Get all unique pairs in a list, including duplicates, in python-CodePudding

How can I obtain all unique pairs in a list, allowing for repeats?

E.g. for the list [20, 20, 19] I'd like (20, 20) and (20, 19).

And for [20, 20, 19, 19, 18] I'd like (20, 20), (20, 19), (20, 18), (19, 19) and (19, 18)

I don't really mind what object is returned (list, tuple, dict, set etc) as long as I can then iterate over each pair, and extract both numbers

CodePudding user response：

You can use itertools.combinations in a set comprehension:

>>> from itertools import combinations
>>> lst_1 = [20, 20, 19]
>>> {comb for comb in combinations(lst_1, r=2)}
{(20, 19), (20, 20)}
>>> lst_2 = [20, 20, 19, 19, 18]
>>> {comb for comb in combinations(lst_2, r=2)}
{(20, 20), (19, 19), (20, 19), (19, 18), (20, 18)}

Or just cast into a set directly:

>>> set(combinations(lst_1, r=2))
{(20, 19), (20, 20)}
>>> set(combinations(lst_2, r=2))
{(20, 20), (19, 19), (20, 19), (19, 18), (20, 18)}

CodePudding user response：

You can use a double for-loop where in each iteration, the previous element is removed. I think this solution would be faster than itertools.combinations method especially if the list is large, since it doesn't have to find all combinations first.

def get_pairs(l):
    out = []
    for i in l:
        l.remove(i)
        for j in l:
            if (i,j) in out or (j,i) in out:
                continue
            else:
                out.append((i,j))
    return out

Output:

>>> get_pairs([20, 20, 19, 19, 18])

[(20, 20), (20, 19), (20, 18), (19, 19), (19, 18)]