Re: no way for simple matrix lookup ?

[Mike Miller]

On Wed, 16 Jun 2004, D. Goel wrote:

> [1] Suppose A is a nxn matrix.  And b and are vectors.  How do i return 
> elements of A specified by b and c?
>
> i.e. Want to return a vector D, such that
> D(i) = A(b(i), c(i))
>
> Is the only way to do that a for loop?  Is that inefficient?

Do you mean that b and c contain integer values in the range between 1 and 
n?  You want for b and c to specify the rows and columns of A to retain?

D = A(b,c)

And your variable D will be a matrix with size(D)=[length(b), length(c)]. 
You can transform it into a vector using D=D(:), or you can do this:

D = reshape(A(b, c), length(b)*length(c), 1)

You don't have to loop over 'i'.

Best,

Mike



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