On Dec 23, 7:04 am, Steven D'Aprano wrote:
> def combinations(seq, n):
> if n == 0:
> yield []
> else:
> for i in xrange(len(seq)):
> for cc in combinations(seq[i+1:], n-1):
> yield [seq[i]]+cc
>
> >>> for c in combinations(range(4), 3):
>
> ... print c
> ...
> [0, 1, 2]
> [0, 1, 3]
> [0, 2, 3]
> [1, 2, 3]
Or if you only need to iterate over each combination once instead of
keeping around more than one of them, below is a more efficient
version that reuses the same list for all combinations. Also it
doesn't require the input sequence to implement slicing so it can be
used e.g. with xrange:
def itercombos(seq, n):
def ifill(combo, i, start, end=len(seq), seq=seq, n=n):
if i == n:
yield combo
else:
i1 = i+1
for j in xrange(start,end):
combo[i] = seq[j]
for combo in ifill(combo, i1, j+1):
yield combo
for combo in ifill(n*[None],0,0):
yield combo
>>> for c in itercombos(xrange(4), 3): print c
[0, 1, 2]
[0, 1, 3]
[0, 2, 3]
[1, 2, 3]
>>> for c in combinations(xrange(4), 3): print c
Traceback (most recent call last):
File "combs.py", line 54, in <module>
for c in combinations(xrange(4), 3): print c
File "combs.py", line 12, in combinations
for cc in combinations(seq[i+1:], n-1):
TypeError: sequence index must be integer, not 'slice'
George
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