Say I have a list, and I want to produce a list of all unique pairs of elements without considering the order. One way to do this is:
mylist = ['W','X','Y','Z']
for i in xrange(len(mylist)):
for j in xrange(i+1,len(mylist)):
print mylist[i],mylist[j]
W X
W Y
W Z
X Y
X Z
Y Z
I want to do this with iterators, I thought of the following, even though it doesn't have brevity:
import copy
it1 = iter(mylist)
for a in it1:
it2 = copy.copy(it1)
for b in it2:
print a,b
But this 开发者_Go百科doesn't even work. What is a more pythonic and efficient way of doing this, with iterators or zip, etc.?
This has already been done and is included in the standard library as of Python 2.6:
import itertools
mylist = ['W', 'X', 'Y', 'Z']
for pair in itertools.combinations(mylist, 2):
print pair # pair is a tuple of 2 elements
Seems pretty Pythonic to me ;-)
Note that even if you're calculating a lot of combinations, the combinations()
function returns an iterator so that you can start printing them right away. See the docs.
Also, you are referring to the result as a Cartesian product between the list and itself, but this is not strictly correct: The Cartesian product would have 16 elements (4x4). Your output is a subset of that, namely only the 2-element combinations (with repetition not allowed) of the values of the list.
@Cameron's answer is correct.
I just wanted to point out that
for i in range(len(mylist)):
do_something_to(mylist[i])
is nastily un-Pythonic; if your operation is read-only (does not have to be stored back to the array), do
for i in mylist:
do_something_to(i)
otherwise
mylist = [do_something_to(i) for i in mylist]
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