Python: Find all pairs in a list whose sum is equal to a target value

Last update on April 21 2025 13:04:08 (UTC/GMT +8 hours)

22. Find All Pairs in a List with Sum Equal to a Given Value

Write a Python program that finds all pairs of elements in a list whose sum is equal to a given value.

Sample Solution:

Python Code:

# Define a function 'find_pairs' that takes a list of numbers 'nums' and a target value 'target_val'.
def find_pairs(nums, target_val):
    # Create a set 'nums_set' to efficiently check for the presence of numbers in the input list.
    nums_set = set(nums)
    # Create an empty list 'pairs' to store pairs of numbers that sum up to the target value.
    pairs = []
    # Iterate over the unique numbers in 'nums_set'.
    for n in nums_set:
        # Calculate the complement of the current number with respect to the target value.
        complement = target_val - n
        # Check if the complement is in the 'nums_set'.
        if complement in nums_set:
            # If the complement exists in the set, add a pair consisting of the current number and its complement to the 'pairs' list.
            pairs.append({n, complement})
    # Return the 'pairs' list containing pairs of numbers that meet the criteria.
    return pairs
nums = [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
print("Original list of numbers:")
print(nums)
target_val = 35
print("Target value:",target_val)
print("Find all pairs in the said list whose sum is equal to a target value:")
print(find_pairs(nums, target_val))
nums = [1, 2, 3, 4, 5]
print("\nOriginal list of numbers:")
print(nums)
target_val = 5
print("Target value:",target_val)
print("Find all pairs in the said list whose sum is equal to a target value:")
print(find_pairs(nums, target_val))

Sample Output:

Original list of numbers:
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
Target value:
35
Find all pairs in the said list whose sum is equal to a target value:
[{20, 15}, {16, 19}, {17, 18}, {17, 18}, {16, 19}, {20, 15}]

Original list of numbers:
[1, 2, 3, 4, 5]
Target value:
5
Find all pairs in the said list whose sum is equal to a target value:
[{1, 4}, {2, 3}, {2, 3}, {1, 4}]

Flowchart:

For more Practice: Solve these Related Problems:

Write a Python program to find and print all unique pairs from a list that add up to a specified target sum.
Write a Python program to use a two-pointer technique on a sorted list to identify pairs whose sum equals a given value.
Write a Python program to implement a function that returns a list of tuple pairs that sum to the target value, ensuring no duplicates.
Write a Python program to use dictionary mapping to find pairs that add up to a target sum in a single pass through the list.

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