Sum of Diagonal Elements of a Matrix in C

Program Description:- Write a program to find the sum of diagonal elements of a matrix in C. 

Procedure to find the sum of diagonal elements of a given matrix,
a) Take a matrix
b) Declare a sum variable and initialized with 0.
c) Iterate through each element of the matrix.
d) When the element is a diagonal element then add it to the sum variable.
e) Display the sum variable.

If the given matrix is a square matrix (whose row and column are equal) then the sum of diagonal elements is also called the trace of the matrix.

Now, let us see the simple C program to find the sum diagonal elements. In this program, matrix elements are given in the program itself.

#include <stdio.h>
// main function
int main()
{
  // matrix A 
  int matrix[3][3] = {{1,2,3},{4,5,6},{7,8,9}};

  // declare sum variable
  int sum = 0;
  
  // iterate through the matrix
  // outer loop for row
  for (int i = 0; i < 3; i++) 
  {
    // inner loop for column
    for (int j = 0; j < 3; j++) 
    {
      // condition to find diagonal element
      if(i == j) {
       // calculate sum
       sum += matrix[i][j];
      }
    }
  }

  // display matrix
  printf("Matrix = \n");
  for(int i=0; i<3; i++)
  {
    for(int j=0; j<3; j++)
    {
      printf("%d ", matrix[i][j]);
    }
    printf("\n"); // new line
  }

  // display sum of diagonal elements
  printf("Sum of diagonal elements = %d\n", sum);

  return 0;
}

Output:-

Matrix =
1 2 3
4 5 6
7 8 9
Sum of diagonal elements = 15

Time complexity:- O(N2)

To iterate through the matrix, it requires two loops therefore if we assume the given matrix is a square matrix then the time complexity for the above program will be O(N2).

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Sum of Diagonal Elements of a Matrix in C using Function by taking input value

In the previous program, matrix elements were initialized within the program itself. Now, let us develop another C program by taking input for matrix elements from the user and using functions.

#include <stdio.h>
// function to find sum of 
// digonal elements of a matrix
int diagonalSum(int matrix[10][10], int row, int column)
{
  // declare variable
  int sum = 0;

  // iterate through the matrix
  // outer loop for row
  for (int i = 0; i < 3; i++) 
  {
    // inner loop for column
    for (int j = 0; j < 3; j++) 
    {
      // condition to find diagonal element
      if(i == j) {
        // calculate sum
        sum += matrix[i][j];
      }
    }
  }

  // return sum
  return sum;
}
// function to read matrix
void readMatrix(int matrix[10][10], int row, int column)
{
  for (int i = 0; i < row; ++i)
  {
    for (int j = 0; j < column; ++j)
    {
      scanf("%d", &matrix[i][j]);
    }
  }
}
// main function
int main()
{
  // declare matrix matrix A, B, & C
  int matrix[10][10]; // first matrix

  // read the size of matrices
  int row, column;
  printf("Enter Row and Column for Matrix: ");
  scanf("%d %d", &row, &column);

  // read matrix A
  printf("Enter Matrix Elements: \n");
  readMatrix(matrix, row, column);

  // find sum of diagonal elements
  printf("Sum of diagonal elements = %d\n", 
    diagonalSum(matrix, row, column) );

  return 0;
}

Output:-

Enter Row and Column for Matrix: 3 2
Enter Matrix Elements:
9 8
7 4
5 6
Sum of diagonal elements = 13

Also See:-

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