Print all pairs with given sum in C++

In this problem, we are given an array of integers and an integer sum and we have to print all pairs of integers whose sum is equal to the sum value.

Let’s take an example to understand the problem :

Input − array = {1, 6, -2, 3} sum = 4

Output − (1, 3) , (6, -2)

Here, we need pairs with the given sum value.

A simple solution to the problem will be checking pairs of elements that generate the sum. This can be done by traversing array and find the number in array that sums up to sum value.

Example

This program will illustrate the solution −

 Live Demo

#include <iostream>
using namespace std;
int printPairsWithSum(int arr[], int n, int sum){
   int count = 0;
   for (int i = 0; i < n; i++)
      for (int j = i + 1; j < n; j++)
         if (arr[i] + arr[j] == sum)
            cout<<"[ "<<arr[i]<<", "<<arr[j]<<" ]\n";
}
int main(){
   int arr[] = {1, 6, -2, 3};
   int n = 4;
   int sum = 4;
   cout<<"Pairs with Sum "<<sum<<" are :\n";
   printPairsWithSum(arr, n, sum);
   return 0;
}

Output

Pairs with Sum 4 are :
[ 1, 3 ]
[ 6, -2 ]

This method is easy to understand but not quite efficient. Another way will be using hashing.

We will initialise a hash table and traverse the array and find pairs in it. On match, we will print the array :

Example

The following program will make you understand the algorithm better −

 Live Demo

#include <bits/stdc++.h>
using namespace std;
void printPairsWithSum(int arr[], int n, int sum){
   unordered_map<int, int> pair;
   for (int i = 0; i < n; i++) {
      int rem = sum - arr[i];
      if (pair.find(rem) != pair.end()) {
         int count = pair[rem];
         for (int j = 0; j < count; j++)
         cout<<"["<<rem<<", "<<arr[i]<<" ]\n";
      }
      pair[arr[i]]++;
   }
}
int main(){
   int arr[] = {1, 6, -2, 3};
   int n = 4;
   int sum = 4;
   cout<<"The pair with sum is \n";
   printPairsWithSum(arr, n, sum);
   return 0;
}

Output

Pairs with Sum 4 are :
[ 1, 3 ]
[ 6, -2 ]