是否有一个原因，一些语言允许负模？ [英] Is there a reason some languages allow a negative modulus?

问题描述

我很好奇这些语言（Java，C ...）这忽略模运算的数学定义。

I am curious about these languages (Java, C ...) which ignore mathematical definition of modulus operation.

这是在一个模块的操作返回负值（即，根据定义，应永诺返回一个正数）

What is the point of returning negative values in a module operation (that, by definition, should allways return a positive number)?

推荐答案

我怀疑其余运营商故意设计成具有这些语义，这我同意是不是非常有用。 （你怎能写，显示了平日周日，反日，反日，...，反周一的开天辟地之前日期的日历程序？）

I doubt that the remainder operator was deliberately designed to have those semantics, which I agree aren't very useful. (Would you ever write a calendar program that shows the weekdays Sunday, Anti-Saturday, Anti-Friday, ..., Anti-Monday for dates before the epoch?)

相反，负余数的方式整数除法的副作用被定义

Rather, negative remainders are a side effect of the way integer division is defined.

A rem B := A - (A div B) * B

如果 A DIV乙定义为 TRUNC（A / B），你都会得到C的％运营商。如果 A DIV乙定义为楼（A / B），你得到Python的％ 运营商。其他定义是可能的。

If A div B is defined as trunc(A/B), you get C's % operator. If A div B is defined as floor(A/B), you get Python's % operator. Other definitions are possible.

所以，真正的问题是：

，因为这是C那样被它的方式。

Because that's the way that C does it.

最初，C没有具体说明 / 应处理负数。它留给它的硬件。

Originally, C didn't specify how / should handle negative numbers. It left it up to the hardware.

在实践中，每一个显著C实现使用了舍去除法，所以在1999年，这些语义被正式提出的C标准的一部分。

In practice, every significant C implementation used truncating division, so in 1999 these semantics were formally made a part of the C standard.

由于它更容易（=便宜）在无符号除法的条款实施。你刚才计算 ABS（A）DIV ABS（B）和翻转的迹象，如果（A℃下）XOR（B＆小于0）。

Because it's easier (=cheaper) to implement in terms of unsigned division. You just calculate abs(A) div abs(B) and flip the sign if (A < 0) xor (B < 0).

地板除法具有从商减去1，如果余数为非零的附加步骤。

Floored division has the additional step of subtracting 1 from the quotient if the remainder is nonzero.

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