Re: Efficient multiplication by a diagonal matrix

[SANDS]

Mario Storti writes:
>I found  myself repeatedly with the following  problem. Given a matrix
>A(n,m)  and a vector  v(n), I  have to  multiply   each row  A(j,:) by
>v(j). This is equivalent to compute:
>
>B = diag(v) * A                     (1)
>
>Now, for large    n, (1) is  very  inefficient ...

Right, try using "Tony's Trick", it's been around for years. 

Assuming v is a column vector (if not do v=v(:);) try

B=v(:,ones(length(v),1)) .* A;

This is a kind-of MATLAB specific thing but I think 
it works in octave o.k.

Scott Sands