Mirror of forum.nim-lang.org

3867 :: Modulo operator returning wrong result?

• dixego (orginal) [2018-05-29T01:07:30+02:00] view original

  When I try to run this:

  echo -1 %% 6
  Run

  it prints 3, however it should evidently be 5. Does the %% not work with negative numbers? Am I missing something?

---

• dom96 (orginal) [2018-05-29T13:03:41+02:00] view original

  I've never seen the %% operator. I've always used mod:

  echo -1 mod 6
  Run

  But this gives -1, not 5. Perhaps this PR is relevant: https://github.com/nim-lang/Nim/pull/7118

---

• def (orginal) [2018-05-29T13:43:08+02:00] view original

  Like all operators ending with %, %% treats its arguments as unsigned numbers. This comes from a time when Nim didn't have unsigned numbers yet. -1 unsigned is 18446744073709551615 and 18446744073709551615 mod 6 is 3.

  https://nim-lang.org/docs/system.html#%%,IntMax32,IntMax32

---

• mratsim (orginal) [2018-05-29T19:33:07+02:00] view original

  -1 is correct, 5 would be the result of the rem operation.

  Note that for most programming language remainder (math) is called modulo (programming) which does not help.

---

• treeform (orginal) [2018-05-29T20:37:23+02:00] view original

  Nim really should have a rem operator and a mod operator as they are slightly different:

  https://en.wikipedia.org/wiki/Modulo_operation

  If you want python's mod operator:

  proc pythonMod(n, M: int): int = ((n mod M) + M) mod M
  assert pythonMod(1, -4) == -3
  assert pythonMod(-1, 4) == 3
  assert pythonMod(-1, -4) == -1
  assert pythonMod(1, 4) == 1
  Run

---