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Re: Performance optimization (allocation inside a for loop)

From: Rob Mahurin
Subject: Re: Performance optimization (allocation inside a for loop)
Date: Thu, 2 Apr 2009 14:10:43 -0400

On Apr 2, 2009, at 1:25 PM, John W. Eaton wrote:
On  2-Apr-2009, Rob Mahurin wrote:

| Any idea why (1:n)(1:n) makes a full matrix more quickly than [1:n]?
| Just looking at timing, it looks like [] waits for something that ()
| () doesn't. I would have expected any difference to go the other way.

What do you mean by "more quickly?"

Well, if I do

lim = 1e3;
n = 1e5;
for i = lim:-1:1; tic;
        retval = (1:n)(1:n); t_paren(i) = toc; endfor
for i = lim:-1:1; tic;
        retval = [1:n]; t_brack(i) = toc; endfor
plot([t_paren; t_brack]')

then t_paren is mostly around 7 milliseconds, but t_brack is mostly around 10.5 milliseconds. With n = 1e6 the times are 70 ms and 100 ms or so.


Rob Mahurin
Department of Physics and Astronomy
University of Tennessee                 865 207 2594
Knoxville, TN 37996                     address@hidden

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