python: getting all pairs of values from list

Question

I’d like a tuple of each pair of values from a list, e.g.

[1,2,3,4]

Would yield:

(1,2)
(1,3)
(1,4)
(2,3)
(2,4)
(3,4)

This seems very one-line recipe ish but I can’t quite get it to work.

Asked By: Wells

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Source

Answer 1

That is actually the combinations of 2 elements from your list. Use itertools.combinations this way:

>>> your_list = [1,2,3,4]
>>> from itertools import combinations
>>> list(combinations(your_list,2))
# [(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]

If you need all pairs, use itertools.product

Or, you can use simple list-comprehension:

>>> your_list = [1,2,3,4]
>>> [(your_list[i], your_list[j]) for i in range(len(your_list)) for j in range(i+1,len(your_list))]
# [(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]

Answered By: mshsayem

Answer 2

If you want to avoid accessing list element using indices, a more Pythonic way is the following:

lst = [1,2,3,4]

listOfPairs = []
for e1 in lst:
    for e2 in lst:
        if e1 >= e2: continue #Flip sign for descending order
        listOfPairs.append((e1, e2))

Say you can’t use the < <= > >= comparators on the elements of the list, eg in a list of functions, you can ~~call the .index() function~~ use the indices from the enumerate() function (sorting according to list order):

#Some nonsense example functions
def f1(): return
def f2(): return
def f3(): return
def f4(): return

lst = [f1, f2, f3, f4]

listOfPairs2 = []
for i1, e1 in enumerate(lst):
    for i2, e2 in enumerate(lst):
        if i1 >= i2: continue
        listOfPairs2.append((e1, e2))

Answered By: wojohn

python: getting all pairs of values from list

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