In depth C language (4) -- definition and use of switch

thorough C Language (4)——switch The definition and use of

Used in the full text C The standard for the C99,ANSI C, If there is no special description, the default is .

C++ It is Dev 5.7.1 Of C++ edition .

C Of Switch and case

Common grammar ：

switch (expression)
	statement

among expression The result of must be an integer value .

Although it is possible to use a statement directly , But the results are as follows ：

switch (expression)
{
    
 	statement-list       
}

Throughout statement-list The label of is as follows ：

case constant-expression: 

every last case The tag must have a unique value ：

Constant expression (constant-expression),constant-expression It refers to the expression evaluated during compilation , It cannot be any variable .

case With the execution flow

case The most special point of the statement is , It does not divide the statement into several execution parts , But through case To determine the entry point of the statement list .

The execution process is as follows ：

1. The calculation is complete expression The content of .
2. The execution flows to its... In the statement list case The tag value is the same as expression Statements with matching values .
3. Start from the execution of the previous statement , Until switch-case The bottom of .

All the statements will be executed .

for instance ：

switch (month)
{
    
    case 12: 
        printf("%d",12);
    case 11:
        printf("%d",11);
}

The result? , It may be different from what you think .

When month be equal to 12 When , give the result as follows ：

1211

This is the result of execution flow , Lead to case The statements behind the tag will be executed , Including others case.

Can you realize the function of dividing the code into different parts ？ Of course. ,break The statement is .

break sentence

break The statement is very simple , Is to make case The flow stops at different positions . Give Way switch Run in a more traditional way .

How to use it is as follows ：

switch (month)
{
    
    case 12: 
        printf("%d",12);
        break;
    case 11:
        printf("%d",11);
    case 10:
        printf("%d",10);
}

give the result as follows ：

12
12
--------------------------------
Process exited after 4.971 seconds with return value 0
 Please press any key to continue . . .

break The habit of

One caveat , final case The need for break Do you ？ Theoretically speaking, traditional break In the syntax , We don't need to be in the last case Add break. Because no case Will continue to execute , But for the consistency of future modifications , Let's add at the end break, Add it later case There is no need to add options .

In order to allow different expression values to be executed , Can make multiple case The label corresponding , As shown below ：

switch (month)
{
    
    case 12: 
        printf("%d",12);
        break;
    case 11:
    case 10:
        printf("%d %d",10,11);
        break;
}

give the result as follows ：

11
1011
--------------------------------
10
1011
--------------------------------

default Clause

If there is a situation , all case The tag values of do not match , But you want to find a condition to cover the bottom , You can use default Clause , Put the label below case Where you can be .

default:

That is to say, you can write like this ：

switch (month)
{
    
    case 12: 
        printf("%d",12);
        break;
    default :
        printf("%s","default");
    case 11:
    case 10:
        printf("%d %d",10,11);
        break;
}

give the result as follows ：

9
default1011
--------------------------------

So it's lost default The meaning of , But it's still interesting hhhh.