Count Index Pairs for Same Array Elements in Python

Suppose we have a list of numbers called nums. We have to find the number of pairs i < j such that nums[i] and nums[j] are same.

So, if the input is like nums = [5, 4, 5, 4, 4], then the output will be 4, as We have index pairs like (0, 2), (1, 3), (1, 4) and (3, 4).

To solve this, we will follow these steps −

• c := a list containing frequencies of each elements present in nums

• count := 0

• for each n in list of all values of c, do

  • count := count + floor of (n *(n - 1)) / 2

• return count

Example

Let us see the following implementation to get better understanding

from collections import Counter
def solve(nums):
   c = Counter(nums)
   count = 0
   for n in c.values():
      count += n * (n - 1) // 2
   return count

nums = [5, 4, 5, 4, 4]
print(solve(nums))

Input

[5, 4, 5, 4, 4]

Output

4