“for"循环后分号的影响 [英] Effect of semicolon after 'for' loop

问题描述

假设我想使用 for 循环在 C 中打印一条消息五次.为什么我在 for 循环后添加分号，如下所示:

Say I want to print a message in C five times using a for loop. Why is it that if I add a semicolon after for loop like this:

for (i=0;i<5;i++);

消息没有打印 5 次，但如果我不把分号放在那里，它会打印吗?

the message does not get printed 5 times, but it does if I do not put the semicolon there?

推荐答案

分号是一个合法的声明，称为 null statement ^* ，意思是什么都不做".由于 for 循环执行单个操作(可能是包含在 {} 中的块)分号被视为循环的主体，从而导致您观察到的行为.

Semicolon is a legitimate statement called null statement ^* that means "do nothing". Since the for loop executes a single operation (which could be a block enclosed in {}) semicolon is treated as the body of the loop, resulting in the behavior that you observed.

以下代码

 for (i=0;i<5;i++);
 {
     printf("hello
");
 }

解释如下:

• 重复五次for (i=0;i<5;i++)
• ...什么都不做(分号)
• 为局部变量打开一个新的作用域{
• ...打印你好"
• 关闭作用域}

如您所见，重复的操作是;，而不是printf.

---

^* 参见 K&R，第 1.5.2 节

As you can see, the operation that gets repeated is ;, not the printf.

---

^* See K&R, section 1.5.2

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