From help-octave-request at bevo dot che dot wisc dot edu Sun Dec 28 14:38:45 2003
Subject: Effectively selecting elements of a matrix
From: Thomas Weber <weber at num dot uni-sb dot de>
To: help-octave at bevo dot che dot wisc dot edu
Date: Sun, 28 Dec 2003 21:38:14 +0100

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:

I tried something like
results_vec = diag(A(vec1, vec2));

which works only for smaller (shorter) vectors vec1 and vec2. Octave
needs to create a new matrix of size 32000x32000 which results in a
out-of-memory error.

I am currently using a for-loop

result_vec = zeros(1,32000);
for k=1:32000
  result_vec(k) = A(vec1(k), vec2(k));
endfor;

but this is really slow.

I am grateful for any ideas.

Best regards 
Thomas



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