A question about efficiency

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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