Python中对负数的取模运算 [英] The modulo operation on negative numbers in Python

问题描述

我在Python中发现了一些有关负数的奇怪行为:

I've found some strange behaviour in Python regarding negative numbers:

>>> -5 % 4
3

谁能解释发生了什么事?

Could anyone explain what's going on?

推荐答案

与C或C ++不同，Python的模运算符(%)始终返回与分母(除数)符号相同的数字.您的表情之所以产生3，是因为

Unlike C or C++, Python's modulo operator (%) always return a number having the same sign as the denominator (divisor). Your expression yields 3 because

(-5)/4 = -1.25->下限(-1.25)= -2

(-5) / 4 = -1.25 --> floor(-1.25) = -2

(-5)％4 =(-2× 4 + 3)％4 = 3.

(-5) % 4 = (-2 × 4 + 3) % 4 = 3.

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

但是在C中，如果 N ≥3，我们得到一个负数，它是无效数字，我们需要通过添加7来手动对其进行修正:

but in C, if N ≥ 3, we get a negative number which is an invalid number, and we need to manually fix it up by adding 7:

int result = (2 - N) % 7;
return result < 0 ? result + 7 : result;

(请参阅 http://en.wikipedia.org/wiki/Modulo_operator 确定了不同语言的结果符号.)

(See http://en.wikipedia.org/wiki/Modulo_operator for how the sign of result is determined for different languages.)

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