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Re: How to speed up large FOR loops? [message #79807 is a reply to message #79804]

Tue, 10 April 2012 11:21

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

On Tuesday, April 10, 2012 12:26:27 PM UTC-4, Saurav Dhital wrote:
> Yes. More specifically, I reform arrays and assign them to the new
> FITS structure. This happens a lot.

This is a problem. A pattern like this,
L = [0]
for i = 0, 1000000-1 do L = [L, i]

will run quickly at first and then slow down as L gets longer and longer.

The trick is to pre-allocate your lists like L. If the size of L is not known in advance, then pre-allocate a reasonable fixed size to begin with, and then grow it by factors of 2 as needed (up to a point).

At the end, you will need to trim the unused portion of L.

I do this kind of pattern all the time, and it can be quite efficient.

Craig

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[Message index]

		Re: How to speed up large FOR loops? By: Saurav Dhital on Tue, 10 April 2012 12:39
		Re: How to speed up large FOR loops? By: Craig Markwardt on Tue, 10 April 2012 11:21
		Re: How to speed up large FOR loops? By: Saurav Dhital on Tue, 10 April 2012 10:23
		Re: How to speed up large FOR loops? By: David Fanning on Tue, 10 April 2012 10:13
		Re: How to speed up large FOR loops? By: Fabzi on Tue, 10 April 2012 09:53
		Re: How to speed up large FOR loops? By: Saurav Dhital on Tue, 10 April 2012 09:26
		Re: How to speed up large FOR loops? By: Fabzi on Tue, 10 April 2012 09:17
		Re: How to speed up large FOR loops? By: Matt Francis on Thu, 12 April 2012 19:41
		Re: How to speed up large FOR loops? By: Saurav on Fri, 20 April 2012 09:48

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