I need to generate all the combinations with length k from a list of length n, and I must do it using recursion.

For Example:

INPUT:  choose_sets([1,2,3,4],3)
OUTPUT: [[1,2,3],[1,2,4],[1,3,4],[2,3,4]]

INPUT:  choose_sets([1,2,3,4],2)
OUTPUT: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]

I'm stuck implementing this in code, so I would be happy for some help. This is my code so far (I'm missing something just don't know what):

def choose_sets(lst,k):

    if k == len(lst):
        return lst
    if k == 0:
        return []
    if k > len(lst):
        return []

    sets=[]
    sub_lst=lst[:]
    sub_lst.remove(sub_lst[0])

    a= choose_sets(sub_lst,k-1)
    for i in a:
        i.append(lst[0])
    sets.append(a)

    b= choose_sets(sub_lst,k)
    sets.append(b)

    return sets

You can get solution from Generator for permutations, combinations, selections of a sequence (Python recipe)

def xuniqueCombinations(items, n):
    if n==0: yield []
    else:
        for i in xrange(len(items)):
            for cc in xuniqueCombinations(items[i+1:],n-1):
                yield [items[i]]+cc

>>> def xuniqueCombinations(items, n):
...     if n==0: yield []
...     else:
...         for i in xrange(len(items)):
...             for cc in xuniqueCombinations(items[i+1:],n-1):
...                 yield [items[i]]+cc
...
>>> for x in xuniqueCombinations( [1,2,3,4],2):
...     print x
[1, 2]
[1, 3]
[1, 4]
[2, 3]
[2, 4]
[3, 4]

Edited 4 year later (7/12/2015)

To run it on Python3 just change xrange to range, Python3's range is Python2's xrange.. Thanks @ederollora to notice me.

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