From help-octave-request at bevo dot che dot wisc dot edu Sun Dec 28 15:44:23 2003
Subject: Re: Effectively selecting elements of a matrix
From: "Jonathan C. Webster" <jwebster03 at snet dot net>
To: Thomas Weber <weber at num dot uni-sb dot de>
CC: help-octave at bevo dot che dot wisc dot edu
Date: Sun, 28 Dec 2003 16:33:21 -0500



Thomas Weber wrote:
> Hello, 
> 
> my problem is as follows: 
> I have a matrix A (size: 50x500) and two vectors (vec1, vec2) of length
> 32000. The elements of the vectors are indices for the matrix, i.e. if
> vec1(4) = 5 and vec2(4) = 10, I want to select A(5,10) into a new
> vector results_vec at position 4:
> results_vec (4) = A(5,10)
>  
> My problem is how to create this new vector effectively:
> 
> 
Ravel the matrix A out into one long vector, A_vec, using fortran indexing if you want.
Then combine vec1 and vec2 into a single index vector into your A_vec.
  .
vec3 = vec1 + size(A)(1) * (vec2 - 1)

Finally, your new reordered vector A3_vec

A3_vec = A_vec(vec3)

I think thats right, but check it.

Jonathan






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