
    printf("%u\n", invert(5732, 6, 3));
    return 0;
}

unsigned invert(unsigned x, int p, int n)
{   
    // 000001111100000
    unsigned mask = ~(~0U << n) << (p + 1 - n);
     
    // xxxxx^^^^^xxxxx
    return x ^ mask;
}

! gcc Chapter2/Exercise\ 2-07/invert.c -o Chapter2/Exercise\ 2-07/invert

! cat Chapter2/Exercise\ 2-08/rightrot.c

#include <stdio.h>

unsigned rightroc(unsigned x, int n);

int main(void)
{
    printf("%u\n", rightroc(5732, 3));
    return 0;
}

unsigned rightroc(unsigned x, int n)
{   
    while (n-- > 0)
        if(x & 1)
            x = (x >> 1) | ~(~0U >> 1);
        else
            x = x >> 1;
    return x;
}

! gcc Chapter2/Exercise\ 2-08/rightrot.c -o Chapter2/Exercise\ 2-08/rightrot

2.10 Assignment Operators and Expressions

Expressions such as

i = i + 2

in which the variable on the left hand side is repeated immediately on the right, can be written in the compressed form

i += 2

The operator += is called an assignment operator.

Most binary operators (operators like + that have a left and right operand) have a corresponding assignment operator op =, where op is one of

• • * / % << >> & ^ |

If expr₁ and expr₂ are expressions, then

expr₁ op = expr₂

is equivalent to

expr₁ = (expr₁) op (expr₂)

except that expr₁ is computed only once. Notice the parentheses around expr₂:

x *= y + 1

means

x = x * (y + 1)

rather than

x = x * y + 1

As an example, the function bitcount counts the number of 1-bits in its integer argument.

/* bitcount: count 1 bits in x */
int bitcount(unsigned x)
{
int b;

for (b = 0; x != 0; x \>\>= 1)  
    if (x & 01)  
        b++;  
return b;  

}

Declaring the argument x to be unsigned ensures that when it is right-shifted, vacated bits will be filled with zeros, not sign bits, regardless of the machine the program is run on.

Quite apart from conciseness, assignment operators have the advantage that they correspond better to the way people think. We say “add 2 to i” or “increment i by 2,” not “take i, add 2, then put the result back in i.” Thus the expression i += 2 is preferable to i = i+2. In addition, for a complicated expression like

yyval[yypv[p3+p4] + yypv[p1+p2]] += 2

the assignment operator makes the code easier to understand, since the reader doesn’t have to check painstakingly that two long expressions are indeed the same, or to wonder why they’re not. And an assignment operator may even help a compiler to produce efficient code.

We have already seen that the assignment statement has a value and can occur in expressions; the most common example is

while ((c = getchar()) != EOF)
…

The other assignment operators (+=, -=, etc.) can also occur in expressions, although this is less frequent.

In all such expressions, the type of an assignment expression is the type of its left operand, and the value is the value after the assignment.

! cat Chapter2/Exercise\ 2-09/bitcount.c

#include <stdio.h>

int bitcount(unsigned x);

int main(void)
{
    printf("%d\n", bitcount(011111));
    return 0;
}

/* bitcount: count 1 bits in x */
int bitcount(unsigned x)
{
    int b;

    for (b = 0; x != 0; x &= (x - 1))
        b++;
    return b;
}

! gcc Chapter2/Exercise\ 2-09/bitcount.c -o Chapter2/Exercise\ 2-09/bitcount

2.11 Conditional Expressions

The statements

if (a > b)
z = a;
else
z = b;

compute in z the maximum of a and b. The conditional expression, written with the ternary operator “?:”, provides an alternate way to write this and similar constructions. In the expression

expr₁ ? expr₂ : expr₃

the expression expr₁ is evaluated first. If it is non-zero (true), then the expression expr₂ is evaluated, and that is the value of the conditional expression. Otherwise expr₃ is evaluated, and that is the value. Only one of expr₂ and expr₃ is evaluated. Thus to set z to the maximum of a and b,

z = (a > b) ? a : b; /* z = max(a, b) */

It should be noted that the conditional expression is indeed an expression, and it can be used wherever any other expression can be. If expr₂ and expr₃ are of different types, the type of the result is determined by the conversion rules discussed earlier in this chapter. For example, if f is a float and n is an int, then the expression

(n > 0) ? f : n

is of type float regardless of whether n is positive.

Parentheses are not necessary around the first expression of a conditional expression, since the precedence of ?: is very low, just above assignment. They are advisable anyway, however, since they make the condition part of the expression easier to see.

The conditional expression often leads to succinct code. For example, this loop prints n elements of an array, 10 per line, with each column separated by one blank, and with each line (including the last) terminated by a newline.

for (i = 0; i < n; i++)
printf(“%6d%c,” a[i], (i%10==9 ¦ ¦ i==n-1) ? ‘\n’ : ’ ’);

A newline is printed after every tenth element, and after the n-th. All other elements are followed by one blank. This might look tricky, but it’s more compact than the equivalent if-else. Another good example is

printf(“You have %d item%s.\n,” n, n==1 ? "" : “s”);

! cat Chapter2/Exercise\ 2-10/lower.c

#include <stdio.h>

int lower(int c);

int main(void)
{
    char s[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    int i;

    i = 0;
    while (s[i] != '\0') {
        printf("%c -> %c\n", s[i], lower(s[i]));
        i++;
    }

    return 0;
}

int lower(int c)
{
    return (c >= 'A' && c<= 'Z') ? c + 'a' - 'A' : c;
}

! gcc Chapter2/Exercise\ 2-10/lower.c -o Chapter2/Exercise\ 2-10/lower

2.12 Precedence and Order of Evaluation

Table 2-1 summarizes the rules for precedence and associativity of all operators, including those that we have not yet discussed. Operators on the same line have the same precedence; rows are in order of decreasing precedence, so, for example, *, /, and % all have the same precedence, which is higher than that of binary + and -. The “operator” () refers to function call. The operators -> and . are used to access members of structures; they will be covered in Chapter 6, along with sizeof (size of an object). Chapter 5 discusses * (indirection through a pointer) and & (address of an object), and Chapter 3 discusses the comma operator.

Note that the precedence of the bitwise operators &, ^, and | falls below == and !=. This implies that bit-testing expressions like

if ((x & MASK) == 0) …

must be fully parenthesized to give proper results.

C, like most languages, does not specify the order in which the operands of an operator are evaluated. (The exceptions are &&, ||, ?:, and ‘,’.) For example, in a statement like

x = f() + g();

f may be evaluated before g or vice versa; thus if either f or g alters a variable on which the other depends, x can depend on the order of evaluation. Intermediate results can be stored in temporary variables to ensure a particular sequence.

Similarly, the order in which function arguments are evaluated is not specified, so the statement

printf(“%d %d\n,” ++n, power(2, n)); /* WRONG */

can produce different results with different compilers, depending on whether n is incremented before power is called. The solution, of course, is to write

++n;
printf(“%d %d\n,” n, power(2, n));

Function calls, nested assignment statements, and increment and decrement operators cause “side effects” some variable is changed as a by-product of the evaluation of an expression. In any expression involving side effects, there can be subtle dependencies on the order in which variables taking part in the expression are updated. One unhappy situation is typified by the statement

a[i] = i++;

The question is whether the subscript is the old value of i or the new. Compilers can interpret this in different ways, and generate different answers depending on their interpretation. The standard intentionally leaves most such matters unspecified. When side effects (assignment to variables) take place within an expression is left to the discretion of the compiler, since the best order depends strongly on machine architecture. (The standard does specify that all side effects on arguments take effect before a function is called, but that would not help in the call to printf above.)

The moral is that writing code that depends on order of evaluation is a bad programming practice in any language. Naturally, it is necessary to know what things to avoid, but if you don’t know how they are done on various machines, you won’t be tempted to take advantage of a particular implementation.

TABLE 2-1. PRECEDENCE AND ASSOCIATIVITY OF OPERATORS

3.1 Statements and Blocks

An expression such as x = 0 or i++ or printf(…) becomes a statement when it is followed by a semicolon, as in

x = 0;
i++;
printf(…);

In C, the semicolon is a statement terminator, rather than a separator as it is in languages like Pascal.

Braces { and } are used to group declarations and statements together into a compound statement, or block, so that they are syntactically equivalent to a single statement. The braces that surround the statements of a function are one obvious example; braces around multiple statements after an if, else, while, or for are another. (Variables can be declared inside any block; we will talk about this in Chapter 4.) There is no semicolon after the right brace that ends a block.

3.2 If-Else

The if-else statement is used to express decisions. Formally, the syntax is

if (expression)
statement₁
else
statement₂

where the else part is optional. The expression is evaluated; if it is true (that is, if expression has a non-zero value), statement₁ is executed. If it is false (expression is zero) and if there is an else part, statement₂ is executed instead.

Since an if simply tests the numeric value of an expression, certain coding shortcuts are possible. The most obvious is writing

if (expression)

instead of

if (expression != 0)

Sometimes this is natural and clear; at other times it can be cryptic.

Because the else part of an if-else is optional, there is an ambiguity when an else is omitted from a nested if sequence. This is resolved by associating the else with the closest previous else-less if. For example, in

if (n > 0)
if (a > b)
z = a;
else
z = b;

the else goes with the inner if, as we have shown by indentation. If that isn’t what you want, braces must be used to force the proper association:

if (n > 0) {
if (a > b)
z = a;
}
else
z = b;

The ambiguity is especially pernicious in situations like this:

if (n >= 0)
for (i = 0; i < n; i++)
if (s [i] > 0) {
printf(“…”);
return i;
}
else /* WRONG */
printf(“error – n is negative\n”);

The indentation shows unequivocally what you want, but the compiler doesn’t get the message, and associates the else with the inner if. This kind of bug can be hard to find; it’s a good idea to use braces when there are nested ifs.

By the way, notice that there is a semicolon after z = a in

if (a > b)
z = a;
else
z = b;

This is because grammatically, a statement follows the if, and an expression statement like “z = a;” is always terminated by a semicolon.

3.3 Else-If

The construction

if (expression)
statement
else if (expression)
statement
else if (expression)
statement
else if (expression)
statement
else
statement

occurs so often that it is worth a brief separate discussion. This sequence of if statements is the most general way of writing a multi-way decision. The expressions are evaluated in order; if any expression is true, the statement associated with it is executed, and this terminates the whole chain. As always, the code for each statement is either a single statement, or a group in braces.

The last else part handles the “none of the above” or default case where none of the other conditions is satisfied. Sometimes there is no explicit action for the default; in that case the trailing

else
statement

can be omitted, or it may be used for error checking to catch an “impossible” condition.

To illustrate a three-way decision, here is a binary search function that decides if a particular value x occurs in the sorted array v. The elements of v must be in increasing order. The function returns the position (a number between 0 and n−1) if x occurs in v, and −1 if not.

Binary search first compares the input value x to the middle element of the array v. If x is less than the middle value, searching focuses on the lower half of the table, otherwise on the upper half. In either case, the next step is to compare x to the middle element of the selected half. This process of dividing the range in two continues until the value is found or the range is empty.

/* binsearch: find x in v [0] <= v [1] <= … <= v [n−1] */
int binsearch(int x, int v [], int n)
{
int low, high, mid;

low = 0;
high = n - 1;
while (low <= high) {
mid = (low+high) / 2;
if (x < v [mid])
high = mid - 1;
else if (x > v [mid])
low = mid + 1;
else /* found match */
return mid;
}
return −1; /* no match */
}

The fundamental decision is whether x is less than, greater than, or equal to the middle element v [mid] at each step; this is a natural for else-if.

! cat Chapter3/Exercise\ 3-01/binsearch.c

#include <stdio.h>

int binsearch(int x, int v[], int n);

int main(void)
{
    int x = 12, n = 10;
    int v[10] = {2, 3, 7, 10, 11, 12, 15, 18, 20, 21};
    printf("binsearch result: %d\n", binsearch(x, v, n));
    return 0;
}

/* binsearch: find x in v[0] <= v[1] <= ... <= v[n-1] */
int binsearch(int x, int v[], int n)
{
    int low, high, mid;

    low = 0;
    high = n - 1;
    while (low < high) {
        mid = (low + high) / 2;
        if (x <= v[mid])
            high = mid;
        else
            low = mid + 1;
    }
    return (x == v[low]) ? low : -1;
}

! gcc Chapter3/Exercise\ 3-01/binsearch.c -o Chapter3/Exercise\ 3-01/binsearch

! Chapter3/Exercise\ 3-01/binsearch

binsearch result: 5

3.4 Switch

The switch statement is a multi-way decision that tests whether an expression matches one of a number of constant integer values, and branches accordingly.

switch (expression) {
case const-expr : statements
case const-expr : statements
default: statements
}

Each case is labeled by one or more integer-valued constants or constant expressions. If a case matches the expression value, execution starts at that case. All case expressions must be different. The case labeled default is executed if none of the other cases are satisfied. A default is optional; if it isn’t there and if none of the cases match, no action at all takes place. Cases and the default clause can occur in any order.

In Chapter 1 we wrote a program to count the occurrences of each digit, white space, and all other characters, using a sequence of if … else if … else. Here is the same program with a switch:

#include \<stdio.h\>
  
main()  \* count digits, white space, others \*  
{  
  int c, i, nwhite, nother, ndigit[10];  
  
  nwhite = nother = 0;  
  for (i = 0; i < 10; i++)  
    ndigit[i] = 0;  
  while ((c = getchar()) != EOF) {  
    switch (c) {  
    case ′0′: case ′1′: case ′2′: case ′3′: case ′4′:  
    case ′5′: case ′6′: case ′7′: case ′8′: case ′9′:  
      ndigit[c-′0′]++;  
      break;  
    case ′ ′:  
    case ′\n′:  
    case ′\t′:  
      nwhite++;  
      break;  
    default:  
      nother++;  
      break;  
    }  
  }  
  printf("digits =");  
  for (i = 0; i < 10; i++)  
    printf(" %d", ndigit[i]);  
  printf(", white space = %d, other = %d\n",  
    nwhite, nother);  
  return 0;  
}

The break statement causes an immediate exit from the switch. Because cases serve just as labels, after the code for one case is done, execution falls through to the next unless you take explicit action to escape. break and return are the most common ways to leave a switch. A break statement can also be used to force an immediate exit from while, for, and do loops, as will be discussed later in this chapter.

Falling through cases is a mixed blessing. On the positive side, it allows several cases to be attached to a single action, as with the digits in this example. But it also implies that normally each case must end with a break to prevent falling through to the next. Falling through from one case to another is not robust, being prone to disintegration when the program is modified. With the exception of multiple labels for a single computation, fall-throughs should be used sparingly, and commented.

As a matter of good form, put a break after the last case (the default here) even though it’s logically unnecessary. Some day when another case gets added at the end, this bit of defensive programming will save you.

! cat Chapter3/Exercise\ 3-02/escape.c

#include <stdio.h>

#define MAXLINE 1000

int getchars(char line[], int maxline);
void escape(char s[], char t[]);
void unescape(char s[], char t[]);

int main(void)
{
    char t[MAXLINE];
    char s1[MAXLINE];
    char s2[MAXLINE];

    printf("Input some characters, then press Ctrl+D:\n");
    while (getchars(t, MAXLINE) == 0)
        ;

    escape(s1, t);
    printf("\nescape result:\n%s\n", s1);
    
    unescape(s2, s1);
    printf("unescape result:\n%s\n", s2);

    return 0;
}

int getchars(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF; ++i)
        if (i < lim - 1)
            s[l++] = c;
    s[l] = '\0';

    return l;
}

void escape(char s[], char t[])
{
    int i, j;

    for (i = 0, j = 0; t[i] != '\0'; ++i) {
        switch (t[i]) {
        case '\n':
            s[j++] =  '\\';
            s[j++] =  'n';
            break;
        case '\t':
            s[j++] =  '\\';
            s[j++] =  't';
            break;
        case '\v':
            s[j++] =  '\\';
            s[j++] =  'v';
            break;
        case '\b':
            s[j++] =  '\\';
            s[j++] =  'b';
            break;
        case '\r':
            s[j++] =  '\\';
            s[j++] =  'r';
            break;
        case '\f':
            s[j++] =  '\\';
            s[j++] =  'f';
            break;
        case '\a':
            s[j++] =  '\\';
            s[j++] =  'a';
            break;
        case '\\':
            s[j++] =  '\\';
            s[j++] =  '\\';
            break;
        case '\?':
            s[j++] =  '\\';
            s[j++] =  '\?';
            break;
        case '\'':
            s[j++] =  '\\';
            s[j++] =  '\'';
            break;
        case '\"':
            s[j++] =  '\\';
            s[j++] =  '\"';
            break;
        default:
            s[j++] = t[i];
            break;
        }
    }
    s[j] = '\0';
}

void unescape(char s[], char t[])
{
    int i, j;

    for (i = 0, j = 0; t[i] != '\0'; ++i) {
        if (t[i] == '\\') {
            switch (t[++i]) {
            case 'n':
                s[j++] =  '\n';
                break;
            case 't':
                s[j++] =  '\t';
                break;
            case 'v':
                s[j++] =  '\v';
                break;
            case 'b':
                s[j++] =  '\b';
                break;
            case 'r':
                s[j++] =  '\r';
                break;
            case 'f':
                s[j++] =  '\f';
                break;
            case 'a':
                s[j++] =  '\a';
                break;
            case '\\':
                s[j++] =  '\\';
                break;
            case '\?':
                s[j++] =  '\?';
                break;
            case '\'':
                s[j++] =  '\'';
                break;
            case '\"':
                s[j++] =  '\"';
                break;
            // actually, cases below should never happen if the input string is correct.
            case '\0':
            default:
                s[j++] = '\\';
                --i;
                break;
            }
        } else {
            s[j++] =  t[i];
        }
    }
    s[j] = '\0';
}

! gcc Chapter3/Exercise\ 3-02/escape.c -o Chapter3/Exercise\ 3-02/escape

! Chapter3/Exercise\ 3-02/escape

Input some characters, then press Ctrl+D:

3.5 Loops—While and For

We have already encountered the while and for loops. In

while (expression)
statement

the expression is evaluated. If it is non-zero, statement is executed and expression is re-evaluated. This cycle continues until expression becomes zero, at which point execution resumes after statement.

The for statement

for (expr₁; expr₂; expr₃)
statement

is equivalent to

expr₁;
while (expr₂) {
statement
expr₃;
}

except for the behavior of continue, which is described in Section 3.7.

Grammatically, the three components of a for loop are expressions. Most commonly, expr₁ and expr₃ are assignments or function calls and expr₂ is a relational expression. Any of the three parts can be omitted, although the semicolons must remain. If expr₁ or expr₃ is omitted, it is simply dropped from the expansion. If the test, expr₂, is not present, it is taken as permanently true, so

for (;;) {
…
}

is an “infinite” loop, presumably to be broken by other means, such as a break or return.

Whether to use while or for is largely a matter of personal preference. For example, in

while ((c = getchar()) == ′ ′ || c == ′\n′ || c == ′\t′)
; /* skip white space characters */

there is no initialization or re-initialization, so the while is most natural.

The for is preferable when there is a simple initialization and increment, since it keeps the loop control statements close together and visible at the top of the loop. This is most obvious in

for (i = 0; i < n; i++)
…

which is the C idiom for processing the first n elements of an array, the analog of the Fortran DO loop or the Pascal for. The analogy is not perfect, however, since the index and limit of a C for loop can be altered from within the loop, and the index variable i retains its value when the loop terminates for any reason. Because the components of the for are arbitrary expressions, for loops are not restricted to arithmetic progressions. Nonetheless, it is bad style to force unrelated computations into the initialization and increment of a for, which are better reserved for loop control operations.

As a larger example, here is another version of atoi for converting a string to its numeric equivalent. This one is slightly more general than the one in Chapter2; it copes with optional leading white space and an optional + or - sign. (Chapter4) shows atof, which does the same conversion for floating-point numbers.

The structure of the program reflects the form of the input:

skip white space, if any
get sign, if any
get integer part and convert it

Each step does its part, and leaves things in a clean state for the next. The whole process terminates on the first character that could not be part of a number.

\#include <ctype.h>  
  
\* atoi: convert s to integer; version 2 \*
int atoi(char s [])  
{  
  int i, n, sign;  
  
  for (i = 0; isspace(s[i]); i++) \* skip white space \*  
    ;  
  sign = (s[i] == ′-′) ? −1 : 1;  
  if (s[i] == ′+′ || s[i] == ′-′) \* skip sign \* 
    i++;  
  for (n = 0; isdigit(s[i]); i++)  
    n = 10 * n + (s[i] - ′0′);  
  return sign * n;  
}

The standard library provides a more elaborate function strtol for conversion of strings to long integers; see Section 5 of Appendix B.

The advantages of keeping loop control centralized are even more obvious when there are several nested loops. The following function is a Shell sort for sorting an array of integers. The basic idea of this sorting algorithm, which was invented in 1959 by D. L. Shell, is that in early stages, far-apart elements are compared, rather than adjacent ones as in simpler interchange sorts. This tends to eliminate large amounts of disorder quickly, so later stages have less work to do. The interval between compared elements is gradually decreased to one, at which point the sort effectively becomes an adjacent interchange method.

\* shellsort: sort v[0]...v[n−1] into increasing order \*
void shellsort(int v[], int n)  
{  
  int gap, i, j, temp;  
  
  for (gap = n/2; gap > 0; gap /= 2)  
    for (i = gap; i < n; i++)  
      for (j=i-gap; j>=0 && v[j]>v[j+gap]; j-=gap) {  
        temp = v[j];  
        v[j] = v[j+gap];  
        v[j+gap] = temp;  
      }  
}

There are three nested loops. The outermost controls the gap between compared elements, shrinking it from n/2 by a factor of two each pass until it becomes zero. The middle loop steps along the elements. The innermost loop compares each pair of elements that is separated by gap and reverses any that are out of order. Since gap is eventually reduced to one, all elements are eventually ordered correctly. Notice how the generality of the for makes the outer loop fit the same form as the others, even though it is not an arithmetic progression.

One final C operator is the comma “,”, which most often finds use in the for statement. A pair of expressions separated by a comma is evaluated left to right, and the type and value of the result are the type and value of the right operand. Thus in a for statement, it is possible to place multiple expressions in the various parts, for example to process two indices in parallel. This is illustrated in the function reverse(s), which reverses the string s in place.

\#include <string.h>  
# reverse: reverse string s in place 
void reverse(char s[])  
{  
  int c, i, j;  
  
  for (i = 0, j = strlen(s)−1; i < j; i++, j--) {  
    c = s[i];  
    s[i] = s[j];  
    s[j] = c;  
  }  
}

The commas that separate function arguments, variables in declarations, etc., are not comma operators, and do not guarantee left to right evaluation.

Comma operators should be used sparingly. The most suitable uses are for constructs strongly related to each other, as in the for loop in reverse, and in macros where a multistep computation has to be a single expression. A comma expression might also be appropriate for the exchange of elements in reverse, where the exchange can be thought of as a single operation:

for (i = 0, j = strlen(s)−1; i < j; i++, j–)
c = s[i], s[i] = s[j], s[j] = c;

! cat Chapter3/Exercise\ 3-03/expand.c

#include <stdio.h>

#define MAXLINE 1000

int getchars(char line[], int maxline);
void expand(char s1[], char s2[]);

int main(void)
{
    char s1[MAXLINE];
    char s2[MAXLINE];

    printf("Input some characters, then press enter:\n");
    while (getchars(s1, MAXLINE) == 0)
        ;

    expand(s1, s2);
    printf("\nThe expanded result is:\n%s\n", s2);

    return 0;
}

int getchars(char s[], int lim)
{
    int c, i, l;

    for (i = 0, l = 0; (c = getchar()) != EOF && c != '\n'; ++i)
        if (i < lim - 1)
            s[l++] = c;
    s[l] = '\0';

    return l;
}

void expand(char s1[], char s2[])
{
    int i, j, k;
    int c;

    for (i = 0, j = 0; s1[i] != '\0'; ++i) {
        if (s1[i+1] == '-' && (c = s1[i+2]) != '\0' &&
            (('a' <= s1[i] && s1[i] <= 'z' && s1[i] <= c && c <= 'z') ||
            ('A' <= s1[i] && s1[i] <= 'Z' && s1[i] <= c && c <= 'Z') ||
            ('0' <= s1[i] && s1[i] <= '9' && s1[i] <= c && c <= '9'))) {
                k = 0;
                while (k <= c - s1[i])
                    s2[j++] = s1[i] + k++;
                i += 2;
        } else {
            s2[j++] = s1[i];
        }
    }
    s2[j] = '\0';
}

! gcc Chapter3/Exercise\ 3-03/expand.c -o Chapter3/Exercise\ 3-03/expand

! Chapter3/Exercise\ 3-03/expand

Input some characters, then press enter:

3.6 Loops—Do-while

As we discussed in Chapter 1, the while and for loops test the termination condition at the top. By contrast, the third loop in C, the do-while, tests at the bottom after making each pass through the loop body; the body is always executed at least once.

The syntax of the do is

do
statement
while (expression);

The statement is executed, then expression is evaluated. If it is true, statement is evaluated again, and so on. When the expression becomes false, the loop terminates. Except for the sense of the test, do-while is equivalent to the Pascal repeat-until statement.

Experience shows that do-while is much less used than while and for. Nonetheless, from time to time it is valuable, as in the following function itoa, which converts a number to a character string (the inverse of atoi). The job is slightly more complicated than might be thought at first, because the easy methods of generating the digits generate them in the wrong order. We have chosen to generate the string backwards, then reverse it.

\* itoa: convert n to characters in s \* 
void itoa(int n, char s[])
{  
  int i, sign;  
  if ((sign = n) < 0) \* record sign \*  
    n = -n; \* make n positive \* 
  i = 0;  
  do {    \* generate digits in reverse order \*
    s[i++] = n % 10 + ′0′;  \* get next digit \*
  } while ((n /= 10) > 0);   \* delete it \*
  if (sign < 0)  
    s[i++] = ′-′;  
  s[i] = ′\0′;  
  reverse(s);  
}

The do-while is necessary, or at least convenient, since at least one character must be installed in the array s, even if n is zero. We also used braces around the single statement that makes up the body of the do-while, even though they are unnecessary, so the hasty reader will not mistake the while part for the beginning of a while loop.

! cat Chapter3/Exercise\ 3-04/itoa.c

#include <stdio.h>
#include <limits.h>

#define MAXLINE 1000

void itoa(int n, char s[]);
void reverse(char s[]);

int main(void)
{
    char s[MAXLINE];
    
    itoa(INT_MIN, s);
    printf("%d is converted to %s.\n", INT_MIN, s);
    itoa(826, s);
    printf("%d is converted to %s.\n", 826, s);
    itoa(-2093, s);
    printf("%d is converted to %s.\n", -2093, s);

    return 0;
}

void itoa(int n, char s[])
{
    int i, sign, remainder;

    sign = n;
    i = 0;
    do {
        remainder = n % 10;
        s[i++] = ((sign < 0) ? -remainder : remainder) + '0';
    } while (n /= 10);
    if (sign < 0)
        s[i++] = '-';
    s[i] = '\0';
    reverse(s);
}

void reverse(char s[])
{
    int i, j;
    int tmp;

    for (j = 0; s[j] != '\0'; ++j)
        ;
    --j;

    for (i = 0; i < j; ++i, --j) {
        tmp = s[i];
        s[i] = s[j];
        s[j] = tmp;
    }
}

! gcc Chapter3/Exercise\ 3-04/itoa.c -o Chapter3/Exercise\ 3-04/itoa

! Chapter3/Exercise\ 3-04/itoa

-2147483648 is converted to -2147483648.
826 is converted to 826.
-2093 is converted to -2093.

! cat Chapter3/Exercise\ 3-05/itob.c

#include <stdio.h>
#include <limits.h>

#define MAXLINE 1000

void itob(int n, char s[], int b);
void reverse(char s[]);

int main(void)
{
    char s[MAXLINE];
    
    itob(INT_MIN, s, 16);
    printf("%d in base %d is %s.\n", INT_MIN, 16, s);
    itob(826, s, 10);
    printf("%d in base %d is %s.\n", 826, 10, s);
    itob(-2093, s, 5);
    printf("%d in base %d is %s.\n", -2093, 5, s);

    return 0;
}

void itob(int n, char s[], int b)
{
    int i, sign, remainder;
    char bases[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    if (b < 2 || b > 36)
        return;

    sign = n;
    i = 0;
    do {
        remainder = n % b;
        s[i++] = ((sign < 0) ? -remainder : remainder) + '0';
    } while (n /= b);
    if (sign < 0)
        s[i++] = '-';
    s[i] = '\0';
    reverse(s);
}

void reverse(char s[])
{
    int i, j;
    int tmp;

    for (j = 0; s[j] != '\0'; ++j)
        ;
    --j;

    for (i = 0; i < j; ++i, --j) {
        tmp = s[i];
        s[i] = s[j];
        s[j] = tmp;
    }
}

! gcc Chapter3/Exercise\ 3-05/itob.c -o Chapter3/Exercise\ 3-05/itob

! Chapter3/Exercise\ 3-05/itob

-2147483648 in base 16 is -80000000.
826 in base 10 is 826.
-2093 in base 5 is -31333.

! cat Chapter3/Exercise\ 3-06/itoa.c

#include <stdio.h>
#include <limits.h>

#define MAXLINE 1000

void itoa(int n, char s[], int w);
void reverse(char s[]);

int main(void)
{
    char s[MAXLINE];
    int width;

    width = 11;
    itoa(INT_MIN, s, width);
    printf("%12d is converted to %s.\n", INT_MIN, s);
    itoa(826, s, width);
    printf("%12d is converted to %s.\n", 826, s);
    itoa(-2093, s, width);
    printf("%12d is converted to %s.\n", -2093, s);

    return 0;
}

void itoa(int n, char s[], int w)
{
    int i, sign, remainder, k, j;

    sign = n;
    i = 0;
    do {
        remainder = n % 10;
        s[i++] = ((sign < 0) ? -remainder : remainder) + '0';
    } while (n /= 10);
    if (sign < 0)
        s[i++] = '-';
    
    while (i < w)
        s[i++] = ' ';

    s[i] = '\0';
    reverse(s);
}

void reverse(char s[])
{
    int i, j;
    int tmp;

    for (j = 0; s[j] != '\0'; ++j)
        ;
    --j;

    for (i = 0; i < j; ++i, --j) {
        tmp = s[i];
        s[i] = s[j];
        s[j] = tmp;
    }
}

! gcc Chapter3/Exercise\ 3-06/itoa.c -o Chapter3/Exercise\ 3-06/itoa

! Chapter3/Exercise\ 3-06/itoa

 -2147483648 is converted to -2147483648.
         826 is converted to         826.
       -2093 is converted to       -2093.

3.7 Break and Continue

It is sometimes convenient to be able to exit from a loop other than by testing at the top or bottom. The break statement provides an early exit from for, while, and do, just as from switch. A break causes the innermost enclosing loop or switch to be exited immediately.

The following function, trim, removes trailing blanks, tabs, and newlines from the end of a string, using a break to exit from a loop when the rightmost non-blank, non-tab, non-newline is found.

\* trim: remove trailing blanks, tabs, newlines \*  
int trim(char s[])  
{  
  int n;  
  
  for (n = strlen(s)−1; n >= 0; n--)  
    if (s[n] != ′ ′ && s[n] != ′\t′ && s[n] != ′\n′)  
      break;  
  s[n+1] = ′\0′;  
  return n;  
}

strlen returns the length of the string. The for loop starts at the end and scans backwards looking for the first character that is not a blank or tab or newline. The loop is broken when one is found, or when n becomes negative (that is, when the entire string has been scanned). You should verify that this is correct behavior even when the string is empty or contains only white space characters.

The continue statement is related to break, but less often used; it causes the next iteration of the enclosing for, while, or do loop to begin. In the while and do, this means that the test part is executed immediately; in the for, control passes to the increment step. The continue statement applies only to loops, not to switch. A continue inside a switch inside a loop causes the next loop iteration.

As an example, this fragment processes only the non-negative elements in the array a; negative values are skipped.

for (i = 0; i < n; i++) {  
  if (a[i] < 0)  \* skip negative elements \*
    continue;  
      ...\* do positive elements \*  
}

The continue statement is often used when the part of the loop that follows is complicated, so that reversing a test and indenting another level would nest the program too deeply.

3.8 Goto and Labels

C provides the infinitely-abusable goto statement, and labels to branch to. Formally, the goto is never necessary, and in practice it is almost always easy to write code without it. We have not used goto in this book.

Nevertheless, there are a few situations where gotos may find a place. The most common is to abandon processing in some deeply nested structure, such as breaking out of two or more loops at once. The break statement cannot be used directly since it only exits from the innermost loop. Thus:

for ( … )
for ( … ) {
…
if (disaster)
goto error;
}
…

error:
clean up the mess

This organization is handy if the error-handling code is non-trivial, and if errors can occur in several places.

A label has the same form as a variable name, and is followed by a colon. It can be attached to any statement in the same function as the goto. The scope of a label is the entire function.

As another example, consider the problem of determining whether two arrays a and b have an element in common. One possibility is

for (i = 0; i < n; i++)
for (j = 0; j < m; j++)
if (a[i] == b[j])
goto found;
/* didn′t find any common element */
…
found:
/* got one: a[i] == b[j] */
…

Code involving a goto can always be written without one, though perhaps at the price of some repeated tests or an extra variable. For example, the array search becomes

found = 0;
for (i = 0; i < n && !found; i++)
for (j = 0; j < m && !found; j++)
if (a[i] == b[j])
found = 1;
if (found)
/* got one: a[i−1] == b[j−1] */
…
else
/* didn′t find any common element */
…

With a few exceptions like those cited here, code that relies on goto statements is generally harder to understand and to maintain than code without gotos. Although we are not dogmatic about the matter, it does seem that goto statements should be used rarely, if at all.

References