当在C使用双逻辑非（！）？ [英] When to use the double logical not (!!) in C?

问题描述

什么是用C否定两次一些有效的用途？例如，是这样的：

What are some valid uses for negating twice in C? For example, something like:

if (!!my_cond) {

}

据我了解，在 !! 将保证 !! my_cond 将是0，如果 my_cond 是假否则为1。当你不断的需求的使用呢？

As I understand, the !! will guarantee that the !!my_cond will be 0 if my_cond is false and 1 otherwise. When would you ever need to use this?

推荐答案

在你正在显示它的背景下，这是没用的，因为本身的价值将评估为 0 或 1 。

In the context that you are showing it, it is useless, since the value itself would evaluate to 0 or 1.

它可以在不会做这样的布尔的转换，如算术或索引

It can be usefull in a context that would not do such a "Boolean" conversion, such as arithmetic or indexing

size_t zerovalues[2] = { 0, 0, };

for (size_t i = 0; i < N; ++i)
    ++zerovalues[!!A[i]];

在最后你不得不 A 的值0的数量 zerovalues​​ [0] 和那些没有在 zerovalues​​数[1]

At the end you'd have the number of values 0 of A in zerovalues[0] and the number of those that aren't in zerovalues[1]

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