Re: [eigen] MatrixX multiply

[Benoit Jacob <jacob@xxxxxxxxxxxxxxx>]

On Thu, 15 Feb 2007, Steven große Deters wrote:

> Thank you. Perhaps you could give me some advise so that I could try to do
> it on my own in the meantime ?
>
> How could an entry-by-entry product of two (large) matrix's by accomplished
> ?
> Would it be reasonable to access the single elements e.g by row -> vector ->
> element  and than multiply the elements. Or would that be to much of a
> performance hit ?

You can certainly do it, but there's a faster way: Eigen allows you to 
directly access the elements in the array of matrix entries, by 
operator[]. You can take advantage of it here, i.e. do a single loop like

	for( int i = 0; i < size()*size(); i++ )
		result_matrix[i] = matrix1[i] * matrix2[i];

As a side-note: if you know at compile-time the size of your matrices and 
if this size isn't too big, you'll get much, much better performance with 
fixed-size matrices, i.e.

Matrix< float, Size > or Matrix4f instead of MatrixXf.

Benoit