为什么负数的模数为正(-7％6 == 5)? [英] Why is modulo of a negative number positive (-7 % 6 == 5)?

问题描述

为什么以下是正确的?

print(-7 % 6 == 5);

这意味着-7 % 6会产生 5 ，但是我会期望看到-1(在JavaScript中就是这种情况).

This means that -7 % 6 yields 5, but I would expect to see -1 (in JavaScript this is the case).

在DartPad中进行验证.

推荐答案

Dart中的模数行为与JavaScript，C，C ++等中的行为不同.它与Python共享其行为，关于此主题，有一个在此处有很好答案的问题.

The modulo behavior in Dart is different from the behavior in JavaScript, C, C++, and others. It shares its behavior with Python and about this topic there is a question with great answers here.

在@kennytm的出色的答案上加了一点，对此的直观解释是，它的作用类似于肯定的因式分解(至少在Dart中如此):

Adding a bit to the superb answer by @kennytm, the intuitive explanation for this is that it works analogous to positive factorisation (at least in Dart):

17 % 6 == 5          <=> -7 % 6 == 5
(2 * 6 + 5) % 6 == 5 <=> (-2 * 6 + 5) % 6 == 5

但是，它可以很容易地以不同的方式合理地实施，但这就是Dart中的工作方式.

However, it could have easily been implemented differently, reasonably, but this is how it works in Dart.

以下是一个好处(引用自前面提到的答案):

A benefit of this is the following (quote from the answer mentioned earlier):

选择它来代替C行为，因为非负结果通常更有用.一个示例是计算工作日.如果今天是星期二(第2天)，那么前 N 天是星期几?在Python中，我们可以使用

It is chosen over the C behavior because a nonnegative result is often more useful. An example is to compute week days. If today is Tuesday (day #2), what is the week day N days before? In Python we can compute with

return (2 - N) % 7

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