# Program to find number of pairs (i, j) such that ith and jth elements are same in Python

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Suppose we have an array nums. We have to find number of pairs (i,j) are there such that nums[i] = nums[j] but i is not same as j.

So, if the input is like nums = [1,3,1,3,5], then the output will be 4, because the pairs are (0,2), (2,0), (1,3) and (3,1)

To solve this, we will follow these steps −

• d := a new map
• for each c in nums, do
• d[c] := (d[c] + 1) when c is present in d otherwise 1
• res := 0
• for each c is in the list of elements (x for all x in d where d[x] > 1), do
• res := res +(d[c] *(d[c]-1))
• return res

## Example

Let us see the following implementation to get better understanding −

def solve(nums):
d = {}
for c in nums:
d[c] = d[c] + 1 if c in d.keys() else 1

res = 0
for c in (x for x in d if d[x] > 1):
res += (d[c] * (d[c]-1))

return res

nums = [1,3,1,3,5]
print(solve(nums))

## Input

[1,3,1,3,5]


## Output

4
Updated on 25-Oct-2021 06:37:43