How to find all ordered pairs of elements in array of integers whose sum lies in a given range of value

Question

Given an array of integers find the number of all ordered pairs of elements in the array whose sum lies in a given range [a,b]

Here is an O(n^2) solution for the same <

Answer 1

from itertools import ifilter, combinations

def countpairs2(array, a, b):
    pairInRange = lambda x: sum(x) >= a and sum(x) <= b
    filtered = ifilter(pairInRange, combinations(array, 2))
    return sum([2 for x in filtered])

I think the Itertools library comes in quite handy. I also noticed you counted pairs twice, for example you counted (1, 3) and (3, 1) as two different combinations. If you don't want that, just change the 2 in the last line to a 1. Note: The last could be changed to return len(list(filtered)) * 2. This CAN be faster, but at the expense of using more RAM.