Q: Feature Wish: "%" Extension

[Tim Peters]

[John Roth]
> The basic sign rules for multiplication are so clear that I don't
> really need to repeat them here. If division is the inverse of
> multiplication, then the sign rules follow directly. Any definition
> of division which does not do that isn't an inverse of multiplication,
> but some other operation masquerading under the same name.

Are you serious?  In a world where 3/4 == 0, how long could any programmer
survive believing that integer division and multiplcation are "inverses"?
The antecedent is dead on arrival.

> This might be a much more useful operation in practice, but
> it is confusing to someone who expects the system to behave
> the way he was taught in grammer school.

I can enumerate a hundred ways in which computer arithmetic confuses
beginners, and I bet you could too.  It even confuses beginners that, say,
4.00 - 1.00 gets displayed as 3 or 3. instead of 3.00.

> In other words, (-i)/j should  -(i/j).

Newbies "know" too that if i < 0 and j > 0, then i/j < 0 (isn't that one of
the "basic sign rules" you're defending?).  But-- oops! --*that* one happens
to be true of floor division, but not of truncating division.  Integer
division (whether truncating, flooring, rounding, or what have you) is an
information-losing process, and the loss of information kills identities
without prejudice.

I'm not in favor of crippling a language for real-life use just to save a
newbie a few seconds' initial confusion.  Besides, newbies are sooooo easy
to confuse no matter what you do <0.5 wink>:

   i   j   i/j (C99 style)
  -8   3    -2
  -7   3    -2
  -6   3    -2
  -5   3    -1
  -4   3    -1
  -3   3    -1
  -2   3     0
  -1   3     0
   0   3     0
   1   3     0
   2   3     0
   3   3     1
   4   3     1
   5   3     1
   6   3     2
   7   3     2
   8   3     2

Ask your favorite newbie why the quotients -2, -1, 1 and 2 show up 3 times
each, but 0 shows up 5 times.  I have asked newbies this.  The usual
reaction is paralysis <wink> -- and rote memorization without understanding
is a recipe for disaster in programming work.  The table for floor division
is much more comforting, even to a newbie:

   i   j   i/j (Python style)
  -9   3    -3
  -8   3    -3
  -7   3    -3
  -6   3    -2
  -5   3    -2
  -4   3    -2
  -3   3    -1
  -2   3    -1
  -1   3    -1
   0   3     0
   1   3     0
   2   3     0
   3   3     1
   4   3     1
   5   3     1
   6   3     2
   7   3     2
   8   3     2

BTW, the "exactly 3 of each quotient" goes a long way toward explaining why
floor division is less error-prone in practice (for newbies and experts):
the "weird lump" around 0 doesn't exist.  Definitions Have Consequences, and
that's a vital lesson for computer newbies to learn. too.

confusion-isn't-to-be-feared-when-it's-a-sign-of-learning-ly y'rs  - tim