C++ find missing in the second array

Given two arrays A and B of sizes n and m. Find the elements from array A that are not present in array B.

Example

Input :

a[] = {1, 2, 3, 5, 4};

b[] = {1, 2, 3};

Output: 5 4 

Explanation:

5 and 4 are present in the first array, but not in the second array.

Input :

a[] = {4, 3, 5, 9, 11};

b[] = {4, 9, 3, 11, 10};

Output: 5

Explanation:

5 is present in the first array, but not in the second array.

1. Method 1

A simple approach is to use two loops and check whether the A[i] is present in array B.

CPP code

#include<bits/stdc++.h>

using namespace std;




void findMissingelement(int a[], int b[],

                 int n, int m) // find missing element in the array B

{

    for (int i = 0; i < n; i++) // outer loop

    {

        int j;

        for (j = 0; j < m; j++) // inner loop

            if (a[i] == b[j]) // if element is found search element in a[i]

                break;




        if (j == m) // if we reach end of the array means element is not found

            cout << a[i] << " "; // print the element

    }

}







int main()

{

    int a[] = { 1, 2, 6, 3, 4, 5 }; // array A

    int b[] = { 2, 4, 3, 1, 0 }; // array B

    int n = sizeof(a) / sizeof(a[0]); // find size of array A

    int m = sizeof(b) / sizeof(b[1]); // find size of array B

    findMissingelement(a, b, n, m);

    return 0;

}

Output

6 5

C code

#include <stdio.h>

void findMissingelement(int a[], int b[],

                 int n, int m) // find missing element in the array B

{

    for (int i = 0; i < n; i++) // outer loop

    {

        int j;

        for (j = 0; j < m; j++) // inner loop

            if (a[i] == b[j]) // if element is found search element in a[i]

                break;




        if (j == m) // if we reach end of the array means element is not found

            printf("%d ",a[i]);  // print the element

    }

}

int main()

{

    int a[] = { 1, 2, 6, 3, 4, 5 }; // array A

    int b[] = { 2, 4, 3, 1, 0 }; // array B

    int n = sizeof(a) / sizeof(a[0]); // find size of array A

    int m = sizeof(b) / sizeof(b[1]); // find size of array B

    findMissingelement(a, b, n, m);

    return 0;

}

Output

6 5

2. Method 2

The second approach uses the idea of unordered_set.

In this, insert all the elements of array B in the set and iterate in the first array. If the pointer reaches the end in the set, the element is not present.

Unorderd_set

It is a container that stores values in the form of keys.

Internally, a set implements a red black tree and hence the search, insert and delete is O(1).

Syntax

unordered_set<data_type> variable_name;

Code of unordered_set implementation

#include <iostream>

#include <unordered_map>

using namespace std;




int main()

{

         

          unordered_set<int> s;




            s.insert(1);

 s.insert(2);

s.insert(3);




          for (auto x : s) // print elements of set

          cout << x  << endl;




}

Output

1

2

3

Method 2 code

#include<bits/stdc++.h>

using namespace std;







void findMissingelement(int a[], int b[],

                int n, int m) // function to find the missing element

{

  

    unordered_set <int> s; // create a set

    for (int i = 0; i < m; i++)

        s.insert(b[i]); // insert array B elements




   // Iterate in array A and search a[i] in set s if set reaches the end the element is not present

    for (int i = 0; i < n; i++)

        if (s.find(a[i]) == s.end())

            cout << a[i] << " "; // print the absent element

}




// Driver code

int main()

{

    int a[] = { 1, 2, 6, 3, 4, 5 };

    int b[] = { 2, 4, 3, 1, 0 };

    int n = sizeof(a) / sizeof(a[0]); // find size of the array a

    int m = sizeof(b) / sizeof(b[1]);// find size of the array b

    findMissingelement(a, b, n, m);

    return 0;

}

Output

6 5

Time complexity - O(n+m)

Space complexity - O(n)