Subtract all items in a list against each other

Question

I have a list in Python that looks like this:

myList = [(1,1),(2,2),(3,3),(4,5)]

And I want to subtract each item with the others, like this:

(1,1) - (2,2)
(1,1) - (3,3)
(1,1) - (4,5)
(2,2) - (3,3)
(2,2) - (4,5)
(3,3) - (4,5)

The expected result would be a list with the answers:

[(1,1), (2,2), (3,4), (1,1), (2,3), (1,2)]

How can I do this? If I approach it with a for loop, I can maybe store the previous item and check it against the one that I'm working with at that moment, but it doesn't really work.

Answer 1

Use itertools.combinations with tuple unpacking to generate the pairs of differences:

>>> from itertools import combinations
>>> [(y1-x1, y2-x2) for (x1, x2), (y1, y2) in combinations(myList, 2)]                    
[(1, 1), (2, 2), (3, 4), (1, 1), (2, 3), (1, 2)]

Answer 2

You could use a list comprehension, with np.subtract to 'subtract' the tuples from each other:

import numpy as np

myList = [(1,1),(2,2),(3,3),(4,5)]

answer = [tuple(np.subtract(y, x)) for x in myList for y in myList[myList.index(x)+1:]]
print(answer)

Output

[(1, 1), (2, 2), (3, 4), (1, 1), (2, 3), (1, 2)]

Answer 3

Using operator.sub with combinations.

>>> from itertools import combinations
>>> import operator
>>> myList = [(1, 1),(2, 2),(3, 3),(4, 5)]
>>> [(operator.sub(*x), operator.sub(*y)) for x, y in (zip(ys, xs) for xs, ys in combinations(myList, 2))]
[(1, 1), (2, 2), (3, 4), (1, 1), (2, 3), (1, 2)]
>>>