David Glasser news at davidglasser.net
Wed Sep 26 18:00:39 EDT 2001

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Bruce Dawson <comments at cygnus-software.com> wrote:

> >>> mylist = [1, 2, 3, 5, 4, 6]
> >>> def compare1(x, y):
> ...  return x < y
> ...
> >>> mylist.sort(compare1)
> >>> mylist
> [1, 2, 3, 5, 4, 6]

The comparison "x < y" can return two values: 1 if x < y, and 0 if
x >= y.  But a sort function really needs to be more specific,
differentiating between x < y, x > y, and x == y.  The standard for sort
functions, based on C's strcmp, is to return:

   -1 if x < y
   0  if x == y
   +1  if x > y

(It appears that any negative value is as good as -1, and any positive
for +1, but the documentation just say those specific values.)  This is,
as you said, so a function can do something along the lines of "return x
- y".

The really good news, though, is that you can ignore most of this by
using Python's cmp function.  For example, to sort by absolute value:

>>> x = [1, 0, -2, 52, -32, 4]
>>> def abssort(x, y):
...    return cmp(abs(x), abs(y))
... 
>>> x.sort(abssort)
>>> x
[0, 1, -2, 4, -32, 52]


-- 
David Glasser
news at davidglasser.net               http://www.davidglasser.net/

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