forum.alglib.net

I need a function to copy a matrix/submatrix a into matrix/submatrix b without transposing a. I wrote the function below to address this, is there a more efficient way to do this? Out of curiosity, why does rmatrixcopy transpose a before copying?

Thanks,
Dave
/*************************************************************************
Copy

Input parameters:
M - number of rows
N - number of columns
A - source matrix, MxN submatrix is copied without transposing
IA - submatrix offset (row index)
JA - submatrix offset (column index)
B - destination matrix, must be large enough to store result
IB - submatrix offset (row index)
JB - submatrix offset (column index)
*************************************************************************/
static void algMatCopy(const alglib::ae_int_t m, const alglib:ae_int_t n, const alglib:real_2d_array &a,
const alglib:ae_int_t ia, const alglib:ae_int_t ja, alglib:real_2d_array &b, const alglib:ae_int_t ib,
const alglib:ae_int_t jb)
{
alglib::real_2d_array aT;
aT.setlength(n,m); // because rmatrixcopy transposes a before copying to b.
alglib::rmatrixcopy(m, n, a, ia, ja, aT, 0, 0);
alglib::rmatrixcopy(m, n, aT, 0, 0, b, ib, jb);
}

Hi!

It does not transpose the matrix! The comment says 'without transposing' because there is a similar functions which performs copy-and-transpose.

So, there is no need to copy the matrix via temporary storage. Actually, the design reminds me of one old joke: 'our CPU implements multiplication via division by the inverse number'.