C language -- 12 branch statement switch

1、 understand switch Branch statement

switch Statement is also a branch statement , It is often used in the case of multiple branches .
such as ：
Input 1, It will output Monday
Input 2, Will output Tuesday
…
Input 7, It will output on Sunday
For such code requirements , If you use if 、else if、else The form of will appear too complex . How to use switch Statement to realize this function ？switch What is the structure of the statement ？

switch( Integer expression )
{
    
	 Statement item ;
}

among , An integer expression must be an integer , And is a constant （ Characters are OK , Because characters are also a kind of integer , Because characters are stored in characters ASCII Code value ）; Statement items are some case sentence .

case  Integral constant expression ;
	 sentence ;

Code implementation ：

int main()
{
    
	int day = 0;
	scanf("%d",&day);
	switch (day)
	{
    
	case 1:
		printf(" Monday \n");
	case 2:
		printf(" Tuesday \n");
	case 3:
		printf(" Wednesday \n");
	case 4:
		printf(" Thursday \n");
	case 5:
		printf(" Friday \n");
	case 6:
		printf(" Saturday \n");
	case 7:
		printf(" Sunday \n");
	}
	return 0;
}

Run the code , The results obtained after the discovery run are as follows ：

1
 Monday 
 Tuesday 
 Wednesday 
 Thursday 
 Friday 
 Saturday 
 Sunday 

obviously , Such a result is wrong . Because after the execution case 1 after , Nothing can stop it , It will always be case 2、case 3… Continue to carry out . If the program is running , The input is 6, Will be output Saturday, Sunday .switch The following expression （day） How many , It will come from case A few in , What determines the entry is the expression day And case Value after .
Modify the program ： At every case Followed by break, To jump out case.
case The decision is the entrance , and break The decision is to export , This is the real branch .

2、 Example ： Input 1-5 Just output the working days , Input 6-7 Just output the rest day .

int main()
{
    
	int day = 0;
	scanf("%d",&day);
	switch (day)
	{
    
	case 1:
	case 2:
	case 3:
	case 4:
	case 5:
		printf(" Working day \n");
		break;
	case 6:
	case 7:
		printf(" Rest Day \n");
		break;
	}
	return 0;
}

The results of the test run are as follows ：

5
 Working day 

break The actual effect of the statement is to divide the statement list into different parts .
Have a good habit of programming ： In the last case Add a clause after the statement break sentence . In case we need to add something else on the basis of the original procedure case sentence , The above input will not appear 1, Print out all the information from Monday to Sunday .

3、default Clause

If the expressed value is the same as all case What if the values of the tags don't match ？ It's nothing , The result is that all statements are skipped , The program does not terminate , No errors reported . Because this situation is C It is not considered a mistake in language , however , What if you don't want to ignore expressions that don't match all the tags ？ At this time, you can add a sentence to the statement list default Clause , Write it down in any one case Where labels can appear .
When switch The value of the expression does not match all case The value of the tag , This default The statement following the clause will execute . Be careful ： Every switch Only one... Can appear in the statement default Clause .

int main()
{
    
	int day = 0;
	scanf("%d",&day);
	switch (day)
	{
    
	default:
		printf(" Input error \n");
		break;
	case 1:
		printf(" Monday \n");
		break;
	case 2:
		printf(" Tuesday \n");
		break;
	case 3:
		printf(" Wednesday \n");
		break;
	case 4:
		printf(" Thursday \n");
		break;
	case 5:
		printf(" Friday \n");
		break;
	case 6:
		printf(" Saturday \n");
		break;
	case 7:
		printf(" Sunday \n");
		break;
	}
	return 0;
}

test result ：

9
 Input error 

4、 practice

Look at the result of the program ？

int main()
{
    
	int n = 1;
	int m = 2;
	switch (n)
	{
    
	case 1 :
		m++;
	case 2:
		n++;
	case 3:
		switch (n)
			//switch You can nest 
		{
    
		case 1:
			n++;
		case 2:
			m++;
			n++;
			break;
		}
	case 4:
		m++;
		break;
	default:
		break;
	}
	printf("m=%d,n=%d\n",m,n);
	return 0;
}

The main thing is to investigate switch Inside break The role of .

The final running results are as follows ：

m=5,n=3