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Why does alglib::rmatrixcopy transpose a before copying?
http://forum.alglib.net/viewtopic.php?f=2&t=4609		 Page 1 of 1
Author:  dvblackham [ Wed Jun 26, 2024 11:47 pm ]
Post subject:  Why does alglib::rmatrixcopy transpose a before copying?
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);
}
Author:  Sergey.Bochkanov [ Thu Jun 27, 2024 1:34 pm ]
Post subject:  Re: Why does alglib::rmatrixcopy transpose a before copying?
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'.
Author:  dvblackham [ Wed Jul 03, 2024 4:23 pm ]
Post subject:  Re: Why does alglib::rmatrixcopy transpose a before copying?
Sergey,

Thank you for your response. I am sorry for the confusion my initial post created. I included a function that I wrote to copy the matrix due to rmatrixcopy transposing the matrix, I simply created a similar header for the function I wrote that calls rmatrixcopy twice in order to deal with the transpose issue. Is there a function that copies without transposing? I couldn't find it.

Thanks,
Dave

Here's a copy from the C++ manual for rmatrixcopy:
/*************************************************************************
Copy

Input parameters:
M - number of rows
N - number of columns
A - source matrix, MxN submatrix is copied and transposed
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)
*************************************************************************/
void rmatrixcopy(const ae_int_t m, const ae_int_t n, const real_2d_array &a, const ae_int_t ia, const ae_int_t ja, real_2d_array &b, const ae_int_t ib, const ae_int_t jb, const xparams _xparams = alglib::xdefault);
Author:  Sergey.Bochkanov [ Thu Jul 04, 2024 8:39 am ]
Post subject:  Re: Why does alglib::rmatrixcopy transpose a before copying?
Hi! It is merely a mistake in the documentation, a copy-paste from rmatrixtranspose(). I will fix it in the next release.
Author:  dvblackham [ Fri Jul 12, 2024 11:26 pm ]
Post subject:  Re: Why does alglib::rmatrixcopy transpose a before copying?
Thanks Sergey
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