Python Memory Error when generating pairs using itertools.combinations

Question

I have an int list with unspecified number. I would like to find the difference between two integers in the list that match a certain value.

from itertools import combinations

#Example of a list
intList = [3, 6, 2, 7, 1]
diffList = [abs(a -b) for a, b in combinations(intList, 2)]

#Given if difference = 2    
print diffList.count(2)

The code snippet worked but when a larger list is given, I am getting MemoryError. Can anyone tell me if there's something wrong with the codes or the error is due to my hardware limitation?

Answer 1

Exactly how large is "a larger list"? If len(intList) is n, len(diffList) will be n*(n-1)//2 (the number of combinations of n things taken 2 at a time). This will consume all your memory if n is large enough.

If you only care about the value 2,

print sum(abs(a-b) == 2 for a, b in combinations(intList, 2))

is one way to do it that consumes very little memory regardless of how large intList is. However, it will still take time proportional to the square of len(intList).