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Re: mean() function [message #46929 is a reply to message #46927]

Tue, 10 January 2006 19:22

Kenneth P. Bowman
Messages: 585
Registered: May 2000

Senior Member

In article <dq1gv6$v05$1@news.nems.noaa.gov>,
Paul Van Delst <Paul.vanDelst@noaa.gov> wrote:

> Kenneth Bowman wrote:
>> In article <1136932449.216202.42760@g44g2000cwa.googlegroups.com>,
>> biocpu@yahoo.com wrote:
>>
>>
>>> y = fltarr(1008879)+35
>>
>>
>> Looks like roundoff error to me
>>
>> IDL> y = fltarr(1008879)+35
>> IDL> print, mean(y)
>> 35.0497
>> IDL> print, mean(y, /double)
>> 35.000000
>> IDL> y = dblarr(1008879)+35
>> IDL> print, mean(y)
>> 35.000000
>
> Huh. I don't see this in single precision (see other post). What version of
> IDL did you use?
>
> paulv

The version I posted (quoted above) is

{ ppc darwin unix Mac OS X 6.2 Jun 20 2005 32 32}

If I run it on my PowerBook (now sadly obsolete ;-) ), which is running
6.1

{ ppc darwin unix Mac OS X 6.1 Jul 14 2004 32 32}

I get exactly what was in the original post (he was running 6.0 on IRIX)

IDL> y = fltarr(1008879) + 35
IDL> print, mean(y)
35.5249
IDL> print, mean(y, /double)
35.000000
IDL> print, version

I suppose math libraries or compilers changed between 6.1 and 6.2.

Once the values you are adding differ by 6-7 orders of magnitude,
precision is completely lost for single precision floats.

IDL> print, total(replicate(1.0, 10^7))
1.00000e+07
IDL> print, total(replicate(1.0, 10^8))
1.67772e+07
IDL> print, total(replicate(1.0D0, 10^8))
1.0000000e+08

Cheers, Ken

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[Message index]

		Re: mean() function By: Maarten[1] on Thu, 12 January 2006 03:07
		Re: mean() function By: Paolo Grigis on Wed, 11 January 2006 06:19
		Re: mean() function By: Nigel Wade on Wed, 11 January 2006 01:49
		Re: mean() function By: Kenneth P. Bowman on Tue, 10 January 2006 19:22
		Re: mean() function By: biocpu on Tue, 10 January 2006 16:18
		Re: mean() function By: Paul Van Delst[1] on Tue, 10 January 2006 15:13
		Re: mean() function By: Paul Van Delst[1] on Tue, 10 January 2006 15:09
		Re: mean() function By: David Fanning on Tue, 10 January 2006 15:07
		Re: mean() function By: K. Bowman on Tue, 10 January 2006 15:00
		Re: mean() function By: Foldy Lajos on Tue, 10 January 2006 14:53
		Re: mean() function By: R.Bauer on Sun, 15 January 2006 00:58
		Re: mean() function By: savoie on Thu, 12 January 2006 08:49

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