The switch Statement

The switch Statement

In the last section, you saw that nested if statements are used when there is more than one decision to be made. The nested if statements will become very complex if there are many decisions that need to be made, however. Sometimes, the programmer will have problems following the complex if statements.

Fortunately there is another statement in C, the switch statement, that you can use to make unlimited decisions or choices based on the value of a conditional expression and specified cases.

The general form of the switch statement is

switch (expression) {
   case expression1:
        statement1;
   case expression2:
        statement2;
   .
   .
   .
   default:
        statement-default;
}

Here the conditional expression, expression, is evaluated first. If the return value of expression is equal to the constant expression expression1, the statement statement1 is executed. If the value of expression is the same as the value of expression2, statement2 is executed. If, however, the value of expression is not equal to any values of the constant expressions labeled by the case keyword, the statement (statement-default) following the default keyword is executed.

You have to use the case keyword to label each case. The default keyword is recommended to be used for the default case. Note that no constant expressions are identical in the switch statement.

The program in Listing 10.4 gives you an example of using the switch statement. The program also demonstrates an important feature of the switch statement.

TYPE
Listing 10.4. Using the switch statement.

1:  /* 10L04.c  Using the switch statement */
2:  #include <stdio.h>
3:
4:  main()
5:  {
6:     int day;
7:
8:     printf("Please enter a single digit for a day\n");
9:     printf("(within the range of 1 to 3):\n");
10:    day = getchar();
11:    switch (day){
12:       case `1':
13:          printf("Day 1\n");
14:       case `2':
15:          printf("Day 2\n");
16:       case `3':
17:          printf("Day 3\n");
18:       default:
19:          ;
20:    }
21:    return 0;
22: }


    OUTPUT
    If I run the executable file 10L04.exe and enter 3, I obtain the following output:

    C:\app>10L04
    Please enter a single digit for a day
    (within the range of 1 to 3):
    3
    Day 3
    C:\app>

    ANALYSIS
    As you can see in line 6, an int variable, day, is declared; it is assigned the input entered by the user in line 10.

In line 11, the value of the integer variable day is evaluated in the switch statement. If the value is equal to one of the values of the constant expressions, the computer starts to execute statements from there. The constant expressions are labeled by prefixing case in front of them.

For instance, I entered 3 and then pressed the Enter key. The numeric value of 3 is assigned to day in line 10. Then, after finding a case in which the value of the constant expression matches the value contained by day, the computer jumps to line 17 to execute the printf() function and display Day 3 on the screen.

Note that under the default label in Listing 10.4, there is an empty (that is, null) statement ending with semicolon (;) in line 19. The computer does nothing with the empty statement.

However, if I enter 1 from my keyboard and then press the Enter key, I get the following output:

C:\app>10L04
Please enter a single digit for a day
(within the range of 1 to 3):
1
Day 1
Day 2
Day 3
C:\app>

    OUTPUT
    From the output, you can see that the statement controlled by the selected case, case 1, and the statements controlled by the rest of the cases are executed, because Day 1, Day 2, and Day 3 are displayed on the screen. Likewise, if I enter 2 from my keyboard, I have Day2 and Day3 shown on the screen.

This is an important feature of the switch statement: The computer continues to execute the statements following the selected case until the end of the switch statement.

You're going to learn how to exit from the switch construct in the next section.