Replace the middle element of matrix with sum of surrounding elements

Namachila November 18, 2025

0 0 6 minutes read

Given a square matrix mat[][] of odd dimensions, the task is to change the value of the middle most element of the matrix to the sum of the elements surrounding it.

Examples:

Input: 
mat[][] = { {2, 1, 7}, 
            {3, 7, 2}, 
            {5, 4, 9} }
Output:
2 1 7
3 10 2
5 4 9

Input: 
mat[][] = {{1, 3, 5, 6, 7},
           {3, 5, 3, 2, 1},
           {1, 2, 3, 4, 5},
           {7, 9, 2, 1, 6},
           {9, 1, 5, 3, 2} }
Output: 
1 3 5 6 7
3 5 3 2 1
1 2 11 4 5
7 9 2 1 6
9 1 5 3 2

Approach: The middle element of the matrix of odd order will be located at the position mat[n / 2][n / 2].

Therefore directly update the element as:

mat[n / 2][n / 2] = mat[n / 2 - 1][n / 2]
                  + mat[n / 2][n / 2 - 1]
                  + mat[n / 2 + 1][n / 2]
                  + mat[n / 2][n / 2 + 1]

Below is the implementation of the above approach:

C++

// C++ program to replace the value of the middle element  
// of the matrix with the sum of surrounding elements 
  
#include <iostream> 
using namespace std; 
const int MAX = 100; 
  
// Function to print the matrix 
void print(int mat[][MAX], int n) 
{ 
    for (int i = 0; i < n; i++) { 
        for (int j = 0; j < n; j++) { 
            cout << mat[i][j] << " "; 
        } 
        cout << endl; 
    } 
} 
  
// Function to change the value of the middle element  
// of the matrix to the sum of surrounding elements 
void changemiddle(int mat[][MAX], int n) 
{ 
    // Change the middle element 
    mat[n / 2][n / 2] 
        = mat[n / 2 - 1][n / 2] 
          + mat[n / 2][n / 2 - 1] 
          + mat[n / 2 + 1][n / 2] 
          + mat[n / 2][n / 2 + 1]; 
  
    // Function call to print the matrix 
    print(mat, n); 
} 
  
// Driver code 
int main() 
{ 
    int mat[][MAX] = { { 2, 1, 7 }, 
                       { 3, 7, 2 }, 
                       { 5, 4, 9 } }; 
  
    changemiddle(mat, 3); 
  
    return 0; 
} 

C

// C program to replace the value of the middle element  
// of the matrix with the sum of surrounding elements 
#include <stdio.h> 
  
#define MAX 100 
  
// Function to print the matrix 
void print(int mat[][MAX], int n) 
{ 
  for (int i = 0; i < n; i++) { 
    for (int j = 0; j < n; j++) { 
      printf("%d ",mat[i][j]); 
    } 
    printf("\n"); 
  } 
} 
  
// Function to change the value of the middle element  
// of the matrix to the sum of surrounding elements 
void changemiddle(int mat[][MAX], int n) 
{ 
  // Change the middle element 
  mat[n / 2][n / 2] 
    = mat[n / 2 - 1][n / 2] 
    + mat[n / 2][n / 2 - 1] 
    + mat[n / 2 + 1][n / 2] 
    + mat[n / 2][n / 2 + 1]; 
  
  // Function call to print the matrix 
  print(mat, n); 
} 
  
// Driver code 
int main() 
{ 
  int mat[][MAX] = { { 2, 1, 7 }, 
                    { 3, 7, 2 }, 
                    { 5, 4, 9 } }; 
  
  changemiddle(mat, 3); 
  
  return 0; 
} 
  
// This code is contributed by kothavvsaakash.

Java

// Java program to replace the value of the middle element  
// of the matrix with the sum of surrounding elements\ 
  
public class GFG{ 
      
    // Function to print the matrix 
    static void print(int mat[][], int n) 
    { 
        for (int i = 0; i < n; i++) { 
            for (int j = 0; j < n; j++) { 
                System.out.print(mat[i][j] + " "); 
            } 
             System.out.println(); 
        } 
    } 
      
    // Function to change the value of the middle element  
    // of the matrix to the sum of surrounding elements 
    static void changemiddle(int mat[][], int n) 
    { 
        // Change the middle element 
        mat[n / 2][n / 2] 
            = mat[n / 2 - 1][n / 2] 
              + mat[n / 2][n / 2 - 1] 
              + mat[n / 2 + 1][n / 2] 
              + mat[n / 2][n / 2 + 1]; 
      
        // Function call to print the matrix 
        print(mat, n); 
    } 
      
    // Driver code 
    public static void main(String []args) 
    { 
        int mat[][] = { { 2, 1, 7 }, 
                           { 3, 7, 2 }, 
                           { 5, 4, 9 } }; 
      
        changemiddle(mat, 3); 
    } 
    // This code is contributed by Ryuga 
} 

Python3

# Python3 program to replace the value 
# of the middle element of the matrix  
# with the sum of surrounding elements  
  
MAX = 100
  
# Function to print the matrix  
def printMatrix(mat, n):  
  
    for i in range(n):  
        for j in range(n):  
            print(mat[i][j], end = " ")  
          
        print() 
  
# Function to change the value of  
# the middle element of the matrix 
# to the sum of surrounding elements  
def changemiddle(mat, n): 
  
    # Change the middle element  
    mat[n // 2][n // 2] = (mat[n // 2 - 1][n // 2] + 
                           mat[n // 2][n // 2 - 1] + 
                           mat[n // 2 + 1][n // 2] + 
                           mat[n // 2][n // 2 + 1]) 
  
    # Function call to print the matrix  
    printMatrix(mat, n)  
  
# Driver Code 
if __name__ == "__main__":  
  
    mat = [ [ 2, 1, 7 ],  
            [ 3, 7, 2 ],  
            [ 5, 4, 9 ]]  
  
    changemiddle(mat, 3) 
  
# This code is contributed  
# by rituraj_jain 

C#

// C# program to replace the value of the middle element  
// of the matrix with the sum of surrounding elements 
   
using System; 
public class GFG{ 
       
    // Function to print the matrix 
    static void print(int[,] mat, int n) 
    { 
        for (int i = 0; i < n; i++) { 
            for (int j = 0; j < n; j++) { 
                Console.Write(mat[i,j] + " "); 
            } 
             Console.Write("\n"); 
        } 
    } 
       
    // Function to change the value of the middle element  
    // of the matrix to the sum of surrounding elements 
    static void changemiddle(int[,] mat, int n) 
    { 
        // Change the middle element 
        mat[n / 2,n / 2] 
            = mat[n / 2 - 1,n / 2] 
              + mat[n / 2,n / 2 - 1] 
              + mat[n / 2 + 1,n / 2] 
              + mat[n / 2,n / 2 + 1]; 
       
        // Function call to print the matrix 
        print(mat, n); 
    } 
       
    // Driver code 
    public static void Main() 
    { 
        int[,] mat = { { 2, 1, 7 }, 
                           { 3, 7, 2 }, 
                           { 5, 4, 9 } }; 
       
        changemiddle(mat, 3); 
    } 
     
} 

PHP

<?php 
// PHP program to replace the value of  
// the middle element of the matrix  
// with the sum of surrounding elements 
  
// Function to print the matrix 
function printmat(&$mat, $n) 
{ 
    for ($i = 0; $i < $n; $i++) 
    { 
        for ($j = 0; $j < $n; $j++)  
        { 
            echo ($mat[$i][$j]); 
            echo (" "); 
        } 
        echo ("\n"); 
    } 
} 
  
// Function to change the value of the 
// middle element of the matrix to the 
// sum of surrounding elements 
function changemiddle(&$mat, $n) 
{ 
    // Change the middle element 
    $mat[$n / 2][$n / 2] = $mat[$n / 2 - 1][$n / 2] +  
                           $mat[$n / 2][$n / 2 - 1] +  
                           $mat[$n / 2 + 1][$n / 2] +  
                           $mat[$n / 2][$n / 2 + 1]; 
  
    // Function call to print the matrix 
    printmat($mat, $n); 
} 
  
// Driver code 
$mat = array(array(2, 1, 7), 
             array(3, 7, 2), 
             array(5, 4, 9)); 
  
changemiddle($mat, 3); 
  
// This code is contributed  
// by Shivi_Aggarwal  
?> 

Javascript

<script> 
  
// JavaScript program to replace the 
// value of the middle element  
// of the matrix with the sum of  
// surrounding elements 
      
    // Function to print the matrix 
    function print(mat,n) 
    { 
        for (let i = 0; i < n; i++) { 
            for (let j = 0; j < n; j++) { 
                document.write(mat[i][j] + " "); 
            } 
             document.write("<br>"); 
        } 
    } 
      
    // Function to change the value of the middle element  
    // of the matrix to the sum of surrounding elements 
    function changemiddle(mat,n) 
    { 
        // Change the middle element 
        mat[(Math.floor(n / 2))][(Math.floor(n / 2))] 
            = mat[(Math.floor(n / 2) - 1)][(Math.floor(n / 2))] 
              + mat[(Math.floor(n / 2))][(Math.floor(n / 2) - 1)] 
              + mat[(Math.floor(n / 2) + 1)][(Math.floor(n / 2))] 
              + mat[(Math.floor(n / 2))][(Math.floor(n / 2) + 1)]; 
        
        // Function call to print the matrix 
        print(mat, n); 
    } 
      
    // Driver code 
    let mat = [ [ 2, 1, 7 ],  
            [ 3, 7, 2 ],  
            [ 5, 4, 9 ]] ; 
      
    changemiddle(mat, 3) 
      
      
// This code is contributed by rag2127 
  
</script> 

Output

2 1 7 
3 10 2 
5 4 9

Time Complexity: O(n * m), Here n is No. of Rows and m is No. of Column.
Auxiliary Space: O(1), as constant extra space is required.

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