Re: A question about efficiency

That makes perfect sense.

Many thanks Andrew,
Tim


Andrew Lumsdaine wrote:

> 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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>
>
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