Switch Case 语句

2024年10月29日 | 12 分钟阅读

Switch case 是一种控制语句，用于实现决策逻辑。它由作为独立决策分支的多个 case 组成。它是具有大量条件和缓存内存问题的 if 语句的更好替代品。

C Switch Case 语句

为了创建 switch case，C 语言提供了 switch、case 和 default 关键字。switch 的语法如下所示。

C/C++/Java/C#/Javascript Switch Case 语句语法

switch(expression){
	case expression: 
		// statements
	case expression:
		// statements
	.
	.
	.
	case expression n:
		// statements
	default:
		// statements
}

注意事项

Switch case 仅允许在 case 表达式中使用整数和字符常量。我们不能使用浮点值。
仅当输入值匹配时才执行 case，否则执行 default case。
Break 关键字可用于中断控制并将控制从 switch 中取出。它是可选的，如果不使用，控制将转移到下一个 case。
Switch case 在开发菜单驱动的应用程序时非常有用。
Switch case 不能有变量表达式，例如 (a+2*b+3)。
不允许有重复的 case 表达式。

Java Switch Case 语句

Java switch 与 C 语言 switch 语句类似，并提供相同的功能。在 Java 中，switch 表达式可以是 byte、short、int、char、String 和 Enum 类型。

switch 表达式的类型必须与 case 的类型匹配，否则会生成错误。

Ruby Switch Case 语句

Ruby 中的 switch case 具有不同的标识符和声明语法。使用 case 关键字代替 switch，使用 when 代替 case，使用 else 代替 default。

Ruby 使用 === 运算符来匹配 case 和 when 表达式。如果没有任何 when 匹配，则执行 Else 表达式。

使用 === 运算符比较 case 指定的表达式和 when 指定的表达式，并执行匹配的 when 子句的代码。

Ruby Switch Case 语法

case expression
[when expression [, expression ...] [then]
   code ]...
[else
   code ]
end

Rust Switch Case 语句（模式匹配）

Rust 不支持 C 风格的 switch case，但具有更好的替代匹配表达式。匹配表达式是一个基于模式匹配的概念，并且具有用于匹配表达式的分支。

它遵循 value => expression 语法，并且匹配表达式匹配每个值并执行相应的表达式。语法如下所示。

Rust 模式匹配语法

match input_expression {
    patterns_1 => result_expression_1,
    patterns_2 => result_expression_2,
    ...
    patterns_n => result_expression_n
	_ => expression
}

我们可以直接与文字进行匹配，并且 _ 充当“任何”case。

Swift Switch Case 语句

Swift 的 switch 语句被认为比 C 类型 switch case 更强大。它用于匹配各种类型的 case，包括范围、元组等。

在 swift 中，switch case 不会穿透到较低的 case。 switch case 的语法如下所示。

Swift Switch Case 语法

switch value
{
case value 1: 
	respond to value 1
case value 2,value 3:
	respond to value 2 or 3
default:
	otherwise, do something else
}

让我们看一个 switch case 示例，该示例打印我们传递的数字的名称。

C Switch Case 示例 1：整数输入

#include <stdio.h>
int main()
{
	// integer input is provided to the switch case.
    int num = 2;
    switch(num){
        case 1:
            printf("One");
            break;
        case 2:
            printf("Two");
            break;
        case 3:
            printf("Three");
            break;
        default:
            printf("value is other than 1, 2, 3");
    }
    return 0;
}

输出

Two

上面的示例根据输入打印 Two，并且由于 break，它会停止进一步的执行并退出 switch 块。

Java Switch Case 示例 1：整数输入

int num = 2;
    switch(num){
        case 1:
            System.out.println("One");
            break;
        case 2:
            System.out.println("Two");
            break;
        case 3:
            System.out.println("Three");
            break;
        default:
            System.out.println("value is other than 1, 2, 3");
    }

输出

Two

Ruby Switch Case 示例：整数输入

$num =  2
case $num
when 1
   puts "One"
when 2
   puts "Two"
when 3 
    puts "Three"
else
   puts "value is other than 1, 2, 3"
end

输出

Two

C# Switch Case 示例：整数输入

int num = 2;
    switch(num){
        case 1:
            Console.WriteLine("One");  
            break;
        case 2:
            Console.WriteLine("Two");  
            break;
        case 3:
            Console.WriteLine("Three");  
            break;
        default:
            Console.WriteLine("value is other than 1, 2, 3");  
            break;
     }

输出

Two

Php Switch Case 示例：整数输入

<?php
 $num = 2;
    switch($num){
        case 1:
            echo "One";
            break;
        case 2:
            echo "Two";
            break;
        case 3:
            echo "Three";
            break;
        default:
            printf("value is other than 1, 2, 3");
    }
?>

输出

Two

Rust Switch Case 示例：整数输入

fn main() {
let num = 2;
    match num {
        1 => println!("One"),
        2 => println!("Two"),
        3 => println!("Three"),
        _ => println!("Ain't special"),
    }
}

输出

Two

Javascript Switch Case 示例：整数输入

var val;
var num = 2;
switch(num){
        case 1:
            alert("One")
            break;
        case 2:
            alert("Two")
            break;
        case 3:
            alert("Three")
            break;
        default:
           alert("value is other than 1, 2, 3");
    }

输出

Two

Swift Switch Case 示例：整数输入

let num = 2;
    switch(num){
        case 1:
            print("One");
            break;
        case 2:
            print("Two");
            break;
        case 3:
            print("Three");
            break;
        default:
            print("value is other than 1, 2, 3");
    }

C Switch Case 示例：整数表达式

它允许我们将整数表达式与 case 一起使用。请参见以下示例。

#include <stdio.h>
int main()
{
    int num = 3;
    switch(num){
        case 1:
            printf("One");
            break;
        case 1+1: // integer expression
            printf("Two");
            break;
        case 2+1:
            printf("Three");
            break;
        default:
            printf("value is other than 1, 2, 3");
    }
    return 0;
}

输出

Three

Java Switch Case 示例：整数表达式

int num = 3;
    switch(num){
        case 1:
            System.out.println("One");
            break;
        case 1+1:
            System.out.println("Two");
            break;
        case 2+1:
            System.out.println("Three");
            break;
        default:
            System.out.println("value is other than 1, 2, 3");
    }

输出

Three

Ruby Switch Case 示例：整数表达式

$num =  3
case $num
when 1
   puts "One"
when 1+1
   puts "Two"
when 2+1 
    puts "Three"
else
   puts "value is other than 1, 2, 3"
end

输出

Three

C# Switch Case 示例：整数表达式

 int num = 3;
    switch(num){
        case 1:
            Console.WriteLine("One");  
            break;
        case 1+1:
            Console.WriteLine("Two");  
            break;
        case 2+1:
            Console.WriteLine("Three");  
            break;
        default:
            Console.WriteLine("value is other than 1, 2, 3");  
            break;
     }

Php Switch Case 示例：整数表达式

$num = 3;
    switch($num){
        case 1:
            echo "One";
            break;
        case 1+1:
            echo "Two";
            break;
        case 2+1:
            echo "Three";
            break;
        default:
            printf("value is other than 1, 2, 3");
    }

输出

Three

Rust Switch Case：整数表达式

Rust 不允许将整数表达式放入匹配表达式中。因此，应用 1+2 或 a+3 等会在代码中生成错误。

Rust Switch Case：整数表达式

var num = 3;
switch(num){
        case 1:
            alert("One")
            break;
        case 1+1:
            alert("Two")
            break;
        case 2+1:
            alert("Three")
            break;
        default:
           alert("value is other than 1, 2, 3");
    }

输出

Three

Swift Switch Case：整数表达式

let num = 3;
    switch(num){
        case 1:
            print("One");
            break;
        case 1+1:
            print("Two");
            break;
        case 2+1:
            print("Three");
            break;
        default:
            print("value is other than 1, 2, 3");
    }

C Switch Case 示例：字符常量

Switch case 同时支持整数或字符常量。字符由其 ascii 值替换。

注意：C 编译器隐式地将字符常量替换为其 ASCII 值，'a' => 97。

#include <stdio.h>
int main()
{
    int num = 'a';
    switch(num){
        case 'a':
            printf("It's a");
            break;
        case 'b':
            printf("It's b");
            break;
        case 'c':
            printf("It's c");
            break;
        default:
            printf("value is other than a, b, c");
}
    return 0;
}

输出

It's a

Java Switch Case 示例：字符常量

Switch case 同时支持整数或字符常量。字符由其 ascii 值替换。

注意：Java 隐式地将字符常量替换为其 ASCII 值，'a' => 97。

int num = 'a';
    switch(num){
        case 'a':
            System.out.println("It's a");
            break;
        case 'b':
            System.out.println("It's b");
            break;
        case 'c':
            System.out.println("It's c");
            break;
        default:
            System.out.println("value is other than a, b, c");
    }

输出

It's a

Ruby Switch Case 示例：字符常量

$num =  'a'
case $num
when 'a'
   puts "It's a"
when 'b'
   puts "It's b"
when 'c' 
    puts "It's c"
else
   puts "value is other than a, b, c"
end

输出

It's a

C# Switch Case 示例：字符常量

int num = 'a';
    switch(num){
        case 'a':
            Console.WriteLine("It's a");  
            break;
        case 'b':
            Console.WriteLine("It's b");  
            break;
        case 'c':
            Console.WriteLine("It's c");  
            break;
        default:
            Console.WriteLine("value is other than a, b, c");  
            break;
     }

输出

It's a

PHP Switch Case 示例：字符常量

$num = 'a';
    switch($num){
        case 'a':
            echo "It's a";
            break;
        case 'b':
            echo "It's b";
            break;
        case 'c':
            echo "It's c";
            break;
        default:
            printf("value is other than a, b, c");
    }

输出

It's a

Rust Switch Case 示例：字符常量

let num = 'a';
    match num {
        'a' => println!("It's a"),
        'b' => println!("It's b"),
        'c' => println!("It's c"),
        _ => println!("Other than a, b, c"),
    }

输出

It's a

Javascript Switch Case 示例：字符常量

var num = 'a';
switch(num){
        case 'a':
            alert("It's a")
            break;
        case 'b':
            alert("It's b")
            break;
        case 'c':
            alert("It's c")
            break;
        default:
           alert("value is other than a, b, c");
    }

输出

It's a

Swift 匹配单个字符的示例

let num = "a";
    switch(num){
        case "a":
            print("It's a");
            break;
        case "b":
            print("It's b");
            break;
        case "c":
            print("It's c");
            break;
        default:
            print("value is other than a, b, c");
    }

输出

It's a

C Switch Case 示例：没有 Break

#include <stdio.h>
int main()
{
    int num = 2;
    switch(num){
        case 1:
            printf("One \n");
        case 2:
            printf("Two \n");
        case 3:
            printf("Three \n");
        default:
            printf("value is other than 1, 2, 3");
    }
    return 0;
}

输出

Two 
Three 
value is other than 1, 2, 3

请参见上面的示例，传递了一个整数输入，该输入与 case 2 匹配，不仅执行了该 case，还执行了该 case 之后的所有其他 case，包括 default。

这意味着在没有 break 的情况下，它还会执行其他 case，直到到达 switch 的末尾或找到 break 为止。

Java Switch Case 示例：没有 Break

int num = 2;
    switch(num){
        case 1:
            System.out.println("One");
        case 2:
            System.out.println("Two");
        case 3:
            System.out.println("Three");
        default:
            System.out.println("value is other than 1, 2, 3");
    }

输出

Two
Three
value is other than 1, 2, 3

Ruby Switch Case 示例：Break

Ruby 不使用 break 语句，并且在 switch case 中使用隐式 break，不需要添加额外的 break。

C# Switch Case 示例：Break

C# 不允许从一个 case 隐式地穿透到另一个 case。我们可以显式地放置 break，或者 C# 使用隐式 break 来防止从一个 case 自动穿透到另一个 case。

如果我们不放置 break 并编译代码，则 C# 会报告一个错误

错误：控制无法从一个 case 标签穿透到另一个 case 标签。

PHP Switch Case 示例：没有 Break

在没有 break 的情况下，它将穿透所有 case，直到 switch 完成或找到 break 为止。

<?php
 $num = 'a';
    switch($num){
        case 'a':
            echo "It's a\n";
        case 'b':
            echo "It's b\n";
        case 'c':
            echo "It's c\n";
        default:
            printf("value is other than a, b, c");
    }
?>

输出

It's a
It's b
It's c
value is other than a, b, c

Javascript Switch Case 示例：没有 Break

它将穿透到最后一个 case 或直到找到 break 为止。它将执行匹配的 case 之后的所有 case。请参见下面的示例。

var num = 'a';
switch(num){
        case 'a':
            alert("It's a")
        case 'b':
            alert("It's b")
        case 'c':
            alert("It's c")
        default:
           alert("value is other than a, b, c");
    }

输出

It's a
It's b
It's c
value is other than a, b, c

Swift Switch Case 示例：没有 Break

即使删除了 break 语句，它也不会穿透。这意味着 swift 不允许 case 穿透到 switch 中的较低位置。

let num = "b";
    switch(num){
        case "a":
            print("It's a");
        case "b":
            print("It's b");
        case "c":
            print("It's c");
        default:
            print("value is other than a, b, c");
    }

有时我们想为多个 case 执行通用语句。请参见以下示例。

C Switch Case 语句示例：通用语句

#include <stdio.h>
int main()
{
    int num = 'A';
    switch(num){
        case 'a':
        case 'A':
            printf("It's a \n");
            break;
        case 'b':
        case 'B':
            printf("It's b \n");
            break;
        case 'c':
        case 'C':
            printf("It's c \n");
            break;
        default:
            printf("value is other than a, b, c");
    }
    return 0;
}

输出

It's a

将多个 case 组合在一起以执行通用任务。无论输入是 'a' 还是 'A'，它都会打印相同的消息（执行通用语句）。

Java Switch Case 示例：通用语句

int num = 'A';
    switch(num){
        case 'a':
        case 'A':
            System.out.println("It's a");
            break;
        case 'b':
        case 'B':
            System.out.println("It's b");
            break;
        case 'c':
        case 'C':
            System.out.println("It's c");
            break;
        default:
            System.out.println("value is other than a, b, c");
    }

输出

It's a

Ruby Switch Case 示例：通用语句

  when 'a':
  when 'A':
       puts "It's a"

Ruby 在这种情况下不起作用，因为它会隐式地中断每个 when，并将控制从 case 中取出。

C# Switch Case 示例：通用语句

int num = 'A';
    switch(num){
        case 'a':
        case 'A':
            Console.WriteLine("It's a");  
            break;
        case 'b':
        case 'B':
            Console.WriteLine("It's b");  
            break;
        case 'c':
        case 'C':
            Console.WriteLine("It's c");
            break;
        default:
            Console.WriteLine("value is other than a, b, c");  
            break;
     }

输出

It's a

Php Switch Case 示例：通用语句

<?php
 $num = 'a';
    switch($num){
        case 'a':
        case 'A':
            echo "It's a\n";
            break;
        case 'b':
        case 'B':
            echo "It's b\n";
            break;
        case 'c':
        case 'C':
            echo "It's c\n";
            break;
        default:
            printf("value is other than a, b, c");
    }
?>

输出

It's a

Rust Switch Case 示例：通用语句

使用 | (或) 运算符可以非常容易地执行通用语句或多个模式。

fn main() {
let num = 'a';
    match num {
       'a' | 'A' => println!("It's a"),
       'b' | 'B' => println!("It's b"),
       'c' | 'C' => println!("It's c"),
        _ => println!("other than a, b or c"),
    }
}

输出

It's a

Javascript Switch Case 示例：通用语句

var num = 'a';
switch(num){
        case 'a':
        case 'A':
            alert("It's a")
            break;
        case 'b':
        case 'B':
            alert("It's b")
            break;
        case 'c':
        case 'C':
            alert("It's c")
            break;
        default:
           alert("value is other than a, b, c");
    }

Swift Switch Case 示例：通用语句

它要求 case 中至少有一个可执行语句，否则会抛出错误消息。请参见以下示例。

let num = "a";
    switch(num){
        case "a":
        case "A":
            print("It's a");
        case "b":
            print("It's b");
        case "c":
            print("It's c");
        default:
            print("value is other than a, b, c");
    }

输出

error: 'case' label in a 'switch' should have at least one executable statement.

为了实现这一点，Swift 提供了一个新概念，即使用 , (逗号) 来提供多个 case。请参见以下示例。

let num = "a";
    switch(num){
        case "a","A":
            print("It's a");
        case "b","B":
            print("It's b");
        case "c":
            print("It's c");
        default:
            print("value is other than a, b, c");
    }

输出

It's a

Java Switch Case 示例：字符串输入

String name = "Sunday";
    switch(name){
        case "Monday":
            System.out.println("Monday");
            break;
        case "Tuesday":
            System.out.println("Tuesday");
            break;
        case "Wednesday":
            System.out.println("Wednesday");
            break;
        case "Thursday":
            System.out.println("Thursday");
            break;
        case "Friday":
            System.out.println("Friday");
            break;
        case "Saturday":
            System.out.println("Saturday");
            break;
        case "Sunday":
            System.out.println("Sunday");
            break;
        default:
            System.out.println("Value is not a valid day");
    }

输出

Sunday

Ruby Switch Case 示例：字符串输入

$num =  "Sunday"
case $num
when "Monday"
   puts "Monday"
when "Tuesday"
   puts "Tuesday"
when "Wednesday"
    puts "Wednesday"
when "Thursday"
    puts "Thursday"
when "Friday"
    puts "Friday"
when "Saturday"
    puts "Saturday"
when "Sunday"
   puts "Sunday"
else
   puts "Value is not a valid day"
end

输出

Sunday

C# Switch Case 示例：字符串输入

String name = "Sunday";
    switch(name){
        case "Monday":
            Console.WriteLine("Monday");  
            break;
        case "Tuesday":
                   Console.WriteLine("Tuesday");  
            break;
        case "Wednesday":
                   Console.WriteLine("Wednesday");  
            break;
        case "Thursday":
                   Console.WriteLine("Thursday");  
            break;
        case "Friday":
                   Console.WriteLine("Friday");  
            break;
        case "Saturday":
                   Console.WriteLine("Saturday");  
            break;
        case "Sunday":
                   Console.WriteLine("Sunday");  
            break;
        default:
                   Console.WriteLine("other than a day");  
                   break;
    }

输出

Sunday

Php Switch Case 示例：字符串输入

<?php
 $name = "Sunday";
    switch($name){
        case "Monday":
            echo "Monday";
            break;
        case "Tuesday":
            echo "Tuesday";
            break;
        case "Wednesday":
            echo "Wednesday";
            break;
        case "Thursday":
            echo "Thursday";
            break;
        case "Friday":
            echo "Friday";
            break;
        case "Saturday":
            echo "Saturday";
            break;
        case "Sunday":
            echo "Sunday";
            break;
        default:
            echo "Other than a day";    
            break;
    }
    ?>

Rust Switch Case 示例：字符串输入

let num = "Sunday";
    match num {
        "Monday"    => println!("Monday"),
        "Tuesday"   => println!("Tuesday"),
        "Wednesday" => println!("Tuesday"),
        "Thursday"  => println!("Tuesday"),
        "Friday"    => println!("Friday"),
        "Saturday"  => println!("Saturday"),
        "Sunday"    => println!("Sunday"),
        _ => println!("Other than day"),
    }

输出

Sunday

Javascript Switch Case 示例：字符串输入

var num = "Sunday";
switch(num){
        case "Monday":
            alert("Monday")
            break;
        case "Tuesday":
            alert("Tuesday")
            break;
        case "Wednesday":
            alert("Wednesday")
            break;
        case "Thursday":
            alert("Thursday")
            break;
        case "Friday":
            alert("Friday")
            break;
        case "Saturday":
            alert("Saturday")
            break;
        case "Sunday":
            alert("Sunday")
            break;
        default:
           alert("Not a valid day");
    }

输出

Sunday

Swift Switch Case 示例：字符串输入

let num = "Sunday";
    switch(num){
        case "Sunday":
            print("Sunday");
            break;
        case "Monday":
            print("Monday");
            break;
        case "Tuesday":
            print("Tuesday");
            break;
        case "Wednesday":
            print("Wednesday");
            break;
        case "Thursday":
            print("Thursday");
            break;
        case "Friday":
            print("Friday");
            break;
        case "Saturday":
            print("Saturday");
            break;
        case "Sunday":
            print("Sunday");
            break;
        default:
            print("It is not a valid day");
    }

输出

Sunday

Ruby Switch Case 示例：输入范围

When 表达式以范围形式给出，并且在指定范围内匹配输入并执行代码。请参见以下示例。

$age =  5
case $age
when 0 .. 2
   puts "baby"
when 3 .. 6
   puts "little child"
when 7 .. 12
   puts "child"
when 13 .. 18
   puts "young"
else
   puts "adult"
end

输出

little child

Rust Switch Case 示例：输入范围

模式值可以位于该范围内，并匹配以评估表达式。

fn main() {
let num = 5;
    match num {    0 ... 2 =>  println!("baby"),
    3 ... 6 => println!("little child"),
    7 ... 12 => println!("child"),
    13 ... 18 => println!("young"),
    _ => println!("adult"),
    }
}

输出

little child

c, c++, java, c#, ruby, php, rust, javascript, python, swift

fn main() { let number = 13; match number { // Match a single value 1 => println!("One!"), // Match several values 2 | 3 | 5 | 7 | 11 => println!("This is a prime"), // Match an inclusive range 13...19 => println!("A teen"), // Handle the rest of cases _ => println!("Ain't special"), } }
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