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Re: no way for simple matrix lookup ?

From: Mike Miller
Subject: Re: no way for simple matrix lookup ?
Date: Wed, 16 Jun 2004 12:20:44 -0500 (CDT)

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'.



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