Duplicate values forming pairs
Input
count_pairs_with_sum([1, 2, 3, 2], 4)
Output
2
Pairs: 1+3 and 2+2 (the two different 2s form one pair).
Full lesson preview
Use combinations to count pairs
Count unordered pairs with a target sum using itertools.combinations. Practice enumerating unique index pairs.
Python practice10 minModules & Standard LibraryBeginnerLast updated March 23, 2026
Problem statement
Given a list of numbers and a target value, count how many unique unordered pairs (i < j) of elements sum exactly to the target. Use itertools.combinations to iterate over index pairs or element pairs to ensure each unordered pair is considered exactly once. Return the count as an integer.
Task
Implement a function that counts how many unique unordered pairs of elements sum to a target, using itertools.combinations.
Examples
Input format
A list of numbers and an integer/number target passed as count_pairs_with_sum(nums, target).
Output format
An integer count of unique unordered pairs whose sum equals the target.
Constraints
Use itertools.combinations to avoid double-counting. The list length may be up to a few thousand; an O(n^2) approach will be fine for beginner-sized inputs, but be mindful of performance.
Samples
Sample 1
Input
count_pairs_with_sum([2, 2, 2], 4)
Output
3
Three unordered pairs can be formed from three 2s: choose any two of them.