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Re: Vectorization quiz

From: James Sherman Jr.
Subject: Re: Vectorization quiz
Date: Thu, 21 Apr 2011 10:37:06 -0400

On Thu, Apr 21, 2011 at 4:49 AM, Daniel Arteaga <address@hidden> wrote:

How could I vectorize the following piece of code?

P = zeros(size(fInf));
for i = 1:length(fInf)
       P(i) = sum( S2( fLin > fInf(i) & fLin < fSup(i) ) );


fInf is a vector of length 1000
fSup is a vector of length 1000
P    is a vector of length 1000
S2   is a vector of length 50000
fLin is a vector of length 50000

Thank you very much in advance,


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Well, I'm not saying that this is the best way, but you could try this:

n = length(fInf);
m = length(fLin);

fLin_repeated = repmat(fLin, [1, n]);
S2_repeated = repmat(S2, [1, n]);
fInf_repeated = repmat(fInf', [m, 1]);
fSup_repeated = repmat(fSup', [m, 1]);

tmp1 = (fLin_repeated > fInf_repeated) & (fLin_repeated < fSup_repeated); 
P = sum( S2_repeated .* tmp1 )';

I particularly don't like the last line.  I feel like there is a function to do the masking part instead of doing the multiplications, but for the life of me I can't remember what it is.  The downside is that you will have 4 different 50000 by 1000 matrices, which may or may not be problematic.

Hope this helps.

P.S. Testing it on my machine, I run out of memory on making the 4th matrix.  To get around this you could either cut up S2 into smaller chunks (maybe to 10000 at a time instead of all 50000) or you could calculate the tmp1 variable, clear fLin_repeated, fSup_repeated, and fLin_repeated, then generate S2_repeated and P.

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