From: Dave Thomas <Dave@...>
Date: 2000-06-06T02:00:13-05:00
Subject: [ruby-talk:03109] Is divmod dangerous?


In (say) Python, the modulus operator (%) is actually a remainder
function too; Python arranges the quotient of integer division so that
the remainder equals the modulo.

In Ruby, % is a true modulo operator, and a.remainder(b) is used to
get the remainder.

                         Ruby                   Python
   a     b  |   a/b   a%b  a.remainder(b) |    a/b      a%b
============|=============================|=================
  13     4  |    3     1       1          |     3        1   
  13    -4  |   -3    -3       1          |    -4       -3
 -13     4  |   -3     3      -1          |    -4        3
 -13    -4  |    3    -1      -1          |     3       -1


Why do we care, you ask?

Well, both Ruby and Python (and Haskell and ...) have divmod, which
returns a quotient and a <something>.

In (say) Python, if

    (q, r) = divmod(x, y)

then

    x = q*y + r


In Ruby, however, divmod returns the quotient and modulo:

   for x in [13, -13]
      for y in [4, -4]
        q, r = x.divmod(y)
        puts q*y + r
      end
   end

   =>  13, 9, -9, -13

This strikes me as dangerous, as I suspect most people would expect
divmod to return a remainder, not a modulus.

Anyone have any opinions?


Dave