Result of -1%7 is different in javascript(-1) and python(6)

Question

The expression -1 % 7 in JavaScript is giving me -1 as the result. Whereas in Python and Haskell, I found the result to be 6.

Can anyone explain why both have different behaviors? Which one is correct?

Answer 1

I'm going to give a slightly different answer. As others have said, functions can do whatever you define them to and m - x = -x mod m. As a prelude, I'll note that Haskell has two "mod" functions, mod and rem which differ in just this aspect. You can make a case that the mod one is preferable mathematically. The rem one corresponds to what you'd get on an x86 processor. There is, in fact, a third one, the Euclidean one, which may be even better as well as described by Raymond Boute in The Euclidean Definitions of the Functions Div and Mod. The third form always returns a positive modulus. (There are, in fact, at least two other choices that can be made.)

So, Javascript's definition is what you get from most machine mod opcodes. In this sense, it might be preferable as this would make it more efficient to implement. Mathematically, Haskell's and Python's definition is better than Javascript's. There's also a third definition which may be slightly better.

One key property that the Euclidean and Haskell/Python definitions both possess is x mod m = y mod m is equivalent to x = y mod m which Javascript's definition lacks. You can verify by calculating 6 % 7 in Javascript.