Python Nested Loop unique pairs

Question

I'm trying to write a nested loop that prints out all possible "unique pairs" of numbers from a certain range. For example, if the range was from 1 to 3 the unique pairs would be:

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

If the range was from 1 to 4 the unique pairs would be:

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

Here's how I did it for 1 to 3:

for i in range(1,4):
    for j in range(2,4):
        if (i != j & j != (i-1)):
            print (i,j)

which prints out (1, 2), (1, 3),(2, 3). But this is a hack because it doesn't work when I change the range to 1,5. It prints out duplicate pairs such as (1,5) and (5,1).

Lukas Graf · Accepted Answer · 2015-03-29 00:00:29Z

10

Use itertools.combinations():

>>> import itertools
>>> print list(itertools.combinations(range(1, 5), r=2))
[(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]

As long as your inputs are unique, there will be no repeated combinations:

itertools.combinations(iterable, r)

Return r length subsequences of elements from the input iterable.

Combinations are emitted in lexicographic sort order. So, if the input iterable is sorted, the combination tuples will be produced in sorted order.

Elements are treated as unique based on their position, not on their value. So if the input elements are unique, there will be no repeat values in each combination.

answered Mar 29, 2015 at 0:00

Lukas Graf

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Comments

Amit Kumar Gupta · Accepted Answer · 2015-03-28 23:57:51Z

10

for i in range(1,4):
    for j in range(i+1,4):  # <-- the key is to make j start at i+1
        print (i,j)

answered Mar 28, 2015 at 23:57

Amit Kumar Gupta

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Comments

Alexandru Godri · Accepted Answer · 2015-03-28 23:57:08Z

6

You are probably looking for something like this:

n = 4
x = [(i,j) for i in range(1,n+1) for j in range(i+1, n+1)]
print x

Cheers, Alex

answered Mar 28, 2015 at 23:57

Alexandru Godri

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4 Comments

user1726343 Over a year ago

Why n+1? Isn't n+1 just 5? Maybe I'm misunderstanding something here?

Alexandru Godri Over a year ago

You need n+1 because range (i, j) goes from i to j - 1. So if we used (1, n) we would have 1 to n - 1. So if n was 4, then we would be missing 4, we would only have 1, 2 and 3.

jkd Over a year ago

@AlexandruGodri I think he is asking why not replace all n+1's with 5's.

Alexandru Godri Over a year ago

Because i wrote it for a general purpose. You just change the "n" to whatever and the code works :)

Shashank · Accepted Answer · 2015-03-29 00:00:39Z

6

You can use itertools.combinations:

>>> from itertools import combinations
>>> print list(combinations(range(1,4), 2))
[(1, 2), (1, 3), (2, 3)]

Python 2 Documentation

Python 3 Documentation

answered Mar 29, 2015 at 0:00

Shashank

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Comments

jinhwanlazy · Accepted Answer · 2015-03-29 00:01:33Z

4

See itertools module.

Maybe what you want is

list(itertools.combinations(range(1,4),2)) == [(1, 2), (1, 3), (2, 3)]

answered Mar 29, 2015 at 0:01

jinhwanlazy

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