Re: A question about efficiency

Hi -- I think this type of operation only makes sense for triangular 
matrices.  I.e., if you think about matrix-vector multiplication:

y_i = sum_j A_ij * x_j

you need all of the values of x to compute each of the values of y.  If 
you change any of the values of x during the computation, you won't get 
the right answer for y -- that is, if you try to update y in-place ala 
an operation like y = A*y.  The only exception is if A is triangular -- 
then as the operation proceeds you no longer need values x_j for j < i 
(or j > i depending on whether you have an upper or lower triangular 
matrix).

Best Regards,
Andrew Lumsdaine

On Jul 23, 2004, at 2:06 AM, Tim wrote:

> Hey guys,
> A simple question here.
>
> How do I apply a matrix transformation 'A' to a vector 'x', without 
> storing the result into another vector?
>
> The only way I can think of doing it now is to do:
> "mult(A,x,y);
> copy(y,x)"
>
> I ask this because I want to be able to transform all the elements in 
> a container of vectors, without getting bogged down by unnecassary 
> calls to copy.
>
> Thanks in advance,
> Timothy Ling
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