CODEWITHSUNDEEP
javaarraysmatrix
Library System
Library System

Reputation: 65

How to check if matrix is symmetric?

I have to split a matrix in half by vertical and to see if it is symmetric. For example, if mat[0][0]==mat[0][1] and mat[1][0]==mat[1][1], the matrix is symemtric. I have managed to check the symmetry on a 2x2 matrix, but I want the function to be available at any kind of matrix, 4x4 or 3x3 for example.

i and j are variables i use to get through the elements of mat and n is the dimension of the matrix. This is the function for a 2x2 matrix. How can I generalise it?

 private static void getSymmetry(int mat[][], int i, int j, int n) {
        int index = 0;
        int sum=0;
        if (n == 2) {
            if (mat[i][j] == mat[i][j + 1]) {
                index = index + 1;
            }
            if (mat[i + 1][j] == mat[i + 1][j + 1]) {
                index = index + 1;
            }
            sum = sum + index;
        }
        System.out.println("Degree of symmetry is " + sum);
    }

Upvotes: 1

Views: 4936

Answers (4)

NJY404
NJY404

Reputation: 358

Here is a less complex and efficient way of checking a symmetric matrix. It breaks the loop whenever there is an unequal dimension or match in the matrix then returns false otherwise true if there were no mismatch.

public boolean isMatrixSymmetric(int [][] matrix){
        for (int x = 0; x < matrix.length; x++) {
            if(matrix.length != matrix[x].length){
                return false;
            }
            for (int y = 0; y < matrix[x].length; y++) {
                if(matrix[y][x] != matrix[x][y]){
                    return false;
                } 
            }
        }
        return true;
}

Upvotes: 0

Vishrant
Vishrant

Reputation: 16698

public class MatrixSymmetry {

  public static void main(String[] args) {

    int[][] matrix = {
        {1, 1, 1},
        {1, 2, 1},
        {1, 1, 1}
    };

    System.out.println(isSymmetric(matrix));
  }

  public static boolean isSymmetric(int[][] matrix) {

    for (int i = 0; i < matrix.length; i++) {

      // you got the row
      int[] row = matrix[i];
      int end = row.length - 1;

      for (int start = 0; start < end; start++) {
        if (row[start] != row[end]) {
          return false;
        }
        end--;
      }

    }
    return true;
  }

}

Upvotes: 0

Serhat Oz
Serhat Oz

Reputation: 798

You can use below approach it checks symmetric for all numbers:

private static boolean isSymmetric(int mat[][]) {
    for(int a = 0; a< mat.length; a++){
        for(int b = 0; b < mat[a].length / 2; b++){
            if(mat[a][b] != mat[a][mat[a].length-b-1]) {
                return false;
            }
        }
    }       
    return true;
}

...

boolean symmetric = isSymmetric(mat);
if(symmetric) {
    System.out.println("symmetric");
} else {
    System.out.println("Not symmetric");
}

Upvotes: 1

NiVeR
NiVeR

Reputation: 9806

Although the provided answers are good, here is another attempt using a known technique called Two-Pointers. I believe though that it is a bit more clear and optimized. 

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        int[][] mat = {
            {1, 2, 2, 1},
            {4, 3, 3, 4},
            {2, 3, 3, 2}
        };

        //assume the matrix is square
        int rows = mat.length, columns = mat[0].length;

        boolean symmetric = true;
        for(int r = 0; r < rows && symmetric; r++){
            //now declare two pointers one from left and one from right
            int left = 0, right = columns - 1;

            while (left < right){
                if(mat[r][left] != mat[r][right]){
                    symmetric = false;
                    break;
                }
                right--;
                left++;
            }
        }

        System.out.println(symmetric? "The matrix is symmetric." : "The matrix isn't symmetric.");
    }
}

Here is the Ideone code.

Upvotes: 0

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