==============
Let us begin with a quick introduction in C. Our aim is to show the essential elements of the language in real programs, but without getting bogged down in details, rules, and exceptions. At this point, we are not trying to be complete or even precise (save that the examples are meant to be correct). We want to get you as quickly as possible to the point where you can write useful programs, and to do that we have to concentrate on the basics: variables and constants, arithmetic, control flow, functions, and the rudiments of input and output. We are intentionally leaving out of this chapter features of C that are important for writing bigger programs. These include pointers, structures, most of C's rich set of operators, several control-flow statements, and the standard library.
This approach and its drawbacks. Most notable is that the complete story on any particular feature is not found here, and the tutorial, by being brief, may also be misleading. And because the examples do not use the full power of C, they are not as concise and elegant as they might be. We have tried to minimize these effects, but be warned. Another drawback is that later chapters will necessarily repeat some of this chapter. We hope that the repetition will help you more than it annoys.
In any case, experienced programmers should be able to extrapolate from the material in this chapter to their own programming needs. Beginners should supplement it by writing small, similar programs of their own. Both groups can use it as a framework on which to hang the more detailed descriptions that begin in Chapter 2.
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 a 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
in 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; the 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 ( ).
The First C Program :
#include <stdio.h> /* include information about standard library */
main() { /* define a function called main that received no
argument values */
/* statements of main are enclosed in braces */
printf("hello, world\n"); /* main calls library function printf */
} /* \n represents the newline character*/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 character 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.
Exercise 1-1. Run the hello, world program on your system. Experiment
with leaving out parts of the program, to see what error messages you get.
Exercise 1-2. Experiment to find out what happens when printf's argument
string contains \c, where c is some character not listed above.
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:
1 -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 scale */
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
where a blank, 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 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-bits 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 1038.
C provides several other data types besides int and float, including:
| Name | Description |
|---|---|
char |
character - a single byte |
short |
short integer |
long |
long integer |
double |
double-precision floating point |
The size of these objects is 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;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 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 will 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 behaviour 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 don'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 the second version:
#include <stdio.h>
/* print Fahrenheit-Celsius table for fahr = 0, 20, ..., 300; floating-point
* version */
main() {
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 = (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.
| String | Description |
|---|---|
%d |
print as decimal integer |
%6d |
print as decimal integer, at least 6 characters wide |
%f |
print as floating point |
%6f |
print as floating point, at least 6 characters wide |
%.2f |
print as floating point, 2 characters after decimal point |
%6.2f |
print as floating point, at least 6 wide and 2 after decimal point |
Among others, printf also recognizes %o for octal, %x for hexadecimal, %c
for character, %s for character string and %% for itself.
Exercise 1-3. Modify the temperature conversion program to print a heading above the table.
Exercise 1-4. Write a program to print the corresponding Celsius to Fahrenheit table.
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 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 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 here.
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
ptintf) 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.
Exercise 1-5. Modify the temperature conversion program to print the table in reverse order, that is, from 300 degrees to 0.
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 replacementtext
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.
We are 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 confirm 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.
Given getchar and putchar, you can write a surprising amount of useful code
without knowing anything more about input and output. The simplest example is a
program that copies its input to its output one character at a time:
read a character
while (charater is not end-of-file indicator)
output the character just read
read a character
Converting this into C gives:
#include <stdio.h>
/* copy input to output; 1st version */
main()
{
int c;
c = getchar();
while (c != EOF) {
putchar(c);
c = getchar();
}
}The relational operator != means "not equal to".
What appears to be a character on the keyboard or screen is of course, like
everything else, stored internally just as a bit pattern. The type char is
specifically meant for storing such character data, but any integer type can be
used. We used int for a subtle but important reason.
The problem is distinguishing the end of input from valid data. The solution is
that getchar returns a distinctive value when there is no more input, a value
that cannot be confused with any real character. This value is called EOF,
for "end of file". We must declare c to be a type big enough to hold any
value that getchar returns. We can't use char since c must be big enough to
hold EOF in addition to any possible char. Therefore we use int.
EOF is an integer defined in <stdio.h>, but the specific numeric value
doesn't matter as long as it is not the same as any char value. By using the
symbolic constant, we are assured that nothing in the program depends on the
specific numeric value.
The program for copying would be written more concisely by experienced C programmers. In C, any assignment, such as
c = getchar();
is an expression and has a value, which is the value of the left hand side
after the assignment. This means that a assignment can appear as part of a
larger expression. If the assignment of a character to c is put inside the
test part of a while loop, the copy program can be written this way:
#include <stdio.h>
/* copy input to output; 2nd version */
main()
{
int c;
while ((c = getchar()) != EOF)
putchar(c);
}The while gets a character, assigns it to c, and then tests whether the
character was the end-of-file signal. If it was not, the body of the while is
executed, printing the character. The while then repeats. When the end of the
input is finally reached, the while terminates and so does main.
This version centralizes the input - there is now only one reference to
getchar and shrinks the program. The resulting program is more compact, and,
once the idiom is mastered, easier to read. You'll see this style often. (It's
possible to get carried away and create impenetrable code, however, a tendency
that we will try to curb.)
The parentheses around the assignment, within the condition are necessary. The precedence of != is higher than that of =, which means that in the absence of parentheses the relational test != would be done before the assignment =. So the statement
c = getchar() != EOF
is equivalent to
c = (getchar() != EOF)
This has the undesired effect of setting c to 0 or 1, depending on whether or
not the call of getchar returned end of file. (More on this in Chapter 2.)
Exercsise 1-6. Verify that the expression getchar() != EOF is 0 or 1.
Exercise 1-7. Write a program to print the value of EOF.
The next program counts characters; it is similar to the copy program.
#include <stdio.h>
/* count characters in input; 1st version */
main( ) {
long nc;
nc = 0;
while (getchar() 1= EOF)
++nc;
printf ("%ld\n", nc l ;
}The statement
++nc;
presents a new operator, ++, which means increment by one. You could instead
write nc = nc+ 1 but ++nc is more concise and often more efficient. There
is a corresponding operator -- to decrement by 1. The operators ++ and --
can be either prefix operators (++nc) or postfix (nc++); these two forms
have different values in expressions, as will be shown in Chapter 2, but ++nc
and nc++ both increment nc. For the moment we will stick to the prefix
form.
The character counting program accumulates its count in a long variable
instead of an int. long integers are at least 32 bits. Although on some
machines, int and long are the same size, on others an int is 16 bits,
with a maximum value of 32767, and it would take relatively little input to
overflow an int counter. The conversion specification %ld tells printf
that the corresponding argument is a long integer. It may be possible to
cope with even bigger numbers by using a double (double precision float). We
will also use a for statement instead of a while, to illustrate another way
to write the loop.
#include <stdio.h>
/* count characters in input; 2nd version */
main()
double nc;
for (nc = 0; gechar() != EOF; ++nc)
;
printf("%.0f\n", nc);
}printf uses %f for both float and double; %.0f suppresses the
printing of the decimal point and the fraction part, which is zero.
The body of this for loop is empty, because all the work is done in the test
and increment parts. But the grammatical rules of C require that a for
statement have a body. The isolated semicolon, called a null statement, is
there to satisfy that requirement. We put it on a separate line to make it
visible.
Before we leave the character counting program, observe that if the input
contains no characters, the while or for test fails on the very first call
to getchar, and the program produces zero, the right answer. This is
important. One of the nice things about while and for is that they test at
the top of the loop, before proceeding with the body. If there is nothing to
do, nothing is done, even if that means never going through the loop body.
Programs should act intelligently when given zero-length input. The while and
for statements help ensure that programs do reasonable things with boundary
conditions.
The next program counts input lines. As we mentioned above, the standard library ensures that an input text stream appears as a sequence of lines, each terminated by a newline. Hence, counting lines is just counting newlines:
#include <stdio.h>
/* count lines in input */
main()
{
int c, nl;
nl = 0;
while ((c = getchar()) != EOF)
if (c == '\n')
++nl;
printf("%d\n", nl);
}The body of the while now consists of an if, which in turn controls the
increment ++nl. The if statement tests the parenthesized condition, and if
the condition is true, executes the statement (or group of statements in
braces) that follows. We have again indented to show what is controlled by
what.
The double equals sign == is the C notation for "is equal to" (like Pascal's
single = or Fortran's .EQ.). This symbol is used to distinguish the
equality test from the single = that C uses for assignment. A word of
caution: newcomers to C occasionally write = when they mean ==. As we will
see in Chapter 2, the result is usually a legal expression, so you will get no
warning.
A character written between single quotes represents an integer value equal to
the numerical value of the character in the machine's character set. This is
called a character constant, although it is just another way to write a small
integer. So, for example, 'A' is a character constant; in the ASCII character
set its value is 65, the internal representation of the character A. Of
course, 'A' is to be preferred over 65: its meaning is obvious, and it is
independent of a particular character set.
The escape sequences used in string constants are also legal in character
constants, so '\n' stands for the value of the newline character, which is 10
in ASCII. You should note carefully that '\n' is a single character, and in
expressions is just an integer; on the other hand, "\n" is a string constant
that happens to contain only one character. The topic of strings versus
characters is discussed further in Chapter 2.
Exercise 1-8: Write a program to count blanks, tabs, and newlines.
Exercise 1-9. Write a program to copy its input to its output, replacing each string of one or more blanks by a single blank.
Exercise 1-10. Write a program to copy its input to its output, replacing
each tab by \t, each backspace by \b, and each backslash by \\. This
makes tabs and backspaces visible in an unambiguous way.
The fourth in our series of useful programs counts lines, words, and
characters, with the loose definition that a word is any sequence of characters
that does not contain a blank, tab or newline. This is a bare-bones version of
the UNIX program wc.
#include <stdio.h>
#define IN 1 /* inside a word */
#define OUT 0 /* outside a word */
/* count lines, words, and characters in input */
main()
{
int c, nl, nw, nc, state;
state = OUT;
nl = nw = nc = 0;
while ((c = getchar()) != EOF) {
++nc;
if (c == '\n')
++nl;
if (c == ' ' || c == '\n' || c = '\t')
state = OUT;
else if (state == OUT) {
state = IN;
++nw;
}
}
printf("%d %d %d\n", nl, nw, nc);
}Every time the program encounters the first character of a word, it counts one
more word. The variable state records whether the program is currently in a
word or not; initially it is "not in a word", which is assigned the value
OUT. We prefer the symbolic constants IN and OUT to the literal values 1
and 0 because they make the program more readable. In a program as tiny as
this, it makes little difference, but in larger programs, the increase in
clarity is well worth the modest extra effort to write it this way from the
beginning. You'll also find that it's easier to make extensive changes in
programs where magic numbers appear only as symbolic constants. The line
nl = nw = nc = 0;
sets all three variables to zero. This is not a special case, but a consequence of the fact that an assignment is an expression with the value and assignments associated from right to left. It's as if we had written
nl = (nw = (nc = 0));
The operator || means OR, so the line
if (c == ' ' || c == '\n' || c = '\t')
says "if c is a blank or c is a newline or c is a tab". (Recall that the
escape sequence \t is a visible representation of the tab character.) There
is a corresponding operator && for AND; its precedence is just higher than
||. Expressions connected by && or || are evaluated left to right, and it
is guaranteed that evaluation will stop as soon as the truth or falsehood is
known. If c is a blank, there is no need to test whether it is a newline or
tab, so these tests are not made. This isn't particularly important here, but
is significant in more complicated situations, as we will soon see.
The example also shows an else, which specifies an alternative action if the
condition part of an if statement is false. The general form is
if (expression)
statement1
else
statement2
One and only one of the two statements associated with an if-else is
performed. If the expression is true, statement1 is executed; if not,
statement2 is executed. Each statement can be a single statement or several
in braces. In the word count program, the one after the else is an if that
controls two statements in braces.
Exercise 1-11. How would you test the word count program? What kinds of input are most likely to uncover bugs if there are any?
Exercise 1-12. Write a program that prints its input one word per line.
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 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
statementn
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 4, 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.
Exercise 1-13. Write a program to print a histogram of the lengths of words in its input. It is easy to draw the histogram with the bars horizontal; a vertical orientation is more challenging.
Exercise 1-14. Write a program to print a histogram of the frequencies of different characters in its input.
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
xy.)
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 base, 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 opening the 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.
Exercise 1.15. Rewrite the temperature conversion program of Section 1.2 to use a function for conversion.
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.
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 apower 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 argument of the array. This is the topic of the next section.
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 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 length */
int getline(char line[], int maxline);
void copy(char to[], char from[]);
/* print the 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 an 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 conversion 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 in the string and terminated with a '\0' to mark the end.
┌───┬───┬───┬───┬───┬────┬────┐
│ h │ e │ l │ l │ o │ \n │ \0 │
└───┴───┴───┴───┴───┴────┴────┘
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 with a '\0', and copies this character into the
output.
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 this 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.
Exercise 1-16. Revise the main routine of the longest-line program so it will correctly print the length of arbitrarily long input lines, and as much as possible of the text.
Exercise 1-17. Write a program to print all input lines that are longer than 80 characters.
Exercise 1-18. Write a program to remove trailing blanks and tabs from each line of input, and to delete entirely blank lines.
Exercise 1-19. Write a function reverse(s) that reverses the character
string s. Use it to write a program that reverses its input a line at a time.
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; 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 the 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 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
writing 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.
Exercise 1-20. Write a program detab that replaces tabs in the input with
the proper number of blanks to space to the next tab stop. Assume a fixed set
of tab stops, say every n columns. Should n be a variable or a symbolic
parameter?
Exercise 1-21. Write a program entab that replaces strings of blanks by
the minimum number of tabs and blanks to achieve the same spacing. Use the same
tab stops as for detab. When either a tab or a single blank would suffice to
reach a tab stop, which should be given preference?
Exercise 1-22. Write a program to "fold" long input lines into two or more
shorter lines after the last non-blank character that occurs before the n-th
column of input. Make sure your program does something intelligent with very
long lines, and if there are no blanks or tabs before the specified column.
Exercise 1-23. Write a program to remove all comments from a C program. Don't forget to handle quoted strings and character constants properly. C comments don't nest.
Exercise 1-24. Write a program to check a C program for rudimentary syntax errors like unmatched parentheses, brackets and braces. Don't forget about quotes, both single and double, escape sequences, and comments. (This program is hard if you do it in full generality.)