Nested if Statements
As you saw in the previous sections, one if statement enables a program to make one decision. In many cases, a program has to make a series of decisions. To enable it to do so, you can use nested if statements.
Listing 10.3 demonstrates the usage of nested if statements.
TYPE
Listing 10.3. Using nested if statements.1: /* 10L03.c Using nested if statements */
2: #include <stdio.h>
3:
4: main()
5: {
6: int i;
7:
8: for (i=-5; i<=5; i++){
9: if (i > 0)
10: if (i%2 == 0)
11: printf("%d is an even number.\n", i);
12: else
13: printf("%d is an odd number.\n", i);
14: else if (i == 0)
15: printf("The number is zero.\n");
16: else
17: printf("Negative number: %d\n", i);
18: }
19: return 0;
20: }
OUTPUT
After running the executable file 10L03.exe, I obtain the following output:
C:\app>10L03
Negative number: -5
Negative number: -4
Negative number: -3
Negative number: -2
Negative number: -1
The number is zero.
1 is an odd number.
2 is an even number.
3 is an odd number.
4 is an even number.
5 is an odd number.
C:\app>
ANALYSIS
Listing 10.3 contains a for loop, starting in line 8 and ending in line 18. According to the expressions of the for statement in line 8, any tasks controlled by the for statement are executed up to 11 times.
First, a decision has to be made based on the return value of the relational expression i > 0 in the if statement of line 9. The i > 0 expression is used to test whether the value of i is positive or negative (including zero.) If the return value is 1, the computer jumps to the second (that is, nested) if statement in line 10.
Note that line 10 contains another relational expression, i%2 == 0, which tests whether the integer variable i is even or odd. Therefore, the second decision of displaying even numbers or odd numbers has to be made according to the return value of the second relational expression, i%2 == 0. The printf() function in line 11 prints out an even number if the return value is 1. Otherwise, the statement in line 13 is executed, and an odd number is shown on the screen.
The computer branches to line 14 if the i > 0 expression returns 0; that is, if the value of i is not greater than 0. In line 14, another if statement is nested within an else phrase, and the relational expression i == 0 is evaluated. If i == 0 is true, which means i contains the value of zero, the string of The number is zero. is displayed on the screen. Otherwise, the value of i is negative, according to the value returned by the i > 0 expression. The statement in line 17 then outputs the negative number to the standard output.
As you can see in the example, the value of i is within the range of 5 to -5. Thus, -5, -4, -3, -2, and -1 are printed out as negative numbers. In addition, the odd numbers 1, 3, and 5, as well as the even numbers 2 and 4, are also printed out.
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