if和switch各语言的写法

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                <p>if在平时写代码的过程中是肯定会用到的，用于对条件进行判断。switch相对来说用的少一点，下面来看代码。</p> 

c/c++的写法

#include <stdio.h>


int main() {

int a = 0;

if (a > 0) {   //if语句及其分支

printf("a is positive");

} else if (a == 0) {

printf("a equals 0\n");

} else {

printf("a is negative");

}



<span class="token keyword">switch</span> <span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token keyword">case</span> <span class="token operator">-</span><span class="token number">1</span><span class="token operator">:</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"a is -1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">break</span><span class="token punctuation">;</span>  <span class="token comment">// 一定要有break,否则会继续执行下面的case</span>
    <span class="token keyword">case</span> <span class="token number">0</span><span class="token operator">:</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"a is 0"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">break</span><span class="token punctuation">;</span>
    <span class="token keyword">case</span> <span class="token number">1</span><span class="token operator">:</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"a is 1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">break</span><span class="token punctuation">;</span>
    <span class="token keyword">default</span><span class="token operator">:</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"input error"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>


<span class="token keyword">switch</span> <span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token keyword">case</span> <span class="token operator">-</span><span class="token number">1</span><span class="token operator">:</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"a is -1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">break</span><span class="token punctuation">;</span>  <span class="token comment">// 一定要有break,否则会继续执行下面的case</span>
    <span class="token keyword">case</span> <span class="token number">0</span><span class="token operator">:</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"a is 0"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">break</span><span class="token punctuation">;</span>
    <span class="token keyword">case</span> <span class="token number">1</span><span class="token operator">:</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"a is 1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">break</span><span class="token punctuation">;</span>
    <span class="token keyword">default</span><span class="token operator">:</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"input error"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
}

java的写法

public class IfSwitch {
    public static void main(String[] args) {
        int a = 0;
        if (a < 0) {
            System.out.println("a is negative");
        } else if (a == 0) {
            System.out.println("a equals 0");
        } else {
            System.out.println("a is positive");
        }

    <span class="token keyword">switch</span> <span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        <span class="token keyword">case</span> <span class="token operator">-</span><span class="token number">1</span><span class="token operator">:</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"a is -1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token keyword">case</span> <span class="token number">0</span><span class="token operator">:</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"a is 0"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token keyword">case</span> <span class="token number">1</span><span class="token operator">:</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"a is 1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token keyword">default</span><span class="token operator">:</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"input is error"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>




}

go的写法

func main()  {

a := 0

if a < 0 {

fmt.Println("a is negative")

} else if a ==0 {

fmt.Println("a equals 0")

} else {

fmt.Println("a is positive")

}



<span class="token keyword">switch</span> a <span class="token punctuation">{<!-- --></span>
<span class="token keyword">case</span> <span class="token operator">-</span><span class="token number">1</span><span class="token operator">:</span>
	<span class="token class-name"><span class="token namespace">fmt<span class="token punctuation">.</span></span>Println</span><span class="token punctuation">(</span><span class="token string">"a is -1"</span><span class="token punctuation">)</span>
<span class="token keyword">case</span> <span class="token number">0</span><span class="token operator">:</span>
	<span class="token class-name"><span class="token namespace">fmt<span class="token punctuation">.</span></span>Println</span><span class="token punctuation">(</span><span class="token string">"a is 0"</span><span class="token punctuation">)</span>
<span class="token keyword">case</span> <span class="token number">1</span><span class="token operator">:</span>
	<span class="token class-name"><span class="token namespace">fmt<span class="token punctuation">.</span></span>Println</span><span class="token punctuation">(</span><span class="token string">"a is 1"</span><span class="token punctuation">)</span>
<span class="token keyword">default</span><span class="token operator">:</span>
	<span class="token class-name"><span class="token namespace">fmt<span class="token punctuation">.</span></span>Println</span><span class="token punctuation">(</span><span class="token string">"input is error"</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>


    <span class="token keyword">switch</span> <span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        <span class="token keyword">case</span> <span class="token operator">-</span><span class="token number">1</span><span class="token operator">:</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"a is -1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token keyword">case</span> <span class="token number">0</span><span class="token operator">:</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"a is 0"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token keyword">case</span> <span class="token number">1</span><span class="token operator">:</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"a is 1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token keyword">default</span><span class="token operator">:</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"input is error"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">switch</span> a <span class="token punctuation">{<!-- --></span>
<span class="token keyword">case</span> <span class="token operator">-</span><span class="token number">1</span><span class="token operator">:</span>
	<span class="token class-name"><span class="token namespace">fmt<span class="token punctuation">.</span></span>Println</span><span class="token punctuation">(</span><span class="token string">"a is -1"</span><span class="token punctuation">)</span>
<span class="token keyword">case</span> <span class="token number">0</span><span class="token operator">:</span>
	<span class="token class-name"><span class="token namespace">fmt<span class="token punctuation">.</span></span>Println</span><span class="token punctuation">(</span><span class="token string">"a is 0"</span><span class="token punctuation">)</span>
<span class="token keyword">case</span> <span class="token number">1</span><span class="token operator">:</span>
	<span class="token class-name"><span class="token namespace">fmt<span class="token punctuation">.</span></span>Println</span><span class="token punctuation">(</span><span class="token string">"a is 1"</span><span class="token punctuation">)</span>
<span class="token keyword">default</span><span class="token operator">:</span>
	<span class="token class-name"><span class="token namespace">fmt<span class="token punctuation">.</span></span>Println</span><span class="token punctuation">(</span><span class="token string">"input is error"</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
}

Python的写法

a = 0
if a < 0:
    print("a is negative")
elif a == 0:
    print("a equals 0")
else:
    print("a is positive")

对比总结：

if和switch的写法大同小异，需要注意的是，c、c++、java的switch分支需要用break，而go的不用break;
python中大概是为了语言简洁，没有switch，使用if也能达到同样的功能。
使用场景：if一般使用在要判断的值有一定范围，而switch适用于要判断的值具有有限个固定值的情况。

if与switch选择的比较

首先要看一个问题，if 语句适用范围比较广，只要是 boolean 表达式都可以用 if 判断；而 switch 只能对基本类型进行数值比较。两者的可比性就仅限在两个基本类型比较的范围内。 说到基本类型的数值比较，那当然要有两个数。
if 语句每一句都是独立的，看下面的语句：

if (a == 1) 
else if (a == 2) 

这样 a 要被读入寄存器两次，1 和 2 分别被读入寄存器一次。于是你是否发现其实 a 读两次是有点多余的，在你全部比较完之前只需要一次读入寄存器就行了，其余都是额外开销。但是 if 语句必须每次都把里面的两个数从内存拿出来读到寄存器，它不知道你其实比较的是同一个 a。

于是 switch case 就出来了，把上面的改成 switch case 版本：

switch (a) {
  case 0:
  break; case 1:
}

因为特定的规则，他一开始就知道你要比 a，于是 a 一次性读取，相比 if 节约了很多开销。

然后根据大量的实际程序测试（不考虑不同的编译器优化程度差异，假设都是最好的优化），那么Switch语句击中第三个选项的时间跟if/else if语句击中第三个选项的时间相同。 击中第一，第二选项的速度if语句快，击中第四以及第四之后的选项的速度switch语句快。 所以，如果所有选项出现概率相同的话，结论就是：5个选项（包括default）的情况下，switch和if/else if相同。低于5个选项if快，高于5给选项switch快！

首先要看一个问题，if 语句适用范围比较广，只要是 boolean 表达式都可以用 if 判断；而 switch 只能对基本类型进行数值比较。两者的可比性就仅限在两个基本类型比较的范围内。 说到基本类型的数值比较，那当然要有两个数。
if 语句每一句都是独立的，看下面的语句：

if (a == 1) ...
else if (a == 2) ... 

这样 a 要被读入寄存器两次，1 和 2 分别被读入寄存器一次。于是你是否发现其实 a 读两次是有点多余的，在你全部比较完之前只需要一次读入寄存器就行了，其余都是额外开销。但是 if 语句必须每次都把里面的两个数从内存拿出来读到寄存器，它不知道你其实比较的是同一个 a。

于是 switch case 就出来了，把上面的改成 switch case 版本：

switch (a) {
  case 0:
  break; case 1:
}

因为特定的规则，他一开始就知道你要比 a，于是 a 一次性读取，相比 if 节约了很多开销。

然后根据大量的实际程序测试（不考虑不同的编译器优化程度差异，假设都是最好的优化），那么Switch语句击中第三个选项的时间跟if/else if语句击中第三个选项的时间相同。 击中第一，第二选项的速度if语句快，击中第四以及第四之后的选项的速度switch语句快。 所以，如果所有选项出现概率相同的话，结论就是：5个选项（包括default）的情况下，switch和if/else if相同。低于5个选项if快，高于5给选项switch快！