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Re: Locate an underflow [message #25204 is a reply to message #25132]

Thu, 24 May 2001 07:36

Paul van Delst
Messages: 364
Registered: March 1997

Senior Member

William Thompson wrote:
>
> Paul van Delst <paul.vandelst@noaa.gov> writes:
>
>> ... If, on running said code, I get a crapload of underflow errors, it's an
>> indication that that either a) the code hasn't been tested very well or b) the
>> programmer didn't really think about the problem enough ...
>
> I disagree. It's exceedingly easy to get underflow errors, and extremely
> difficult to program around them. For example, a simple Gaussian
>
> Y = A*exp(-((X-X0)/Sig)^2)
>
> is almost guaranteed to generate underflow errors. At some point this is going
> to be indistinguishable from zero. You'd have to jump through hoops to avoid
> getting the completely useless underfloat messages.

Not really. what about something like (assuming double precision):

tolerance = (MACHAR(/DOUBLE)).EPS

y = DBLARR( N_ELEMENTS( X ) )
xarg = ((X-X0)/Sig)^2
index = WHERE( xarg < tolerance, count )
IF ( count GT 0 ) THEN $
y = A*exp(-xarg[index])

Or, as I mentioned in my reply to Craig:

y = A * gaussian_function( (X-X0)/Sig )

(or similar) which contains all the bits and pieces for checking. Craig also provided a
method of avoiding the underflows.

paulv

--
Paul van Delst A little learning is a dangerous thing;
CIMSS @ NOAA/NCEP Drink deep, or taste not the Pierian spring;
Ph: (301)763-8000 x7274 There shallow draughts intoxicate the brain,
Fax:(301)763-8545 And drinking largely sobers us again.
Alexander Pope.

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[Message index]

		Locate an underflow By: francesco.spada on Tue, 22 May 2001 14:59
		Re: Locate an underflow By: George N. White III on Fri, 25 May 2001 05:08
		Re: Locate an underflow By: thompson on Thu, 24 May 2001 08:42
		Re: Locate an underflow By: Paul van Delst on Thu, 24 May 2001 07:21
		Re: Locate an underflow By: Paul van Delst on Thu, 24 May 2001 07:36
		Re: Locate an underflow By: thompson on Wed, 23 May 2001 14:13
		Re: Locate an underflow By: thompson on Wed, 23 May 2001 14:01
		Re: Locate an underflow By: Craig Markwardt on Wed, 23 May 2001 11:25
		Re: Locate an underflow By: Paul van Delst on Wed, 23 May 2001 09:59
		Re: Locate an underflow By: davidf on Wed, 23 May 2001 08:16

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