transpose double[][] matrix with a java function?

Question

Do anybody have a function with which I can transpose a Matrix in Java which has the following form:

double[][]

I have function like this:

public static double[][] transposeMatrix(double [][] m){
    for (int i = 0; i < m.length; i++) {
        for (int j = i+1; j < m[0].length; j++) {
            double temp = m[i][j];
            m[i][j] = m[j][i];
            m[j][i] = temp;
        }
    }

    return m;
}

but its wrong somewhere.

Answer 1

    public static double[][] transposeMatrix(double [][] m){
        double[][] temp = new double[m[0].length][m.length];
        for (int i = 0; i < m.length; i++)
            for (int j = 0; j < m[0].length; j++)
                temp[j][i] = m[i][j];
        return temp;
    }

Answer 2

If you would like to use an external library, Apache Commons Math provides the utility to transpose a matrix. Please refer to it official site.

First, you have to create a double array double[][] arr, as you have already done. Then, the transposed 2d matrix can be achieved like this

MatrixUtils.createRealMatrix(arr).transpose().getData()

Answer 3

Since Java 8, you can do this:

public static double[][] transposeMatrix(final double[][] matrix) {

    return IntStream.range(0, matrix[0].length)
        .mapToObj(i -> Stream.of(matrix).mapToDouble(row -> row[i]).toArray())
        .toArray(double[][]::new);
}

Answer 4

Java Class to Transpose a matrix :-

import java.util.Scanner;

public class Transpose {

    /**
     * @param args
     */

    static int col;
    static int row;
    static int[][] trans_arr = new int[col][row];

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Scanner sc = new Scanner(System.in);
        int m = sc.nextInt();
        col = m;
        int n = sc.nextInt();
        row = n;

        int[][] arr = new int[row][col];
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                arr[i][j] = sc.nextInt();
                System.out.print(arr[i][j] + " ");
            }
            System.out.println();
        }
        int[][] trans_arr = new int[col][row];
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                trans_arr[j][i] = arr[i][j];
            }
        }
        for (int i = 0; i < col; i++) {
            for (int j = 0; j < row; j++) {
                System.out.print(trans_arr[i][j] + " ");

            }
            System.out.println();
        }

    }

}

Answer 5

Here is the method

public static double[][] transpose(double arr[][]){
    int m = arr.length;
    int n = arr[0].length;
    double ret[][] = new double[n][m];

    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
            ret[j][i] = arr[i][j];
        }
    }

    return ret;
}

Answer 6

Here is a code to transpose a two dimensional matrix "In Place" (not using another data structure to save output) and hence is more memory efficient:

Same below algorithm can be used for int or char or string data types as well.

  public static double[][] transposeDoubleMatrix(double[][] matrix) {
        int n = matrix.length;
        for (int i = 0; i < n; i++) {
            for (int j = i; j < n; j++) {
                double tmp = matrix[j][i];
                matrix[j][i] = matrix[i][j];
                matrix[i][j] = tmp;
            }
        }
        return matrix;
    }

Answer 7

Here's a small change!

for (int j = i; j < m[0].length; j++)