• bearophile (18/25) Dec 20 2007 Sorry for raising this thread again (with a different name) after so muc...

bearophile <bearophileHUGS lycos.com> writes:

Bill Baxter:
 sort!("a > b")(array);
 I just wanted to say that this is BRILLIANT!  Zero call overhead,
 zero syntax overhead, compile-time lambda functions!

Sorry for raising this thread again (with a different name) after so much time,
I want to say something that comes from my experience.
Python list.sort()/sorted(iterable) accepts an optional cmp() function too, but
later they have added the "key" optional parameter too, and today most of the
times you use key instead of cmp.

http://docs.python.org/lib/typesseq-mutable.html

key specifies a function of one argument that is used to extract a comparison
key from each list element, like key=str.lower.

An example:
 l = [7,2,2,-10,10,-10,-3,4,-9,-7]
 sorted(l)



[-10, -10, -9, -7, -3, 2, 2, 4, 7, 10]
 l.sort(key=abs)
 l



[2, 2, -3, 4, 7, -7, -9, -10, 10, -10]

Using key requires more memory (for the "decorated" elements), but it's often
much simpler to use.
A good sorting routine has many conflicting goals:
- easy to use with a short and simple syntax, for 99% of situations where you
have a small array and you both don't need max sorting speed nor to use as
little memory as possible.
- very fast and memory-lean when used on LOT of data.
- able to sort really fast already partially sorted arrays.
- etc.

The key() parameter is useful for the first situation, that is very common. For
all the other situations you can accept a more fussy syntax, that allows you to
use less memory and/or to sort faster. So I think the "a > b" idea is cute and
useful where you need to save memory, but in many easy situations a key
function as parameter is better (simpler to use).
(I have developed a large library of D function/classes, mostly for
functional-style programming (but useful for other things too), it has both a
much faster sort than the built-in one, and a sort/sorted functions (for arrays
and AAs, iterable classes, etc) that accept an optional key() function too.
Maybe such stuff may be useful for Phobos/Tango).

Bye,
bearophile