Find All Pairs with Given Sum in List in Python - Simple Guide

To find all pairs in a list that add up to a given sum in Python, you can use a loop with a set to track seen numbers and check if the complement (sum minus current number) exists. Use for loops and a set to efficiently find pairs without duplicates.

Syntax

Use a loop to go through each number in the list. For each number, calculate the complement by subtracting it from the target sum. Check if this complement is already in a set of seen numbers. If yes, you found a pair. Add the current number to the set for future checks.

python

def find_pairs_with_sum(numbers, target_sum):
    seen = set()
    pairs = []
    for num in numbers:
        complement = target_sum - num
        if complement in seen:
            pairs.append((complement, num))
        seen.add(num)
    return pairs

Example

This example shows how to find all pairs in a list that add up to 10. It prints each pair found.

python

def find_pairs_with_sum(numbers, target_sum):
    seen = set()
    pairs = []
    for num in numbers:
        complement = target_sum - num
        if complement in seen:
            pairs.append((complement, num))
        seen.add(num)
    return pairs

numbers = [2, 4, 3, 5, 7, 8, 9]
target_sum = 10
pairs = find_pairs_with_sum(numbers, target_sum)
for pair in pairs:
    print(pair)

Output

(3, 7)
(2, 8)

Common Pitfalls

One common mistake is counting the same pair twice, like (3,7) and (7,3). Using a set to track seen numbers helps avoid this. Another mistake is not handling duplicates in the list properly, which can cause repeated pairs.

Also, using nested loops to check every pair works but is slower for large lists.

python

def wrong_find_pairs(numbers, target_sum):
    pairs = []
    for i in range(len(numbers)):
        for j in range(i + 1, len(numbers)):
            if numbers[i] + numbers[j] == target_sum:
                pairs.append((numbers[i], numbers[j]))
    return pairs

# This works but is slower and can be inefficient for big lists

# Correct approach uses a set to track seen numbers and avoid duplicates

Quick Reference

• Use a set to track numbers seen so far.
• Calculate complement as target_sum - current_number.
• Add pairs only when complement is found in the set.
• This method runs in O(n) time, better than nested loops.

Key Takeaways

Use a set to track seen numbers for efficient pair finding.

Calculate complement as target sum minus current number to find pairs.

Avoid nested loops for better performance on large lists.

Watch out for duplicate pairs by controlling order and storage.

Return pairs as tuples for clear and simple output.