switch statement

Executes code according to the value of an integral argument.

Used where one or several out of many branches of code need to be executed according to an integral value.

Syntax

Explanation

The body of a switch statement may have an arbitrary number of case: labels, as long as the values of all constant_expressions are unique (after conversion to the promoted type of expression). At most one default: label may be present (although nested switch statements may use their own default: labels or have case: labels whose constants are identical to the ones used in the enclosing switch).

If expression evaluates to the value that is equal to the value of one of constant_expressions after conversion to the promoted type of expression, then control is transferred to the statement that is labeled with that constant_expression.

If expression evaluates to a value that doesn't match any of the case: labels, and the default: label is present, control is transferred to the statement labeled with the default: label.

If expression evaluates to a value that doesn't match any of the case: labels, and the default: label is not present, none of the switch body is executed.

The break statement, when encountered anywhere in statement, exits the switch statement:

switch(1) {
    case 1 : puts("1"); // prints "1",
    case 2 : puts("2"); // then prints "2" ("fall-through")
}

switch(1) {
    case 1 : puts("1"); // prints "1"
             break;     // and exits the switch
    case 2 : puts("2");
             break;
}

switch (expr)
{
    int i = 4; // not a VLA; OK to declare here
    f(i); // never called
//  int a[i]; // error: VLA cannot be declared here
  case 0:
    i = 17;
default:;
    int a[i]; // OK to declare VLA here
    printf("%d\n", i); // prints 17 if expr == 0, prints indeterminate value otherwise
}

Keywords

switch, case, default

Example

#include <stdio.h>
 
void func(int x)
{
   printf("func(%d): ", x);
   switch(x)
   {
      case 1: printf("case 1, ");
      case 2: printf("case 2, ");
      case 3: printf("case 3.\n"); break;
      case 4: printf("case 4, ");
      case 5: printf("case 5, ");
      default: printf("default.\n");
   }
}
 
int main(void)
{
   for(int i = 1; i < 10; ++i) func(i);
}

func(1): case 1, case 2, case 3.
func(2): case 2, case 3.
func(3): case 3.
func(4): case 4, case 5, default.
func(5): case 5, default.
func(6): default.
func(7): default.
func(8): default.
func(9): default.

References