[Tutor] listing all combinations of elements of a list
alan.gauld at freenet.co.uk
Mon Dec 13 15:07:35 CET 2004
> def combination(items)
> list = 
> for i in range(0,len(items)):
> for j in range(0,len(items)):
for i in items:
for j in items:
Is both shorter and faster - no len() function calls.
Or if you want to use indices:
size = len(items) # only calculate once, it won't change!
lst =  # don't use builtin function names for
for i in range(0,size)
for j in range(i+1,size)
That saves a lot of len() calls and a comparison.
Also it avoids iterating over the whole list each time.
> My problems with this code being that a) code I write is usually
> inefficient, b) it doesn't extend to subsets of size > 2, and c) it
> nested loops, which I have gathered from some previous discussions
> this list to be less than ideal.
I've tidied up a little but I think nested loops are a necessary evil
this case - although somebody is sure to prove me wrong! :-)
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