Java Program to Efficiently compute sums of diagonals of a matrix
Last Updated :
31 May, 2022
Given a 2D square matrix, find the sum of elements in Principal and Secondary diagonals. For example, consider the following 4 X 4 input matrix.
A00 A01 A02 A03
A10 A11 A12 A13
A20 A21 A22 A23
A30 A31 A32 A33
The primary diagonal is formed by the elements A00, A11, A22, A33.
- Condition for Principal Diagonal: The row-column condition is row = column.
The secondary diagonal is formed by the elements A03, A12, A21, A30.
- Condition for Secondary Diagonal: The row-column condition is row = numberOfRows – column -1.
Examples :
Input :
4
1 2 3 4
4 3 2 1
7 8 9 6
6 5 4 3
Output :
Principal Diagonal: 16
Secondary Diagonal: 20
Input :
3
1 1 1
1 1 1
1 1 1
Output :
Principal Diagonal: 3
Secondary Diagonal: 3
Method 1 (O(n ^ 2) :
In this method, we use two loops i.e. a loop for columns and a loop for rows and in the inner loop we check for the condition stated above:
Java
import java.io.*;
public class GFG {
static void printDiagonalSums( int [][]mat,
int n)
{
int principal = 0 , secondary = 0 ;
for ( int i = 0 ; i < n; i++) {
for ( int j = 0 ; j < n; j++) {
if (i == j)
principal += mat[i][j];
if ((i + j) == (n - 1 ))
secondary += mat[i][j];
}
}
System.out.println( "Principal Diagonal:"
+ principal);
System.out.println( "Secondary Diagonal:"
+ secondary);
}
static public void main (String[] args)
{
int [][]a = { { 1 , 2 , 3 , 4 },
{ 5 , 6 , 7 , 8 },
{ 1 , 2 , 3 , 4 },
{ 5 , 6 , 7 , 8 } };
printDiagonalSums(a, 4 );
}
}
|
Output:
Principal Diagonal:18
Secondary Diagonal:18
Time Complexity: O(N*N), as we are using nested loops to traverse N*N times.
Auxiliary Space: O(1), as we are not using any extra space.
Method 2 (O(n) :
In this method we use one loop i.e. a loop for calculating sum of both the principal and secondary diagonals:
Java
import java.io.*;
public class GFG {
static void printDiagonalSums( int [][]mat,
int n)
{
int principal = 0 , secondary = 0 ;
for ( int i = 0 ; i < n; i++) {
principal += mat[i][i];
secondary += mat[i][n - i - 1 ];
}
System.out.println( "Principal Diagonal:"
+ principal);
System.out.println( "Secondary Diagonal:"
+ secondary);
}
static public void main (String[] args)
{
int [][]a = { { 1 , 2 , 3 , 4 },
{ 5 , 6 , 7 , 8 },
{ 1 , 2 , 3 , 4 },
{ 5 , 6 , 7 , 8 } };
printDiagonalSums(a, 4 );
}
}
|
Output :
Principal Diagonal:18
Secondary Diagonal:18
Time Complexity: O(N), as we are using a loop to traverse N times.
Auxiliary Space: O(1), as we are not using any extra space.
Please refer complete article on Efficiently compute sums of diagonals of a matrix for more details!
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