[notfish]

In what languages is n % 2 -1 for negative odd numbers?

Edit: apparently JS, java, and C all do this. That’s horrifying

[jagged-chisel]

Horrifying? It’s mathematically correct.

[seritools]

it's a semantics problem, not a maths problem - modulus and remainder are not the same operation. This easily trips up people since `%` is often called "modulo", yet is implemented as remainder operation in many languages

https://stackoverflow.com/questions/13683563/whats-the-diffe...

[michael1999]

It's actually really awkward. Math usually considers (-7 mod 5) === (2 mod 5). But in C, (-7 % 5 != 2 % 5).

[arcastroe]

No. Math considers -7 = 3 modulo 5. it's a ring that repeats every 5 units. -7 + 5 + 5 = 3.

Think of a clock which is a ring of size 12. In a clock, going backwards 15 hours (-15) is the same as going backwards 3 hours (-3) which is the same as going forwards 9 hours.

-15 = -3 = 9 modulo 12

[michael1999]

You are correct. I thinko'd and missed the edit window. I meant to say:

Math usually considers (-7 mod 5) === (3 mod 5). But in C, (-7 % 5 != 3 % 5).

The issue is that -7 and 3 are congruent, but the % operator keeps the sign. So -7 % 5 yields -2, not +3. Those are congruent, but not equal. I've never had a use for this behaviour, but I've definitely had to work around it. The lazy way is ((x % n) + n) % n which is safe (assuming n > 0).

[arcastroe]

+1. I wholeheartedly agree. That "lazy way" looks all too familiar haha.

[affinepplan]

wrong. it's not any more correct than 1. that's the key part of an "equivalence" class is that the elements are "equivalent"