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switch statement

Executes code according to the value of an integral argument.

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

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Syntax

- -
attr-spec-seq(optional) switch ( expression ) statement
- - - -
attr-spec-seq - (C23)optional list of attributes, applied to the switch statement
expression - any expression of integer type (char, signed or unsigned integer, or enumeration)
statement - any statement (typically a compound statement). case: and default: labels are permitted in statement, and break; statement has special meaning.
- - - - -
case constant-expression : statement (1) (until C23)
attr-spec-seq(optional) case constant-expression : statement(optional) (1) (since C23)
default : statement (2) (until C23)
attr-spec-seq(optional) default : statement(optional) (2) (since C23)
- - -
constant-expression - any integer constant expression
attr-spec-seq - (C23)optional list of attributes, applied to the label

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).

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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.

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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.

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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.

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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;
-}
- -

As with all other selection and iteration statements, the switch statement establishes block scope: any identifier introduced in the expression goes out of scope after the statement.

-

If a VLA or another identifier with variably-modified type has a case: or a default: label within its scope, the entire switch statement must be in its scope (in other words, a VLA must be declared either before the entire switch or after the last label):

-
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
-}
 (since C99)

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:
-      case 6: printf("case 5 or case 6, ");
-      default: printf("default.\n");
-   }
-}
- 
-int main(void)
-{
-   for(int i = 1; i < 9; ++i) func(i);
-}

Output:

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

References

See also

-
C++ documentation for switch statement
-

- © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
- https://en.cppreference.com/w/c/language/switch -

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