该函数输出的说明 [英] Explanation for this function's output

问题描述

我正在做一些复习题，问我以下内容的输出是什么"，并且在理解有关此功能的内容时遇到了一些麻烦:

I am doing review questions which ask me "What is the output of the following," and I am having some trouble understanding something about this function:

int a = 1, b = 1, c = -1;
c = --a && b++;
printf("%d %d %d", a, b, c);

输出是010.我的问题是关于第2行，c = --a && b++.这条线是如何处理的，以及它如何工作/更改值?如果是c = --a || b++?根据我的理解，我认为输出为020.

The output is 010. My question is about line 2, c = --a && b++. How is this line processed, and how does it work/change the values? And if it were c = --a || b++? From my understanding I thought the output would be 020.

推荐答案

理解结果的关键概念是

The key concept to understanding the result is short-circuit evaluation of Boolean operators (&& and ||) -- if, after evaluating the left-hand side of a Boolean operator, the value of the right-hand side cannot affect the overall result, then it will not be evaluated and any side-effects it would produce will not happen.

在第一种情况下，由于--a评估为0(= false)，因此不评估... && ...的第二部分，因为" false AND any "将始终为false.具体来说，b++不会执行，因此其值在输出中仍为1.

In the first case, since --a evaluates to 0 (=false) the second part of ... && ... is not evaluated, since "false AND anything" will always be false. Specifically, b++ is never executed, and so its value remains 1 in the output.

在--a || b++的情况下，整个表达式 的值不能由左侧确定(" false或某物"仍然为true )，因此对b++ 进行了评估(并且副作用是，增加b会发生).

In the case of --a || b++, the value of the whole expression cannot be determined by the left-hand side ("false OR something" can still be true) so the b++ is evaluated (and it's side-effect, incrementing b, happens).

完全了解结果所需的另一个概念是前后递增/递减运算符之间的差异.如果--或++出现在变量之前(如--a中所示)，则变量将先减小或增大，然后使用 new 值评估变量的值.整个表情.如果--或++出现在变量后 (如b++)，则该变量的 current 值将用于计算表达式和增量/递减发生在这种情况发生之后.

The other concept needed to fully understand the results is the difference between pre- and post-increment/decrement operators. If the -- or ++ appears before the variable (as in --a) then the variable is decremented or incremented first and new value is used to evaluate the whole expression. If the -- or ++ appears after the variable (as in b++) then the current value of the variable is used to evaluate the expression and the increment/decrement happens after this has happened.

应注意，试图合并同一变量的--/++ 的两个或多个实例(例如a++ + ++a)的表达式很可能会调用不确定的行为-结果可能因平台，编译器，编译器甚至一天中的时间而异.

It should be noted that expressions that try to combine two or more instances of --/++ of the same variable (e.g. a++ + ++a) are quite likely to invoke undefined behaviour -- the result may vary by platform, compiler, compiler and even the time of day.

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