为什么我会从负数模运算中得到不正确的输出 [英] Why do I get an incorrect output from a modulus operation with negative number

问题描述

我在Dart上尝试了以下代码:我得到28.5

  void main(){双模= -1.5％30.0;打印(模数);} 

Javascript中的相同代码返回-1.5

  let modulo = -1.5％30;console.log(modulo);  

与Dart中上面的javascript代码等效吗?

解决方案

num.remainder 说(再次强调我的意思):

此操作的结果 r 满足: this ==(this〜/other)* other + r .结果，其余的 r 与分隔符 this 的符号相同.

因此，如果您使用:

  void main(){双模=(-1.5).remainder(30.0);打印(模数);} 

您将获得 -1.5 .

请注意，两个值在数学上都是正确的；有两种不同的答案，分别对应执行整数除法时可以计算负商的两种不同方式.您可以选择将负商数舍入为零(也称为舍位)或舍入为负无穷大(底数). remainder 对应于一个截断的除法，而％对应于一个底除的除法.

I tried this code on Dart: I get 28.5

void main() {
  double modulo = -1.5 % 30.0;
  print(modulo);
}

The same code in Javascript returns -1.5

let modulo =  -1.5 % 30;
console.log(modulo);

What is the equivalent of the javascript code above in Dart ?

解决方案

The documentation for num.operator % states (emphasis mine):

Returns the remainder of the Euclidean division. The Euclidean division of two integers a and b yields two integers q and r such that a == b * q + r and 0 <= r < b.abs().

...

The sign of the returned value r is always positive.

See remainder for the remainder of the truncating division.

Meanwhile, num.remainder says (again, emphasis mine):

The result r of this operation satisfies: this == (this ~/ other) * other + r. As a consequence the remainder r has the same sign as the divider this.

So if you use:

void main() {
  double modulo = (-1.5).remainder(30.0);
  print(modulo);
}

you'll get -1.5.

Note that both values are mathematically correct; there are two different answers that correspond to the two different ways that you can compute a negative quotient when performing integer division. You have a choice between rounding a negative quotient toward zero (also known as truncation) or toward negative infinity (flooring). remainder corresponds to a truncating division, and % corresponds to a flooring division.

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