理解C语言-06-逻

C语言的逻辑控制主要有7种:

1> goto 最强大，但一般只在特殊环境下使用。

2> if else

3> ?:

4> switch case

5> for

6> while

7> do while

自从dijkstra的论文Go To Statement Considered Harmful以后，C语言代码很少看到goto了。（一般用在多次资源分配的错误处理上）

但，从计算机的角度来说，缺少goto（jmp指令）还真没法干活。其实，goto是最符合我们的设计流程图的。

将一个流程图用goto来实现也最直观。goto能真正让我们做到心之所动，剑之所至。

接下来，分别考虑将2-7转化为对应的goto语言版本（等价于转化为对应汇编版本）。

2 if语句

if （条件1）

代码块1

else

代码块2

对应goto：

t = 条件1；

if （t为真）goto true；

代码块2

goto finish

true：

代码块1

finish：

3 ?: 和if else是等价的。

只不过if语句为代码块，？：为表达式。

变量= (条件1)? 表达式1：表达式2；

goto版本：

t = 条件1；

if (t为真)

goto true；

变量=表达式2

goto finish；

true：

变量=表达式1

finish：

注意：?:里面的表达式尽量简单，过于复杂的话，改用if语句实现，这样方便调试。

4 switch-case

switch （条件取值变量）

case 元素1 ：

语句块1；

break；

case 元素2：

语句块2；

break；

...

case 元素N

语句块N；

break；

default：

默认处理。

break；

}

goto版本：

跳转表={标签1，标签2，...,标签N }

goto 跳转表[元素index]

标签1：

语句块1

goto Finish

标签2：

语句块2

goto Finish

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