为什么不同编程语言中的模数不同？
Perl
print 2 % -18;
-->
-16
Tcl
puts [expr {2 % -18}]
-->
-16
但是 VBScript
wscript.echo 2 mod -18
-->
2
为什么有区别？
Perl
print 2 % -18;
-->
-16
Tcl
puts [expr {2 % -18}]
-->
-16
but VBScript
wscript.echo 2 mod -18
-->
2
Why the difference?
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维基百科答案在这里相当有帮助。
一个简短的总结是，任何整数都可以定义为
a = qn + r
，其中所有这些字母都是整数，并且
0 <= |r| < |n|。
几乎每种编程语言都要求 (a/n) * n + (a%n) = a。 因此模数的定义几乎总是取决于整数除法的定义。 整数除以负数有两种选择：2/-18 = 0 或 2/-18 = -1。 根据您的语言的情况，通常会更改 % 运算符。
这是因为 2 = (-1) * -18 + (-16) 和 2 = 0 * -18 + 2。
对于 Perl 来说，情况很复杂。 手册页说：“请注意，当使用整数在范围内时，”％ “让您可以直接访问由 C 编译器实现的模运算符。该运算符对于负操作数的定义并不明确，但执行速度会更快。”因此，如果使用整数，它可以选择 Perl 的任一选项（如 C）在适用范围。 如果使用整数不在范围内，手册上说“如果$b是负数，则$a％$b是$a减去$b的不小于$a的最小倍数（即结果将小于或等于零）。
The wikipedia answer is fairly helpful here.
A short summary is that any integer can be defined as
a = qn + r
where all of these letters are integers, and
0 <= |r| < |n|.
Almost every programming language will require that (a/n) * n + (a%n) = a. So the definition of modulus will nearly always depend on the definition of integer division. There are two choices for integer division by negative numbers 2/-18 = 0 or 2/-18 = -1. Depending on which one is true for your language will usually change the % operator.
This is because 2 = (-1) * -18 + (-16) and 2 = 0 * -18 + 2.
For Perl the situation is complicated. The manual page says: "Note that when use integer is in scope, "%" gives you direct access to the modulus operator as implemented by your C compiler. This operator is not as well defined for negative operands, but it will execute faster. " So it can choose either option for Perl (like C) if use integer is in scope. If use integer is not in scope, the manual says " If $b is negative, then $a % $b is $a minus the smallest multiple of $b that is not less than $a (i.e. the result will be less than or equal to zero). "
维基百科的“模运算”页面对此进行了很好的解释。 我不会尝试在这里做得更好，因为我可能会犯一个微妙但重要的错误。
问题在于您可以用不同的方式定义“余数”或“模数”，并且不同的语言选择了不同的选项来实现。
Wikipedia's "Modulo operation" page explains it quite well. I won't try to do any better here, as I'm likely to make a subtle but important mistake.
The rub of it is that you can define "remainder" or "modulus" in different ways, and different languages have chosen different options to implement.
将一个数字和一个除数（其中一个为负数）相除后，至少有两种方法将它们分成商和余数，这样商 * 除数 + 余数 = 数字：您可以将商向负无穷大舍入，或趋于零。
许多语言只选择一种。
我忍不住指出 Common Lisp 两者都提供。
After dividing a number and a divisor, one of which is negative, you have at least two ways to separate them into a quotient and a remainder, such that quotient * divisor + remainder = number: you can either round the quotient towards negative infinity, or towards zero.
Many languages just choose one.
I can't resist pointing out that Common Lisp offers both.
当然，python 明确地告诉你
python, of course, explicitly informs you