switch Statement (C)

The switch and case statements help control complex conditional and branching operations. The switch statement transfers control to a statement within its body.

Syntax

selection-statement:
  switch ( expression ) statement

labeled-statement:
  case constant-expression : statement
  default : statement

Remarks

A switch statement causes control to transfer to one labeled-statement in its statement body, depending on the value of expression.

The values of expression and each constant-expression must have an integral type. A constant-expression must have an unambiguous constant integral value at compile time.

Control passes to the case statement whose constant-expression value matches the value of expression. The switch statement can include any number of case instances. However, no two constant-expression values within the same switch statement can have the same value. Execution of the switch statement body begins at the first statement in or after the matching labeled-statement. Execution proceeds until the end of the body, or until a break statement transfers control out of the body.

Use of the switch statement usually looks something like this:

switch ( expression )
{
    // declarations
    // . . .
    case constant_expression:
        // statements executed if the expression equals the
        // value of this constant_expression
        break;
    default:
        // statements executed if expression does not equal
        // any case constant_expression
}

You can use the break statement to end processing of a particular labeled statement within the switch statement. It branches to the end of the switch statement. Without break, the program continues to the next labeled statement, executing the statements until a break or the end of the statement is reached. This continuation may be desirable in some situations.

The default statement is executed if no case constant-expression value is equal to the value of expression. If there's no default statement, and no case match is found, none of the statements in the switch body get executed. There can be at most one default statement. The default statement doesn't have to come at the end. It may appear anywhere in the body of the switch statement. A case or default label can only appear inside a switch statement.

The type of switch expression and case constant-expression must be integral. The value of each case constant-expression must be unique within the statement body.

The case and default labels of the switch statement's body are significant only in the initial test that determines where execution starts in the statement body. switch statements can be nested. Any static variables are initialized before executing into any switch statements.

The following examples illustrate switch statements:

switch( c )
{
    case 'A':
        capital_a++;
    case 'a':
        letter_a++;
    default :
        total++;
}

All three statements of the switch body in this example are executed if c is equal to 'A', since no break statement appears before the following case. Execution control is transferred to the first statement (capital_a++;) and continues in order through the rest of the body. If c is equal to 'a', letter_a and total are incremented. Only total is incremented when c doesn't equal 'A' or 'a'.

switch( i )
{
    case -1:
        n++;
        break;
    case 0 :
        z++;
        break;
    case 1 :
        p++;
        break;
}

In this example, a break statement follows each statement of the switch body. The break statement forces an exit from the statement body after one statement is executed. If i is equal to -1, only n is incremented. The break following the statement n++; causes execution control to pass out of the statement body, bypassing the remaining statements. Similarly, if i is equal to 0, only z is incremented; if i is equal to 1, only p is incremented. The final break statement isn't strictly necessary, since control passes out of the body at the end of the compound statement. It's included for consistency.

A single statement can carry multiple case labels, as the following example shows:

switch( c )
{
    case 'a' :
    case 'b' :
    case 'c' :
    case 'd' :
    case 'e' :
    case 'f' :  convert_hex(c);
}

In this example, if constant-expression equals any letter between 'a' and 'f', the convert_hex function is called.

Microsoft-specific

Microsoft C doesn't limit the number of case values in a switch statement. The number is limited only by the available memory. ANSI C requires at least 257 case labels be allowed in a switch statement.

The default for Microsoft C is that the Microsoft extensions are enabled. Use the /Za compiler option to disable these extensions.

See also

switch Statement (C++)