C语言 switch 语句用法

The C switch Statement

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

Control passes to the statement whose case constant-expression matches the value of switch ( expression ). The switchstatement can include any number of case instances,
but no two case constants within the same switch statement can have the same value. Execution of the statement body begins at the selected statement and 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 case within the switch statement and to branch to the end of the switch statement. Without break,
the program continues to the next case, executing the statements until a break or the end of the statement is reached. In some situations, this continuation may be desirable.
The default statement is executed if no case constant-expression is equal to the value of switch ( expression ). If the defaultstatement
is omitted, and no case match is found, none of the statements in the switch body are executed. There can be at most one default statement. The default statement need not come at the end;
it can appear anywhere in the body of theswitch 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 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 anyswitch statements.

Note
Declarations can appear at the head of the compound statement forming the switch body, but initializations included in the declarations are not performed. The switch statement transfers control directly to an executable statement within the body, bypassing the lines that contain initializations.

The following examples illustrate switch statements:

switch( c ) 
{
    case 'A':
        capa++;
    case 'a':
        lettera++;
    default :
        total++;
}

如果c==‘A’那么switch中的三条语句都将执行，因为在case后面没有break语句。 Execution control is transferred to the first statement (capa++;) and continues in order through the rest of the body. If c is equal to 'a', lettera and total are
incremented. Only total is incremented if c is not equal to '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 statementn++; 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 is not strictly necessary, since control passes out of the body at the end of the compound statement, but it is included for consistency.

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

case 'a' :
case 'b' :
case 'c' :
case 'd' :
case 'e' :
case 'f' :  hexcvt(c);

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

Microsoft Specific
Microsoft C does not 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.
END Microsoft Specific