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Re: How to handle slow array loops?

From: James Sherman Jr.
Subject: Re: How to handle slow array loops?
Date: Wed, 1 Jun 2011 22:56:40 -0400

On Wed, Jun 1, 2011 at 9:01 PM, <address@hidden> wrote:

please consider this simplified example:

function h = loop(range)

h = zeros(1,1000 + 1);
xx = rand(2, range);

for i=1:range
 for j=1:2
   t = round(xx(j, i) * 1000 + 1);
   h(t) = h(t) + 1;


'tic; h = loop(1000000); toc'

on 2.4GHz Pentium 4 with 1GB RAM takes more then 340 seconds

Every added zero in that range adds another zero to test time

As this loop in Fortran [1] performs ~3000 times faster I'm
what can I do to speed up this Octave code?

Also some explanation why this is so slow is much appreciated

As to why, the short answer is that your comparing compiled code to a script.  I'm not an expert on this, so someone more knowledgeable that me can explain in more detail.

As to how to speed up this code, you could do something like:
function h = loop(range)

xx = rand(2, range);

t = round(xx(:) * 1000 + 1);
h = histc(t, 1:1001);


I think will do the same thing that your function does.

Hope this helps.

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