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Sum of main diagonal elements in a Matrix which are prime
  • Last Updated : 07 Dec, 2020

Given a matrix mat[][] of R rows and C columns. The task is to find the sum of all elements from the main diagonal which are prime numbers
Note: The main diagonals are the ones that occur from Top Left of Matrix Down To Bottom Right Corner.

Examples: 

Input: R = 3, C = 3, mat[][] = {{1, 2, 3}, {0, 1, 2}, {0, 4, 2}} 
Output:
Explanation: 
Elements from main diagonal are { 1, 1, 2}, out of these only ‘2’ is a prime number. 
Therefore, the sum of diagonal elements which are prime = 2.

Input: R = 4, C = 4, mat[][] = { {1, 2, 3, 4}, { 0, 7, 21, 12}, { 1, 2, 3, 6}, { 3, 5, 2, 31}} 
Output: 41 
Explanation: 
Elements from main diagonal are { 1, 7, 3, 31}, out of these {7, 3, 31} are prime numbers. 
Therefore, the sum of diagonal elements which are prime = 7 + 3 + 31 = 41. 

Naive Approach: 



  1. Traverse the given matrix and check if the current element belongs to main diagonal or not.
  2. If element belongs to the main diagonal and it is a prime number then add value of the element to totalSum.
  3. After, traversal of the matrix print the value of totalSum.

Below is the implementation of the above approach:

C++

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// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function checks whether a number
// is prime or not
bool isPrime(int n)
{
    if (n < 2) {
        return false;
    }
 
    // Iterate to check primarility of n
    for (int i = 2; i < n; i++) {
        if (n % i == 0)
            return false;
    }
 
    return true;
}
 
// Function calculates the sum of
// prime elements of main diagonal
void primeDiagonalElementSum(
    int* mat,
    int r, int c)
{
 
    // Initialise total sum as 0
    int totalSum = 0;
 
    // Iterate the given matrix mat[][]
    for (int i = 0; i < r; i++) {
 
        for (int j = 0; j < c; j++) {
 
            int temp = *((mat + i * c) + j);
 
            // If element belongs to main
            // diagonal and is prime
            if ((i == j) && isPrime(temp))
                totalSum += (temp);
        }
    }
 
    // Print the total sum
    cout << totalSum << endl;
}
 
// Driver Code
int main()
{
    int R = 4, C = 5;
 
    // Given Matrix
    int mat[4][5] = { { 1, 2, 3, 4, 2 },
                      { 0, 3, 2, 3, 9 },
                      { 0, 4, 1, 2, 8 },
                      { 1, 2, 3, 6, 6 } };
 
    // Function Call
    primeDiagonalElementSum((int*)mat, R, C);
    return 0;
}

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Java

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// Java program for the above approach
class GFG{
 
// Function checks whether a number
// is prime or not
static boolean isPrime(int n)
{
    if (n < 2)
    {
        return false;
    }
 
    // Iterate to check primarility of n
    for(int i = 2; i < n; i++)
    {
       if (n % i == 0)
           return false;
    }
 
    return true;
}
 
// Function calculates the sum of
// prime elements of main diagonal
static void primeDiagonalElementSum(int [][]mat,
                                    int r, int c)
{
     
    // Initialise total sum as 0
    int totalSum = 0;
 
    // Iterate the given matrix mat[][]
    for(int i = 0; i < r; i++)
    {
       for(int j = 0; j < c; j++)
       {
          int temp = mat[i][j];
           
          // If element belongs to main
          // diagonal and is prime
          if ((i == j) && isPrime(temp))
              totalSum += (temp);
       }
    }
 
    // Print the total sum
    System.out.print(totalSum + "\n");
}
 
// Driver Code
public static void main(String[] args)
{
    int R = 4, C = 5;
 
    // Given Matrix
    int mat[][] = { { 1, 2, 3, 4, 2 },
                    { 0, 3, 2, 3, 9 },
                    { 0, 4, 1, 2, 8 },
                    { 1, 2, 3, 6, 6 } };
 
    // Function Call
    primeDiagonalElementSum(mat, R, C);
}
}
 
// This code is contributed by gauravrajput1

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Python3

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# Python3 program for the above approach
 
# Function checks whether a number
# is prime or not
def isPrime(n):
 
    if (n < 2):
        return False
  
    # Iterate to check primarility of n
    for i in range(2, n):
        if (n % i == 0):
            return False
  
    return True
 
# Function calculates the sum of
# prime elements of main diagonal
def primeDiagonalElementSum(mat, r, c):
  
    # Initialise total sum as 0
    totalSum = 0;
  
    # Iterate the given matrix mat[][]
    for i in range(r):
        for j in range(c):
            temp = mat[i][j]
  
            # If element belongs to main
            # diagonal and is prime
            if ((i == j) and isPrime(temp)):
                totalSum += (temp)
  
    # Print the total sum
    print(totalSum)
 
# Driver code
if __name__=="__main__":
     
    R = 4
    C = 5
  
    # Given Matrix
    mat = [ [ 1, 2, 3, 4, 2 ],
            [ 0, 3, 2, 3, 9 ],
            [ 0, 4, 1, 2, 8 ],
            [ 1, 2, 3, 6, 6 ] ]
  
    # Function call
    primeDiagonalElementSum(mat, R, C)
     
# This code is contributed by rutvik_56

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C#

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// C# program for the above approach
using System;
 
class GFG{
 
// Function checks whether a number
// is prime or not
public static bool isPrime(int n)
{
    if (n < 2)
    {
        return false;
    }
 
    // Iterate to check primarility of n
    for(int i = 2; i < n; i++)
    {
       if (n % i == 0)
           return false;
    }
    return true;
}
 
// Function calculates the sum of
// prime elements of main diagonal
public static void primeDiagonalElementSum(int [,]mat,
                                           int r, int c)
{
     
    // Initialise total sum as 0
    int totalSum = 0;
 
    // Iterate the given matrix mat[][]
    for(int i = 0; i < r; i++)
    {
       for(int j = 0; j < c; j++)
       {
          int temp = mat[i, j];
           
          // If element belongs to main
          // diagonal and is prime
          if ((i == j) && isPrime(temp))
              totalSum += (temp);
       }
    }
 
    // Print the total sum
    Console.WriteLine(totalSum);
}
 
// Driver Code
public static void Main(String[] args)
{
    int R = 4, C = 5;
 
    // Given Matrix
    int [,]mat = { { 1, 2, 3, 4, 2 },
                   { 0, 3, 2, 3, 9 },
                   { 0, 4, 1, 2, 8 },
                   { 1, 2, 3, 6, 6 } };
 
    // Function Call
    primeDiagonalElementSum(mat, R, C);
}
}
 
// This code is contributed by SoumikMondal

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Output: 

3

 

Time Complexity: O(R*C*K), where K is the maximum element in the matrix. 
Auxiliary Space: O(1)

Efficient Approach: We can optimize the naive approach by optimizing the primarility test of the number. Below are the steps for optimizing the primarility test:

  1. Instead of checking till N, we can check till sqrt(N) as the larger factor of N must be a multiple of smaller factor that has been already checked.
  2. The algorithm can be improved further by observing that all primes are of the form 6k ± 1, with the exception of 2 and 3. This is because all integers can be expressed as (6k + i) for some integer k and for i = -1, 0, 1, 2, 3, or 4.
  3. As 2 divides (6k + 0), (6k + 2), (6k + 4); and 3 divides (6k + 3). So a more efficient method is to test if N is divisible by 2 or 3, then to check through all the numbers of form 6k ± 1.

Below is the implementation of the above approach:

C++

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// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function checks whether a number
// is prime or not
bool isPrime(int n)
{
    // Corner cases
    if (n <= 1)
        return false;
    if (n <= 3)
        return true;
 
    // This is checked so that we can skip
    // middle five numbers in below loop
    if (n % 2 == 0 || n % 3 == 0)
        return false;
 
    for (int i = 5; i * i <= n; i = i + 6)
        if (n % i == 0 || n % (i + 2) == 0)
            return false;
 
    return true;
}
 
// Function calculates the sum of
// prime elements of main diagonal
void primeDiagonalElementSum(
    int* mat,
    int r, int c)
{
 
    // Initialise total sum as 0
    int totalSum = 0;
 
    // Iterate the given matrix mat[][]
    for (int i = 0; i < r; i++) {
 
        for (int j = 0; j < c; j++) {
 
            int temp = *((mat + i * c) + j);
 
            // If element belongs to main
            // diagonal and is prime
            if ((i == j) && isPrime(temp))
                totalSum += (temp);
        }
    }
 
    // Print the total sum
    cout << totalSum << endl;
}
 
// Driver Code
int main()
{
    int R = 4, C = 5;
 
    // Given Matrix
    int mat[4][5] = { { 1, 2, 3, 4, 2 },
                      { 0, 3, 2, 3, 9 },
                      { 0, 4, 1, 2, 8 },
                      { 1, 2, 3, 6, 6 } };
 
    // Function Call
    primeDiagonalElementSum((int*)mat, R, C);
    return 0;
}

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Java

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// Java program for the above approach
import java.util.*;
 
class GFG{
 
// Function checks whether a number
// is prime or not
static boolean isPrime(int n)
{
     
    // Corner cases
    if (n <= 1)
        return false;
    if (n <= 3)
        return true;
 
    // This is checked so that we can skip
    // middle five numbers in below loop
    if (n % 2 == 0 || n % 3 == 0)
        return false;
 
    for(int i = 5; i * i <= n; i = i + 6)
        if (n % i == 0 || n % (i + 2) == 0)
            return false;
 
    return true;
}
 
// Function calculates the sum of
// prime elements of main diagonal
static void primeDiagonalElementSum(int[][] mat,
                                    int r, int c)
{
 
    // Initialise total sum as 0
    int totalSum = 0;
 
    // Iterate the given matrix mat[][]
    for(int i = 0; i < r; i++)
    {
        for(int j = 0; j < c; j++)
        {
            int temp = mat[i][j];
 
            // If element belongs to main
            // diagonal and is prime
            if ((i == j) && isPrime(temp))
                totalSum += (temp);
        }
    }
 
    // Print the total sum
    System.out.print(totalSum + "\n");
}
 
// Driver Code
public static void main(String[] args)
{
    int R = 4, C = 5;
 
    // Given Matrix
    int mat[][] = { { 1, 2, 3, 4, 2 },
                    { 0, 3, 2, 3, 9 },
                    { 0, 4, 1, 2, 8 },
                    { 1, 2, 3, 6, 6 } };
 
    // Function call
    primeDiagonalElementSum(mat, R, C);
}
}
 
// This code is contributed by Rajput-Ji

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Python3

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# Python3 program for the above approach
 
# Function checks whether a number
# is prime or not
def isPrime(n):
 
    # Corner cases
    if (n <= 1):
        return False
    if (n <= 3):
        return True
 
    # This is checked so that we can skip
    # middle five numbers in below loop
    if (n % 2 == 0 or n % 3 == 0):
        return False
         
    i = 5
    while i * i <= n:
        if (n % i == 0 or n % (i + 2) == 0):
            return False
             
        i += 6
         
    return True
 
# Function calculates the sum of
# prime elements of main diagonal
def primeDiagonalElementSum(mat, r, c):
 
    # Initialise total sum as 0
    totalSum = 0
 
    # Iterate the given matrix mat[][]
    for i in range(r):
        for j in range(c):
            temp = mat[i][j]
             
            # If element belongs to main
            # diagonal and is prime
            if ((i == j) and isPrime(temp)):
                totalSum += (temp)
 
    # Print the total sum
    print(totalSum)
     
# Driver Code
R, C = 4, 5
 
# Given Matrix
mat = [ [ 1, 2, 3, 4, 2 ],
        [ 0, 3, 2, 3, 9 ],
        [ 0, 4, 1, 2, 8 ],
        [ 1, 2, 3, 6, 6 ] ]
 
# Function Call
primeDiagonalElementSum(mat, R, C)
 
# This code is contributed by divyeshrabadiya07

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C#

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// C# program for the above approach
using System;
class GFG{
 
// Function checks whether a number
// is prime or not
static bool isPrime(int n)
{
     
    // Corner cases
    if (n <= 1)
        return false;
    if (n <= 3)
        return true;
 
    // This is checked so that we can skip
    // middle five numbers in below loop
    if (n % 2 == 0 || n % 3 == 0)
        return false;
 
    for(int i = 5; i * i <= n; i = i + 6)
        if (n % i == 0 || n % (i + 2) == 0)
            return false;
 
    return true;
}
 
// Function calculates the sum of
// prime elements of main diagonal
static void primeDiagonalElementSum(int[,] mat,
                                    int r, int c)
{
 
    // Initialise total sum as 0
    int totalSum = 0;
 
    // Iterate the given matrix [,]mat
    for(int i = 0; i < r; i++)
    {
        for(int j = 0; j < c; j++)
        {
            int temp = mat[i,j];
 
            // If element belongs to main
            // diagonal and is prime
            if ((i == j) && isPrime(temp))
                totalSum += (temp);
        }
    }
 
    // Print the total sum
    Console.Write(totalSum + "\n");
}
 
// Driver Code
public static void Main(String[] args)
{
    int R = 4, C = 5;
 
    // Given Matrix
    int [,]mat = { { 1, 2, 3, 4, 2 },
                    { 0, 3, 2, 3, 9 },
                    { 0, 4, 1, 2, 8 },
                    { 1, 2, 3, 6, 6 } };
 
    // Function call
    primeDiagonalElementSum(mat, R, C);
}
}
 
// This code is contributed by Rohit_ranjan

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Output: 

3

 

Time Complexity: O(R*C*sqrt(K)), where K is the maximum element in the matrix. 
Auxiliary Space: O(1)
 

 

 

 

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