变量x等于a或b [英] Variable x equals a or b

问题描述

这是:

if(x == a || b){//do something}

与:

if(x == a || x == b){//do something}

?

我认为不是，因为在第一种情况下，我们评估x是否等于a，以及b是true还是false.

在第二种情况下，我们评估x是否等于a和x是否等于b.而且我了解到，在惰性评估中，如果x等于a，则我们将不进行进一步评估. /p>

但是有人认为在第一种情况下，我们问x是否等于a或b，所以我想确定.

解决方案

否.

在C ++中，这是

:

x == a || b  // Same as (x == a) || b

等效于此:

(x == a) || (bool)b

如果x和a相等，则

取值为true；如果将b转换为bool，则取值为true.另一方面，在C语言中，它等效于此:

(x == a) || (b != 0)

如果x和a相等，或者如果b不同于0，则计算为true(在这里，我们必须隐式假设b是整数类型，否则不会)编译).

另一方面，这:

(x == a || x == b) // Same as ((x == a) || (x == b))

当x和a相等或x和b相等时(即，如果x等于a或b)，

评估为true在C ++和C中都是如此.

Is this:

if(x == a || b){//do something}

same as:

if(x == a || x == b){//do something}

?

I think it is not, because in the first case we evaluate if x equals a and if b is true or false.

In the second case, we evaluate if x equals a and if x equals b. And I understand that in the lazy evaluation if x equals a than we are not evaluating further.

But somebody thinks that in the first case we ask if x equals a or b, so I wanna make sure.

解决方案

No.

In C++, this:

x == a || b  // Same as (x == a) || b

Is equivalent to this:

(x == a) || (bool)b

Which evaluates to true if x and a are equal OR if b evaluates to true when converted to bool. In C, on the other hand, it is equivalent to this:

(x == a) || (b != 0)

Which evaluate to true if x and a are equal OR if b is different from 0 (here we must make the implicit assumption that b is of integral type, otherwise this won't compile).

On the other hand, this:

(x == a || x == b) // Same as ((x == a) || (x == b))

Evaluates to true when either x and a are equal OR x and b are equal (i.e., if x is either equal to a or equal to b) both in C++ and in C.

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