C31. C Switch Statement

The switch Statement

In everyday programming, we'll often need to compare an expression against a series of values to see which one it currently matches. We saw in Section 5.2 that a cascaded if statement can be used for this purpose. For example, the following cascaded if statement prints the English word that corresponds to a numerical grade:

if (grade == 4)

printf(“Excellent”);

else if (grade == 3)

   printf(“Good”);

  else if (grade == 2) printf(“Average”);

  else if (grade == 1)

printf(“Poor”);

 else if (grade == 0)  printf(“Failing”);

 else

printf(“Illegal grade”);

As an alternative to this kind of cascaded if statement. C provides the switch statement. The following switch is equivalent to our cascaded if:

switch (grade) {

case 4: printf(“Excellent”); break;

case 3: printf (“Good”); break;

case 2: printf(“Average”); break;

case 1: printf(“Poor”); break;

case 0: printf(“Failing”); break;

default: printf(“Illegal grade”); break;

}

When this statement is executed, the value of the variable grade is tested against 4, 3, 2, 1, and 0. If it matches 4. for example, the message Excellent is printed, then the break statement transfers control to the statement following the switch. If the value of grade doesn't match any of the choices listed, the default case applies, and the message Illegal grade is printed.

A switch statement is often easier to read than a cascaded it statement. Moreover, switch statements are often faster than if statements, especially when there are more than a handful of cases.

In its most common form, the switch statement has the form

switch ( expression ) {

case constant-expression : statements

case constant-expression : statements

default : statements

}

The switch statement is fairly complex; let's look at its components one by one:

Controlling expression. The word switch must be followed by an integer expression in parentheses. Characters are treated as integers in C and thus can be tested in switch statements. Floating-point numbers and strings don't qualify, however.

Case labels. Each case begins with a label of the form.