Find Union of Two Arrays in C++

In this tutorial, we will learn how to find the union of two unsorted arrays in C++. Before that let’s first understand what is array.

Array: is a derived data type that contains the same type of data. Like integer array stores values of only integer type, float array stores values of only float type.

Derived data type: is a data type that is defined by the user itself. Other derived data types are Structure, Class, Union, Enumeration, and Pointers.

The union of two arrays: is the set of all elements that are either in A or in B.

Example: 

Array1: { 1, 2, 3, 4, 5 }

Array2: { 4, 5, 6, 7, 8, 9, 10 }

The union of given two arrays: { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }. Because all these elements are present either in Array1 or in Array2.

(*Note: Array should not contain 1 element twice.)

Program to Find Union of Two Unsorted Arrays in C++

#include<iostream>
using namespace std;

int main()
{
  int n1,n2,i,j,flag;
  cout<<"Enter the no. of elements of the 1st array: ";
  cin>>n1;
  
  /* declaring arr1 of size n1 */
  int arr1[n1];
  cout<<"\nEnter the elements of the 1st array: ";
  for(i=0;i<n1;i++)
  {
    cin>>arr1[i];
  }
  
  cout<<"\nEnter the no. of elements of the 2nd array: ";
  cin>>n2;
  
  /* declaring arr2 of size n2 */
  int arr2[n2];
  cout<<"\nEnter the elements of the 2nd array: ";
  for(i=0;i<n2;i++)
  {
    cin>>arr2[i];
  }
  
  /* printing elements that are either in array1 or in array2 */
  cout<<"\nUnion of the two arrays: ";
  
  /* First print all the elements of array1 */
  for(i=0;i<n1;i++)
  {
    cout<<arr1[i]<<" ";
  }
  
  /* Then print all the elements that are in array2 but not in array1 */
  for(j=0;j<n2;j++)
  {
    flag=0;
    for(i=0;i<n1;i++)
    {
      if(arr1[i]==arr2[j])
      {
        flag=1;
        break;
      }
    }
    /* flag!=1 means element of array2 is not present in array1 */
    if(flag!=1)
    {
      cout<<arr2[j]<<" ";
    }
    
  }
  
  return 0;
}

Input/Output:

Enter the no. of elements of the 1st array: 4
Enter the elements of the 1st array: -3 0 4 7

Enter the no. of elements of the 2nd array: 6
Enter the elements of the 2nd array: 4 1 9 7 2 8

Union of the two arrays: -3 0 4 7 1 9 2 8

Time Complexity

O(n1*n2), where n1 is the no. of elements of the first array and n2 is no. of elements of the second array.

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