Find Pairs in Array with given Sum in C

March 19, 2025

Find Pairs in Array with given Sum in C Language

On this page, we will look into a coding question where we will learn how to Find Pairs in an Array with a given sum in C Programing Language.
There might be different approaches to solve this question, one you will find here. If your approach is a bit different post it in the comment section.

Find Pairs in Array with given Sum in C

Find Pairs in Array with given Sum in C

To find pairs in an array with a given sum in C, we can use two methods

  1. Brute Force (Time complexity: O(n^2) )
  2. Using Sorting (Time complexity: O(n log n) )
Both methods have their own advantages and disadvantages. The brute force method is simple to implement but can be slow for large arrays. The sorting method is faster for large arrays, but requires extra space for sorting and modifying the original array.
Find Pairs in Array with given Sum in C

Method 1 : Brute Force Method

  • In the brute force method, we compare each element in the array with all the other elements in the array to check if their sum is equal to the given sum.
  • This method has a time complexity of O(n^2) as we have to compare each element with every other element

Reverse an array or string

Sorting of Array

Array Rotation

Sort the array in Waveform

Boyer-Moore’s Voting Algorithm

C code for brute force method

Run 
#include <stdio.h>

void findPair (int nums[], int n, int target)
{
  int flag = 0;

  for (int i = 0; i < n; i++)
    {
      for (int j = i + 1; j < n; j++)
	{
	  if (nums[i] + nums[j] == target)
	    {
	      printf ("Pair found (%d, %d) \n", nums[i], nums[j]);
	      flag = 1;
	    }
	}
    }
  if (flag == 0)
    {
      printf ("Pair not found");
    }
}

int main ()
{
  int nums[] = {5, 2, 3, 4, 1, 6, 7};
  int target = 7;

  int n = sizeof (nums) / sizeof (nums[0]);

  findPair (nums, n, target);

  return 0;
}

Output

Pair found (5, 2) 
Pair found (3, 4) 
Pair found (1, 6)

Method 2 : Sorting Method

  • The array is first sorted in ascending order in the sorting technique.
  • Then, we employ two pointers, one of which points to the first element and the other to the last element.
  • These two pointers values are added, and we then check to see if the result equals the specified sum.
    The left pointer is increased if the value is less than the total, and the right pointer is decreased if the value is higher.
  • This method has a time complexity of O(n log n) due to the sorting algorithm used.

C code for Sorting Method

Run

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <math.h>

int cmpfunc (const void *a, const void *b)
{
  return (*(int *) a - *(int *) b);
}

 
// Function to find a pair in an array with a given sum using sorting
void findPair(int nums[], int n, int target)
{
    // sort the array in ascending order
    qsort (nums, n, sizeof (int), cmpfunc);
 
    // maintain two indices pointing to endpoints of the array
    int low = 0;
    int high = n - 1;
    int flag = 0;
    // reduce the search space `nums[low…high]` at each iteration of the loop
 
    // loop till the search space is exhausted
    while (low < high)
    {
        // sum found
        if (nums[low] + nums[high] == target)
        {
            printf("Pair found (%d, %d) \n",nums[low] ,nums[high]);
            flag=1;
            
        }
        
 
        // increment `low` index if the total is less than the desired sum;
        // decrement `high` index if the total is more than the desired sum
        (nums[low] + nums[high] < target)? low++: high--;
    }
    if(flag==0)
    {
        printf("Pair not found");
    }
 
    // we reach here if the pair is not found
    
}
 
int main()
{
    int nums[] = {5, 2, 3, 4, 1, 6, 7};
    int target = 7;
 
    int n = sizeof(nums)/sizeof(nums[0]);
 
    findPair(nums, n, target);
 
    return 0;
}

Output

Pair found (1, 6) 
Pair found (2, 5) 
Pair found (3, 4)