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Re: mean() function [message #46924 is a reply to message #46900]

Wed, 11 January 2006 06:19

Paolo Grigis
Messages: 171
Registered: December 2003

Senior Member

>
> [...]
>
> 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

I have no problem with that, understanding the argument of finite
precision and all that, but there is one thing which has me puzzling
(all commands issued from the same linux machine)

IDL Version 5.4 (linux x86). (c) 2000, Research Systems, Inc.

IDL> help,total(replicate(1.,1d8))
<Expression> FLOAT = 1.67772e+07
IDL> exit

IDL Version 5.5a (linux x86). (c) 2001, Research Systems, Inc.

IDL> help,total(replicate(1.,1d8))
<Expression> FLOAT = 6.71089e+07
IDL> exit

IDL Version 5.6 (linux x86 m32). (c) 2002, Research Systems, Inc.

IDL> help,total(replicate(1.,1d8))
<Expression> FLOAT = 1.00000e+08
IDL> exit

I was thinking that the result of the operation would depend
on the hardware used, but I would have guessed that on the same
machine no difference would be seen between different versions
of IDL, since all of them should represent floats in the same way...

So what's happening here? Different compiler optimizations?

Ciao,
Paolo

>
> 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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