5、 C language judgment statement

1、C Judge

• Be careful ： Required when the condition is true , The statement executed when the condition is false is optional
• Equivalence relation ：
  • true== Nonzero == Non empty
  • false==null== zero

1. if,else if, else sentence

• if Statement can be followed by else if,else sentence
• else Statements must be placed at the end
• once else if Statement matching succeeded , other else if Statement and else Statement will not be executed
• Code implementation ：

  #include<stdio.h>
  #include<stdlib.h>
  int a = 10;
  int b = 20;
  int c = 30;
  int main(){
        
      //if sentence 
      if(a <= 10){
        
          printf("a Less than or equal to 10\n");
      }
      printf("a The value of is ：%d\n",a);
  
      //if else sentence 
      if(b == 15){
        
          printf("b be equal to 15");
      }
      else{
        
          printf("b It's not equal to 15\n");
      }
      
      //if else if else sentence 
      if(c > 30){
        
          printf("C Greater than 30");
      }
      else if(c < 30){
        
          printf("c Less than 30");
      }
      else{
        
          printf("c be equal to 30");
      }
  }
  

• Running results ：

  a Less than or equal to 10
  a The value of is ：10
  b It's not equal to 15
  c be equal to 30
  

2.switch sentence

• The rules ：
  • expression Must be constant , An integer or enumeration type
  • When one case When the statement is executed , Subsequent statements will be executed , Until I met break sentence , Jump out of switch sentence
  • default Statements must be placed at the end , Execute when none of the above statements match
• Format ：

  switch(expression){
        
      case 1：
           Code block ;
          break;
      case 2:
           Code block ;
          break;
      default：
           Code block ;
          break;// Can not add 
  }
  

• Code implementation ：

  #include<stdio.h>
  #include<stdlib.h>
  int a = 10;
  int b = 10;
  int main(){
        
      switch (a)
      {
        
      case 10:
          printf("a The value of is 10\n");
          //switch Statement nesting 
          switch (b)
          {
        
          case 100:
              printf("b The value of is 100\n");
              break;
          
          default:
              printf("b The value is not 100\n");
              break;
          }
          break;
      case 20:
          printf("a The value of is 20\n");
          break;
      default:
          printf("a The value is not 10, Neither 20\n");
          break;// Can not add 
      }
  }
  

• Running results ：

  a The value of is 10
  b The value is not 100
  

3. Ternary operator

• Format ：condition ？block1 : block2
• Operation process ： When condition To true , function block1, Otherwise run block2
• Code implementation :

  #include<stdio.h>
  #include<stdlib.h>
  
  int main(){
        
      int num;
  
      printf(" Please enter a number ：\n");
      scanf("%d",&num);
      (num%2 == 0) ? printf(" even numbers \n") : printf(" Odd number \n");
      return 0;
  }
  

• Running results ：

  ./test1
   Please enter a number ：
  2
   even numbers