In-place matrix transpose of an M*N matrix in Java not possible?

Question

Is it possible to do in-place matrix transpose of an M*N matrix in Java? I don't think it's possible in Java because, in a 3*5 matrix, the element at 3*5 would ideally be swapped with the element at 5*3, but such a position does not exist. In say, C/C++, it would be possible since memory is contiguous.

Also, there are other algorithms like the one posted here for M*N matrices, which I believe are not applicable to Java, but how's it any different from swapping elements at A[i][j] with A[j][i] since even the rolling window algorithm in the link above takes auxiliary memory?

Answer 1

You have to be careful with the term in-place. Strictly speaking, the transpose operator 'creates' a new matrix with potentially new dimensions. In the case of the article you linked, in-place just means using the same memory for the initial and final matrices, which is of course possible with a clever representation.

A trivial solution might be:

public class Matrix {
    private int[][] backingArray;
    private boolean transposed;
    public Matrix(int[][] _backingArray) {
        this.backingArray = _backingArray
    }

    public Matrix(int[] data, rows, columns) {
        backingArray = new int[rows][columns]
        for (int i = 0; i < rows; i++)
            for (int j = 0; j < columns; j++)
                backingArray[i][j] = data[i*columns + j]
    }

    public int getElement(i, j) {
        if (transposed)
            return backingArray[j][i];
        return backingArray[i][j];
    }

    public void transpose() {
        transpose = !transpose;
    }
}

EDIT: Corrected some bad syntax

EDIT: Made it so that a 1-D matrix can be read in as per the OPs question (?)