C语言 if...else语句

if语句后面可以跟随可选的else语句，该语句在布尔表达式为false时执行。

if…else - 语法

C编程语言中if.else语句语法为

if(boolean_expression) {
   /* statement(s) will execute if the boolean expression is true */
} else {
   /* statement(s) will execute if the boolean expression is false */
}

if…else - 流程图

if…else - 示例

#include <stdio.h>

int main () {

   /* local variable definition */
   int a=100;

   /* check the boolean condition */
   if( a < 20 ) {
      /* if condition is true then print the following */
      printf("a is less than 20\n" );
   } else {
      /* if condition is false then print the following */
      printf("a is not less than 20\n" );
   }

   printf("value of a is : %d\n", a);

   return 0;
}

编译并执行上述代码时，将生成以下结果-

a is not less than 20;
value of a is : 100

if…else if … else - 语法

If语句后面可以跟随可选的Else If.Else语句，这对于使用单个If.Else If语句测试各种条件非常有用。

C编程语言中if.else语句的语法是-

if(boolean_expression 1) {
   /* Executes when the boolean expression 1 is true */
} else if( boolean_expression 2) {
   /* Executes when the boolean expression 2 is true */
} else if( boolean_expression 3) {
   /* Executes when the boolean expression 3 is true */
} else {
   /* executes when the none of the above condition is true */
}

if…else if … else - 示例

#include <stdio.h>

int main () {

   /* local variable definition */
   int a=100;

   /* check the boolean condition */
   if( a == 10 ) {
      /* if condition is true then print the following */
      printf("Value of a is 10\n" );
   } else if( a == 20 ) {
      /* if else if condition is true */
      printf("Value of a is 20\n" );
   } else if( a == 30 ) {
      /* if else if condition is true  */
      printf("Value of a is 30\n" );
   } else {
      /* if none of the conditions is true */
      printf("None of the values is matching\n" );
   }

   printf("Exact value of a is: %d\n", a );

   return 0;
}

编译并执行上述代码时，将生成以下结果:

None of the values is matching
Exact value of a is: 100