In-Place Matrix Transpose



I was looking around today for an algorithm for In-Place Matrix Transposition and didn't manage to find anything, so this is what I came up with. I'll analyze and document it fully later, but I just wanted to get it down.

It's wasteful in that it will repeat a lot of operations, especially for "thin" matrices (worst case: vector). But it will work, never-the-less. Worst case run time is at most $latex O(n^2)$ where $latex n$ is the size of the matrix $latex n = rows times columns$

Edit: No longer arbitrarily repeating cycles ( was leading to identity operation, duh! ).

    template 
    void CMatrix::inPlaceTranspose(  )
    {
        using std::swap;

        resize(cols,rows);

        int i, j, k, k_start, k_new;
        int n = rows;
        int m = cols;

        int length = rows*cols;

        for(k_start=1; k_start < length; k_start++)
        {
            T       temp    = data[k_start];
            bool    abort   = false;
            k_new = k = k_start;

            // go through the cycle once and ensure that the starting point is
            // minimal
            do
            {
                if( k_new < k_start )
                {
                    abort = true;
                    break;
                }

                k       = k_new;
                i       = k%n;
                j       = k/n;
                k_new   = i*m + j;
            }while(k_new != k_start);


            // if the current value is not the minimum of the cycle, then don't
            // perform the cycle
            if(abort)
                continue;


            // otherwise, perform the cycle
            k_new = k = k_start;
            do
            {
                swap( data[k_new], temp );

                k       = k_new;
                i       = k%n;
                j       = k/n;
                k_new   = i*m + j;
            }while(k_new != k_start);

            swap( data[k_new], temp );
        }
    }

And the result:
In Place Transpose

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