How to find the intersection of two arrays in C++

Overview

The union of two sets, A and B, is the set of elements in A,B, or in both A and B.

Set has a property in which all the elements in the set are unique.

Example

A = {3,5,7 8,4,9} 
B = {1,2,3,5,9,7,8,6}

A ∩ B = { 3, 5, 9, 7, 8 }

Code

#include <iostream>
using namespace std;
#define sizeOfA 6
#define sizeOfB 8
int findCommonElement(int setA[], int sizeOfSetA, int setB[], int sizeOfSetB)
{
  int commonElemnt =0;
  for(int i=0; i<sizeOfSetA; i++)
  {
      for(int j=0; j<sizeOfSetB; j++)
      {
         if(setA[i]==setB[j])
         {
           commonElemnt++;
         }
      }
  }
}
int *findIntersection(int setA[], int sizeOfSetA, int setB[], int sizeOfSetB, int commonElement)
{
  int *IntersectionArray;
  //  find size of new Array 
  int sizeOfIntersectionArray= commonElement; 
  IntersectionArray= new int[sizeOfIntersectionArray];
  int k=0; // to insert elemnt in the union array we use this iterator
  for(int i=0; i<sizeOfSetB; i++)
  {
    for(int j=0; j<sizeOfSetA; j++)
    { 
        if (setA[j]==setB[i])
        {
          IntersectionArray[k]=setA[j];
          k++;
        }
    }
  }
  return IntersectionArray;
}
int main() 
{
  int * IntersectionArray;
  // declare two array A and B 
  int A[sizeOfA] = {3,5,7,8,4,9};
  int B[sizeOfB] = {1,2,3,5,9,7,8,6};
  int commonElement=findCommonElement(A,sizeOfA, B,sizeOfB);// get number of Common Element of both arrays
  int sizeOfNewArray=commonElement-1;// calculate size ofnew Array
  IntersectionArray=findIntersection(A,sizeOfA,B,sizeOfB, commonElement);
  //Print the array element
  cout<<"{ ";
  for(int i =0; i<sizeOfNewArray; i++)
  {
    if(i==sizeOfNewArray-1)
    {
      cout<<IntersectionArray[i]<<" }";
    }
    else
    {
      cout<<IntersectionArray[i]<<", ";
      
    }
  }
  return 0;
}

Explanation

Line 3 and 4: We define the size of setA and setB.
Line 5 to 18: We define the function findCommonElement that can find the count of the common elements between two sets.
Line 19 to 39: We define the function findIntersection that can find the intersection of two set setA and setB.
Line 44 and 45: We define two arrays, setA and setB.
Line 48: We call the function findIntersection and save the array return by function in the IntersectionArray pointer variable.
Line 50 to 62: We print the intersection of setA and setB.

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