关于C中switch语句的有趣事实

前提条件– C语言中的switch语句
Switch是一个控制语句，它允许一个值更改执行控制。

// Following is a simple program to demonstrate syntax of switch.
#include 
int main()
{
   int x = 2;
   switch (x)
   {
       case 1: printf("Choice is 1");
               break;
       case 2: printf("Choice is 2");
                break;
       case 3: printf("Choice is 3");
               break;
       default: printf("Choice other than 1, 2 and 3");
                break;  
   }
   return 0;
} 

输出：

Choice is 2

以下是有关switch语句的一些有趣事实。

1)switch中使用的表达式必须是整数类型(int，char和enum)。不允许使用任何其他类型的表达式。

// float is not allowed in switch
#include 
int main()
{
   float x = 1.1;
   switch (x)
   {
       case 1.1: printf("Choice is 1");
                 break;
       default: printf("Choice other than 1, 2 and 3");
                break;  
   }
   return 0;
} 

输出：

Compiler Error: switch quantity not an integer

在Java，开关中也允许使用String(请参阅此)

2)匹配大小写之后的所有语句都将执行，直到到达break语句为止。

// There is no break in all cases
#include 
int main()
{
   int x = 2;
   switch (x)
   {
       case 1: printf("Choice is 1\n");
       case 2: printf("Choice is 2\n");
       case 3: printf("Choice is 3\n");
       default: printf("Choice other than 1, 2 and 3\n");
   }
   return 0;
} 

输出：

Choice is 2
Choice is 3
Choice other than 1, 2 and 3

// There is no break in some cases
#include 
int main()
{
   int x = 2;
   switch (x)
   {
       case 1: printf("Choice is 1\n");
       case 2: printf("Choice is 2\n");
       case 3: printf("Choice is 3\n");
       case 4: printf("Choice is 4\n");
               break;
       default: printf("Choice other than 1, 2, 3 and 4\n");
                break;
   }
   printf("After Switch");
   return 0;
}

输出：

Choice is 2
Choice is 3
Choice is 4
After Switch

3)默认块可以放置在任何地方。默认位置无关紧要，如果找不到匹配项，则仍将执行默认位置。

// The default block is placed above other cases.
#include 
int main()
{
   int x = 4;
   switch (x)
   {
       default: printf("Choice other than 1 and 2");
                break;        
       case 1: printf("Choice is 1");
               break;
       case 2: printf("Choice is 2");
                break;
   }
   return 0;
}

输出：

Choice other than 1 and 2

4)标签中使用的整数表达式必须是常量表达式

// A program with variable expressions in labels
#include 
int main()
{
    int x = 2;
    int arr[] = {1, 2, 3};
    switch (x)
    {
        case arr[0]: printf("Choice 1\n"); 
        case arr[1]: printf("Choice 2\n");
        case arr[2]: printf("Choice 3\n");
    }
    return 0;
}

输出：

Compiler Error: case label does not reduce to an integer constant

5)永远不会执行以上情况下编写的语句switch语句之后，控制权转移到匹配的情况下，不执行之前编写的语句。

// Statements before all cases are never executed
#include 
int main()
{
   int x = 1;
   switch (x)
   {
       x = x + 1;  // This statement is not executed
       case 1: printf("Choice is 1");
               break;
       case 2: printf("Choice is 2");
                break;
       default: printf("Choice other than 1 and 2");
                break;                   
   }
   return 0;
} 

输出：

Choice is 1

6)两个案例标签不能具有相同的值

// Program where two case labels have same value
#include 
int main()
{
   int x = 1;
   switch (x)
   {
       case 2: printf("Choice is 1");
               break;
       case 1+1: printf("Choice is 2");
                break;
   }
   return 0;
} 

输出：

Compiler Error: duplicate case value

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