The if statement

The if statement

If life were a straight line, it would be very boring. The same thing is true for programming. It would be too dull if the statements in your program could only be executed in the order in which they appear.

In fact, an important task of a program is to instruct the computer to branch (that is, jump) to different portions of the code and work on different jobs whenever the specified conditions are met.

However, in most cases, you don't know in advance what will come next. What you do know is that something is bound to happen if certain conditions are met. Therefore, you can just write down tasks and conditions in the program. The decisions of when to perform the tasks are made by the conditional branching statements.

In C, the if statement is the most popular conditional branching statement; it can be used to evaluate the conditions as well as to make the decision whether the block of code controlled by the statement is going to be executed.

The general form of the if statement is

if (expression) {
   statement1;
   statement2;
   .
   .
   .
}

Here expression is the conditional criterion. If expression is logical TRUE (that is, nonzero), the statements inside the braces ({ and }), such as statement1 and statement2, are executed. If expression is logical FALSE (zero), then the statements are skipped.

Note that the braces ({ and }) form a block of statements that is under the control of the if statement. If there is only one statement inside the block, the braces can be ignored. The parentheses (( and )), however, must always be used to enclose the conditional expression.

For instance, the following expression

if (x > 0)
  printf("The square root of x is: %f\n", sqrt(x));

tells the computer that if the value of x is greater than zero (that is, positive), it should calculate the square root of x by calling the sqrt() function, and then print out the result. Here the conditional criterion is the relational expression x > 0, which returns 1 for true and 0 for false.

Listing 10.1 gives you another example of using the if statement.

TYPE
Listing 10.1. Using the if statement in decision making.

1:  /* 10L01.c  Using the if statement */
2:  #include <stdio.h>
3:
4:  main()
5:  {
6:     int i;
7:
8:     printf("Integers that can be divided by both 2 and 3\n");
9:     printf("(within the range of 0 to 100):\n");
10:    for (i=0; i<=100; i++){
11:       if ((i%2 == 0) && (i%3 == 0))
12:         printf("   %d\n", i);
13:   }
14:   return 0;
15: }

    OUTPUT
    After 10L01.exe, the executable of the program in Listing 10.1, is created and run from a DOS prompt, the following output is printed on the screen:

C:\app> 10L01
Integers that can be divided by both 2 and 3
(within the range of 0 to 100):
   0
   6
   12
   18
   24
   30
   36
   42
   48
   54
   60
   66
   72
   78
   84
   90
   96
C:\app>

    ANALYSIS
    As you see in Listing 10.1, line 6 declares an integer variable, i. Lines 8 and 9 print out two headlines. Starting in line 10, the for statement keeps looping 101 times.

Within the for loop, the if statement in lines 11 and 12 evaluates the logical expression (i%2 == 0) && (i%3 == 0). If the expression returns 1 (that is, the value of i can be divided by both 2 and 3 completely), the value of i is displayed on the screen by calling the printf() function in line 12. Otherwise, the statement in line 12 is skipped.

Note that the braces ({ and }) are discarded in the if statement because there is only one statement under the control of the statement.

The result shown on the screen gives all integers within the range of 0 to 100 that can be divided by both 2 and 3.