How to rewrite array from row-order to column-order?

Question

I have this double for-loop, where I have both row-order and column-order array indexing, which should be bad for performance.

  for (int row = 0; row < h         


        

Answer 1

Since the question is tagged C++, I'll contribute an answer that shows how accessing / manipulating column-major matrices can be done using Boost.Multiarray (it may be useful to others who face a similar problem). I consider Boost to be an extension to the C++ standard library. Feel free to ignore this answer if you don't like/use Boost. :-)

#include 
#include 
#include 

// Prints the contents of a matrix to standard output
template  void printMatrix(const M& matrix)
{
    int height = matrix.shape()[0];
    int width = matrix.shape()[1];
    for (int row=0; row Matrix;
    typedef boost::general_storage_order<2> Storage;
    Matrix matrix(
        data,
        boost::extents[height][width],
        Storage(ordering, ascending)
    );

    // Access source data as if it's row major
    printMatrix(matrix);
    std::cout << "\n";

    // Transpose source data to an actual row-major matrix
    // boost::multi_array is row-major by default
    boost::multi_array matrix2(boost::extents[height][width]);
    std::copy(matrix.begin(), matrix.end(), matrix2.begin());
    printMatrix(matrix2);
}

Output:

0 1 2 3
4 5 6 7
8 9 10 11

0 1 2 3
4 5 6 7
8 9 10 11

As you can see, you can leave the source data in its column-major format, and use boost::multi_array_ref with custom storage specifications to manipulate the data directly (as if it were row-major) using the matrix[row][col] notation.

If the matrix is going to be traversed often in row-major fashion, then it might be better to transpose it to an actual row-major matrix, as shown in the last part of my example.