The if-else Statement

The if-else Statement

In the if statement, when the conditional expression is logical TRUE, the computer will jump to the statements controlled by the if statement and execute them right away. If the expression is false, the computer will ignore those statements controlled by the if statement.

From time to time, you will want the computer to execute some other specified statements when the conditional expression of the if statement is logical FALSE. By doing so, you can use another conditional branching statement in C—the if-else statement.

As an expansion of the if statement, the if-else statement has the following form:

if (expression) {
   statement1;
   statement2;
   .
   .
   .
}
else {
   statement_A;
   statement_B;
   .
   .
   .
}

Here if expression is logical TRUE, the statements controlled by if, including statement1 and statement2, are executed. The statements, such as statement_A and statement_B, inside the statement block and following the else keyword are executed if expression is not logical TRUE.

The program in Listing 10.2 shows how to use the if-else statement.
TYPE
Listing 10.2. Using the if-else statement.

1:  /* 10L02.c  Using the if-else statement */
2:  #include <stdio.h>
3:
4:  main()
5:  {
6:     int i;
7:
8:     printf("Even Number   Odd Number\n");
9:     for (i=0; i<10; i++)
10:       if (i%2 == 0)
11:          printf("%d", i);
12:       else
13:          printf("%14d\n", i);
14:
15:    return 0;
16: }

    OUTPUT
    The following result is obtained by running the executable file 10L02.exe:

    C:\app>10L02
    Even Number   Odd Number
    0             1
    2             3
    4             5
    6             7
    8             9
    C:\app>

    ANALYSIS
    Line 6 of Listing 10.2 declares an integer variable, i. The printf() function in line 8 displays a headline on the screen.

The integer variable i is initialized in the first expression field of the for statement in line 9. Controlled by the for statement, the if-else statement in lines 10_13 is executed 10 times. According to the if-else statement, the printf() function in line 11 prints out even numbers if the relational expression i%2 == 0 in line 10 returns 1 (that is, TRUE). If the relational expression returns 0 (that is, FALSE), the printf() function controlled by the else keyword in line 12 outputs odd numbers to the standard output.

Because the if-else statement is treated as a single statement, the braces { and } are not needed to form a block of statement in the for statement. Likewise, there are no braces used in the if-else statement because the if and else keywords each control a single statement, respectively, in lines 11 and 13.

Note that the minimum width of 14 is specified in the printf() function in line 13, so the output of the odd numbers is listed to the right side of the even numbers, as you can see in the output section. Because the program in Listing 10.2 checks numbers in a range of 0 to 9, the output shows that 0, 2, 4, 6, and 8 are even numbers, and 1, 3, 5, 7, and 9 are odd ones.