条件语句中逗号的优点是什么？ [英] What is the advantage of commas in a conditional statement?

问题描述

我们可以写一个 if 语句为

if (a == 5, b == 6, ... , thisMustBeTrue)

并且只有最后一个条件应该输入 if 正文是否可以满足。

and only the last condition should be satisfiable to enter the if body.

为什么允许这样做？

推荐答案

稍微改变你的例子，假设是这个

Changing your example slightly, suppose it was this

if ( a = f(5), b = f(6), ... , thisMustBeTrue(a, b) )

（注意 = 而不是 == ）。在这种情况下，逗号保证从左到右的评估顺序。相比之下，这个

(note the = instead of ==). In this case the commas guarantee a left to right order of evaluation. In constrast, with this

if ( thisMustBeTrue(f(5), f(6)) )

你不知道之前是否调用 f（5）或在 f（6）之后。

you don't know if f(5) is called before or after f(6).

更正式地说，逗号允许你写一个序列表达式（a，b，c），就像使用; 编写一系列语句一样 A; b; ℃; 。
就像; 创建一个序列点（完整表达式结束）逗号也是如此。只有序列点控制评估顺序，请参阅此帖子。

More formally, commas allow you to write a sequence of expressions (a,b,c) in the same way you can use ; to write a sequence of statements a; b; c;. And just as a ; creates a sequence point (end of full expression) so too does a comma. Only sequence points govern the order of evaluation, see this post.

但当然，在这种情况下，你实际上写的是

But of course, in this case, you'd actually write this

a = f(5);
b = f(6);    
if ( thisMustBeTrue(a, b) )

所以什么时候以逗号分隔的顺序表达式优先于; 分隔的语句序列？几乎从来没有我会说。当你希望右侧替换为单个表达时，也许在宏中。

So when is a comma separated sequence of expressions preferrable to a ; separated sequence of statements? Almost never I would say. Perhaps in a macro when you want the right-hand side replacement to be a single expression.

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