小朋友学C语言（29）：switch case语句

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本文通过示例对比了switch-case语句与if-else-if语句在选择结构中的使用方式，并展示了忘记添加break语句可能导致的问题。

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switch case语句与if elseif语句类似，都是从多个选择条件里选取一个来执行。

（一）先来看一个if elseif程序

#include <stdio.h>

int main() 
{
    int number;
    printf("Please input an integer between 1~7: ");
    scanf("%d", &number);

    printf("Today is ");
    if(1 == number) 
    {
        printf("Monday!");
    }
    else if(2 == number) 
    {
        printf("Tuesday!");
    }
    else if(3 == number) 
    {
        printf("Wednesday!");
    }
    else if(4 == number) 
    {
        printf("Thursday!");
    }
    else if(5 == number) 
    {
        printf("Friday!");
    }
    else if(6 == number) 
    {
        printf("Saturday!");
    }
    else if(7 == number) 
    {
        printf("Sunday!");
    }
    else
    {
        printf("a wrong day! Please input an integer between 1~7!");
    }

    return 0;
}

运行结果：

Please input an integer between 1~7: 7
Today is Sunday!

（二）再用switch case语句实现相同的功能

#include <stdio.h>

int main() 
{
    int number;
    printf("Please input an integer between 1~7: ");
    scanf("%d", &number);

    printf("Today is ");
    switch(number) 
    {
        case 1:
            {
                printf("Monday!");
                break;      
            }
        case 2:
            {
                printf("Tuesday!");
                break;      
            }
        case 3:
            {
                printf("Wednesday!");
                break;      
            }
        case 4:
            {
                printf("Thursday!");
                break;      
            }
        case 5:
            {
                printf("Friday!");
                break;      
            }
        case 6:
            {
                printf("Saturday!");
                break;      
            }
        case 7:
            {
                printf("Sunday!");
                break;      
            }
        default:
            {
                printf("a wrong day! Please input an integer between 1~7!");
                break;              
            }
    }

    return 0;
}

运行结果1：

Please input an integer between 1~7: 7
Today is Sunday!

运行结果2：

Please input an integer between 1~7: 10
Today is a wrong day! Please input an integer between 1~7!

注意：
switch case的结构为

switch(表达式或变量或常量)
{
    case 条件1:
        {
            执行语句;
            break;
        }
    case 条件2:
        {
            执行语句;
            break;
        }
    ……
    default:
        {
            执行语句;
            break;
        }
}

这里“break;”表示中断，若忘了写，程序会继续执行下面的条件。

（三）试试下面这个程序

#include <stdio.h>

int main() 
{
    int number;
    printf("Please input an integer between 1~7: ");
    scanf("%d", &number);

    printf("Today is ");
    switch(number) 
    {
        case 1:
            {
                printf("Monday!");  
            }
        case 2:
            {
                printf("Tuesday!");     
            }
        case 3:
            {
                printf("Wednesday!");       
            }
        case 4:
            {
                printf("Thursday!");    
            }
        case 5:
            {
                printf("Friday!");      
            }
        case 6:
            {
                printf("Saturday!");    
            }
        case 7:
            {
                printf("Sunday!");      
            }
        default:
            {
                printf("a wrong day! Please input an integer between 1~7!");           
            }
    }

    return 0;
}

运行结果：

Please input an integer between 1~7: 3
Today is Wednesday!Thursday!Friday!Saturday!Sunday!a wrong day! Please input an integer between 1~7!

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