008 C language foundation: C judgment

One ： summary

Judgment structures require programmers to specify one or more conditions to evaluate or test , And the statement to execute when the condition is true （ Essential ） And the statement to execute when the condition is false （ Optional ）.
C Language assumes that any non-zero and non null value is true, Put zero or null Is assumed to be false.

 

Two ：if sentence

One if sentence Consists of a Boolean expression followed by one or more statements .

if(boolean_expression){
    
/* The statement that will be executed if the Boolean expression is true */
}

if…else sentence
One if sentence You can follow with an optional else sentence ,else Statement is executed when the Boolean expression is false .

if(){
    

}else{
    

}

Pay attention to distinguish between , and Python There is a difference in grammar ：

if condition_1:
	statement_block_1
elif condition_2:
	statement_block_2
else:
	statement_block_3

 

Two ：switch sentence

Allows testing when a variable is equal to multiple values . Each value is called a case, And the variables to be tested will be applied to each switch case Inspection .

grammar ：

switch(expression){
    
	case constant-expression  :
	   statement(s);
	   break; /*  Optional  */
	case constant-expression  :
	   statement(s);
	   break; /*  Optional  */
  
	/*  You can have any number of  case  sentence  */
	default : /*  Optional  */
	   statement(s);
}

switch Statements must follow the following rules ：

1. switch Statement expression Must be an integer or enumeration type , Or a class type , among class There is a single conversion function that converts it to an integer or enumeration type .
2. In a switch There can be any number of case sentence . Every case Followed by a value to compare and a colon .
3. case Of constant-expression Must be with switch Variables in have the same data type , And it must be a constant or literal quantity .
4. When the variable being tested equals case When the constant in ,case The following statement will be executed , Until I met break The statement so far .
5. When you meet break When the sentence is ,switch End , The control flow will jump to switch The next line after the statement .
6. Not every one case All need to include break. If case The statement does not contain break, The control flow will continue Follow up case, Until I met break until .
7. One switch Statements can have an optional default case, Appear in the switch Ending .default case Can be used on all of the above case To perform a task when it's not true .default case Medium break Statements are not required .

example ：

#include <stdio.h>
int main(){
    
	char grade = 'B';
	switch(grade){
    
		case 'A':
			printf(" great ！ \n");
		case 'B':
		case 'C':
			printf(" well done ！ \n");
		case 'D':
			printf(" Passed  \n");
		case 'E':
			printf(" Not through  \n");
		default:
			printf(" Please try again  \n");
	}
	printf(" Your score is ：%c \n", grade);		//  If this is %d, Will output 66, Corresponding acsii Tabular B
	return 0;
}
 result ：
	┌──(rootkali)-[~/Desktop/c_test]
	└─# vim switch.c          
	┌──(rootkali)-[~/Desktop/c_test]
	└─# gcc switch.c -o switch
	┌──(rootkali)-[~/Desktop/c_test]
	└─# ./switch              
	 well done ！ 
	 Your score is ：B 

 

3、 ... and ：?: Operator

Exp1 ? Exp2 : Exp3;

among ,Exp1、Exp2 and Exp3 Is an expression . Please note that , Use and location of colons .
? The value of the expression is determined by Exp1 Decisive . If Exp1 It's true , The calculation Exp2 Value , The result is the whole ? Value of expression . If Exp1 For false , The calculation Exp3 Value , The result is the whole ? Value of expression .