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Re: mean() function [message #46956 is a reply to message #46938]

Sun, 15 January 2006 05:53

Foldy Lajos
Messages: 268
Registered: October 2001

Senior Member

HI,

sometimes there is a trade-off between speed and correctness.
Here is another example:

IDL> print, !version
{ x86 linux unix linux 6.2 Jun 20 2005 32 64}

IDL> print, smooth([1, 1.0e20, 1, 1, 1, 1], 3)
1.00000 3.33333e+19 3.33333e+19 0.00000 0.00000 1.00000

Instead of calculating the average for each 3-element window, IDL uses
the first average only, then slides the window by adding/subtracting
elements from the window edge. For width w it uses only 2 add/sub per
window instead of w additions, which is a huge speed-up for large widths,
but can give incorrect result for rare cases.

regards,
lajos

On Sun, 15 Jan 2006, Reimar Bauer wrote:

> mean is not useable if it results in this
>
> IDL> print,mean( make_array(500000,val=35,/float) )
> 35.0413
> IDL> print,mean( make_array(400000,val=35,/float) )
> 35.0000
>
> I prefer a slower routine if this is right.
>
> no one would accept 1.0 + 1.0 result = 1.5
>

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[Message index]

		mean() function By: biocpu on Tue, 10 January 2006 14:34
		Re: mean() function By: Foldy Lajos on Mon, 16 January 2006 02:27
		Re: mean() function By: Maarten[1] on Mon, 16 January 2006 00:01
		Re: mean() function By: Foldy Lajos on Sun, 15 January 2006 05:53
		Re: mean() function By: Maarten[1] on Fri, 13 January 2006 00:57
		Re: mean() function By: Kenneth P. Bowman on Thu, 12 January 2006 09:07

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