Count pairs with Bitwise XOR as EVEN number

Given an array of N integers, the task is to find the number of pairs (i, j) such that A[i] ^ A[j] is even.
Examples:

Input: A[] =  { 5, 4, 7, 2, 1}
Output: 4
Since pair of A[] =
( 5, 4 ) = 1( 5, 7 ) = 2( 5, 2 ) = 7( 5, 1 ) = 4
( 4, 7 ) = 3( 4, 2 ) = 6( 4, 1 ) = 5
( 7, 2 ) = 5( 7, 1 ) = 6
( 2, 1 ) = 3
Total XOR even pair  = 4

Input: A[] = { 7, 2, 8, 1, 0, 5, 11 }
Output: 9
Since pair of A[] =
( 7, 2 ) = 5( 7, 8 ) = 15( 7, 1 ) = 6( 7, 0 ) = 7( 7, 5 ) = 2( 7, 11 ) = 12
( 2, 8 ) = 10( 2, 1 ) = 3( 2, 0 ) = 2( 2, 5 ) = 7( 2, 11 ) = 9
( 8, 1 ) = 9( 8, 0 ) = 8( 8, 5 ) = 13( 8, 11 ) = 3
( 1, 0 ) = 1( 1, 5 ) = 4( 1, 11 ) = 10
( 0, 5 ) = 5( 0, 11 ) = 11
( 5, 11 ) = 14

A naive approach is to check for every pair and print the count of pairs which are even.

Below is the implementation of the above approach:

C++



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// C++ program to count pairs
// with XOR giving a even number
#include <iostream>
using namespace std;
  
// Function to count number of even pairs
int findevenPair(int A[], int N)
{
    int i, j;
  
    // variable for counting even pairs
    int evenPair = 0;
  
    // find all pairs
    for (i = 0; i < N; i++) {
        for (j = i + 1; j < N; j++) {
  
            // find XOR operation
            // check even or even
            if ((A[i] ^ A[j]) % 2 == 0)
                evenPair++;
        }
    }
  
    // return number of even pair
    return evenPair;
}
  
// Driver Code
int main()
{
  
    int A[] = { 5, 4, 7, 2, 1 };
    int N = sizeof(A) / sizeof(A[0]);
  
    // calling function findevenPair
    // and print number of even pair
    cout << findevenPair(A, N) << endl;
  
    return 0;
}

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Java

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// Java program to count pairs
// with XOR giving a even number
import java.io.*;
  
class GFG
{
  
// Function to count number of even pairs
static int findevenPair(int []A, int N)
{
    int i, j;
  
    // variable for counting even pairs
    int evenPair = 0;
  
    // find all pairs
    for (i = 0; i < N; i++) 
    {
        for (j = i + 1; j < N; j++)
        {
  
            // find XOR operation
            // check even or even
            if ((A[i] ^ A[j]) % 2 == 0)
                evenPair++;
        }
    }
  
    // return number of even pair
    return evenPair;
}
  
// Driver Code
public static void main (String[] args) 
{
    int A[] = { 5, 4, 7, 2, 1 };
    int N = A.length;
      
    // calling function findevenPair
    // and print number of even pair
    System.out.println(findevenPair(A, N));
}
}
  
// This code is contributed by inder_verma..

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Python3

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# Python3 program to count pairs
# with XOR giving a even number
  
   
# Function to count number of even pairs
def findevenPair(A, N):
  
    # variable for counting even pairs
    evenPair = 0
   
    # find all pairs
    for i in range(0, N):
        for j in range(i+1, N):
              
            # find XOR operation
            # check even or even
            if ((A[i] ^ A[j]) % 2 == 0):
                evenPair+=1
  
    # return number of even pair
    return evenPair;
   
# Driver Code
def main():
    A = [ 5, 4, 7, 2, 1 ]
    N = len(A)
   
    # calling function findevenPair
    # and prnumber of even pair
    print(findevenPair(A, N))
   
if __name__ == '__main__':
    main()
# This code is contributed by PrinciRaj1992 

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C#

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// C# program to count pairs
// with XOR giving a even number
using System;
  
class GFG
{
  
// Function to count number of
// even pairs
static int findevenPair(int []A, int N)
{
    int i, j;
  
    // variable for counting even pairs
    int evenPair = 0;
  
    // find all pairs
    for (i = 0; i < N; i++) 
    {
        for (j = i + 1; j < N; j++)
        {
  
            // find XOR operation
            // check even or even
            if ((A[i] ^ A[j]) % 2 == 0)
                evenPair++;
        }
    }
  
    // return number of even pair
    return evenPair;
}
  
// Driver Code
public static void Main () 
{
    int []A = { 5, 4, 7, 2, 1 };
    int N = A.Length;
      
    // calling function findevenPair
    // and print number of even pair
    Console.WriteLine(findevenPair(A, N));
}
}
  
// This code is contributed
// by inder_verma..

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PHP

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<?php
// PHP program to count pairs
// with XOR giving a even number
  
// Function to count number 
// of even pairs
function findevenPair(&$A, $N)
{
  
    // variable for counting even pairs
    $evenPair = 0;
  
    // find all pairs
    for ($i = 0; $i < $N; $i++)
    {
        for ($j = $i + 1; $j < $N; $j++) 
        {
  
            // find XOR operation
            // check even or even
            if (($A[$i] ^ $A[$j]) % 2 == 0)
                $evenPair++;
        }
    }
  
    // return number of even pair
    return $evenPair;
}
  
// Driver Code
$A = array(5, 4, 7, 2, 1 );
$N = sizeof($A);
  
// calling function findevenPair
// and print number of even pair
echo (findevenPair($A, $N)); 
  
// This code is contributed
// by Shivi_Aggarwal 
?>

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Output:

4

An efficient solution is to Count pairs with Bitwise XOR as ODD number i.e. oddEvenpairs. Then return totalPairs – oddEvenPairs where totalPairs = (N * (N-1) / 2) and oddEvenPairs = count * (N – count). As, pairs that will give Even Bitwise XOR are :

Even, Even
Odd, Odd

So, find the count of pairs with both odd and even elements and subtract from total no. of pairs.

Below is the implementation of the above approach:

C++

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// C++ program to count pairs
// with XOR giving a even number
#include <iostream>
using namespace std;
  
// Function to count number of even pairs
int findEvenPair(int A[], int N)
{
    int count = 0;
  
    // find all pairs
    for (int i = 0; i < N; i++) {
        if (A[i] % 2 != 0)
            count++;
    }
  
    int totalPairs = (N * (N - 1) / 2);
    int oddEvenPairs = count * (N - count);
  
    // return number of even pair
    return totalPairs - oddEvenPairs;
}
  
// Driver Code
int main()
{
    int a[] = { 5, 4, 7, 2, 1 };
    int n = sizeof(a) / sizeof(a[0]);
  
    // calling function findEvenPair
    // and print number of even pair
    cout << findEvenPair(a, n) << endl;
  
    return 0;
}

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Java

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// Java  program to count pairs 
// with XOR giving a even number
  
import java.io.*;
  
class GFG {
    // Function to count number of even pairs 
static int findEvenPair(int A[], int N) 
    int count = 0
  
    // find all pairs 
    for (int i = 0; i < N; i++) { 
        if (A[i] % 2 != 0
            count++; 
    
  
    int totalPairs = (N * (N - 1) / 2); 
    int oddEvenPairs = count * (N - count); 
  
    // return number of even pair 
    return totalPairs - oddEvenPairs; 
  
// Driver Code 
      
    public static void main (String[] args) {
      
    int a[] = { 5, 4, 7, 2, 1 }; 
    int n = a.length; 
    // calling function findEvenPair 
    // and print number of even pair 
    System.out.println(findEvenPair(a, n)); 
    }
//This code is contributed by akt_mit    
}

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Python3

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# python program to count pairs
# with XOR giving a even number
  
# Function to count number of even pairs
def findEvenPair(A, N):
    count = 0
   
    # find all pairs
    for i in range(0,N):
        if (A[i] % 2 != 0):
            count+=1
   
    totalPairs = (N * (N - 1) / 2)
    oddEvenPairs = count * (N - count)
   
    # return number of even pair
    return (int)(totalPairs - oddEvenPairs)
  
# Driver Code
def main():
    a = [ 5, 4, 7, 2, 1 ]
    n = len(a)
   
    # calling function findEvenPair
    # and pr number of even pair
    print(findEvenPair(a, n))
   
if __name__ == '__main__':
    main()
      
# This code is contributed by 29AjayKumar

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C#

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// C# program to count pairs 
// with XOR giving a even number
   
using System;
   
public class GFG {
    // Function to count number of even pairs 
    static int findEvenPair(int []A, int N) 
    
        int count = 0; 
  
        // find all pairs 
        for (int i = 0; i < N; i++) { 
            if (A[i] % 2 != 0) 
                count++; 
        
  
        int totalPairs = (N * (N - 1) / 2); 
        int oddEvenPairs = count * (N - count); 
  
        // return number of even pair 
        return totalPairs - oddEvenPairs; 
    
  
    // Driver Code 
       
    public static void Main() {
       
    int []a = { 5, 4, 7, 2, 1 }; 
    int n = a.Length; 
    // calling function findEvenPair 
    // and print number of even pair 
    Console.Write(findEvenPair(a, n)); 
    }
}
  
// This code is contributed by 29AjayKumar

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PHP

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<?php
// PHP program to count pairs
// with XOR giving a even number
  
// Function to count number of even pairs
function findEvenPair($A, $N)
{
    $count = 0;
  
    // find all pairs
    for ($i = 0; $i < $N; $i++)
    {
        if ($A[$i] % 2 != 0)
            $count++;
    }
  
    $totalPairs = ($N * ($N - 1) / 2);
    $oddEvenPairs = $count * ($N - $count);
  
    // return number of even pair
    return $totalPairs - $oddEvenPairs;
}
  
// Driver Code
$a = array(5, 4, 7, 2, 1);
$n = sizeof($a);
  
// calling function findEvenPair
// and print number of even pair
echo findEvenPair($a, $n) . "\n";
  
// This code is contributed 
// by Akanksha Rai
?>

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Output:

4

Time Complexity : O(n)



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