comp.lang.idl-pvwave archive: archive » Re: machine precision

Home » Public Forums » archive » Re: machine precision

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: machine precision [message #66506 is a reply to message #66505]

Mon, 18 May 2009 04:53

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Wox writes:

> When checking whether two foating point variables are equal, one has
> to do this:
>
> pres = (machar()).eps
> bequal = abs(f1-f2) lt pres
>
> This can go wrong however, as illustrated by the example below. Do I
> need to do error propagation on this? This means that every time f1
> and f2 are calculated differently, I have to calculate a different
> uncertainty? This seems like a lot of work, not to mention the machine
> precision in calculation the propagation of uncertainty... Is there a
> more general rule of thumb I can use?
>
> vec1=[1.,2,3,4,5]
> vec2=vec1
> pres=(machar()).eps
> norm1=sqrt(total(vec1^2,1,/pres))
> norm2=sqrt(total(vec2^2,1,/pres))
> f1=total(vec1*vec2,/pres) ; inner product
> f2=norm1*norm2 ; product of the norms
> ; f1 and f2 must be equal so
> if abs(f1-f2) ge pres then print,'wrong wrong wrong...'

Uh, Wox, I think you need to read this article again:

http://www.dfanning.com/math_tips/sky_is_falling.html

I think if you try:

vec1 = [1.D, 2, 3, 4, 5]

you might find a different result. :-)

Cheers,

David

--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")

Report message to a moderator

[Message index]

		Re: machine precision By: Wout De Nolf on Wed, 20 May 2009 06:08
		Re: machine precision By: Carsten Lechte on Wed, 20 May 2009 03:44
		Re: machine precision By: jameskuyper on Wed, 20 May 2009 03:28
		Re: machine precision By: Carsten Lechte on Wed, 20 May 2009 01:46
		Re: machine precision By: David Fanning on Tue, 19 May 2009 20:01
		Re: machine precision By: jameskuyper on Tue, 19 May 2009 18:41
		Re: machine precision By: Kenneth P. Bowman on Tue, 19 May 2009 11:15
		Re: machine precision By: jeffnettles4870 on Tue, 19 May 2009 08:16
		Re: machine precision By: jameskuyper on Mon, 18 May 2009 18:44
		Re: machine precision By: Wout De Nolf on Mon, 18 May 2009 07:02
		Re: machine precision By: Wout De Nolf on Mon, 18 May 2009 06:55
		Re: machine precision By: jameskuyper on Mon, 18 May 2009 06:33
		Re: machine precision By: David Fanning on Mon, 18 May 2009 06:30
		Re: machine precision By: Wout De Nolf on Mon, 18 May 2009 06:04
		Re: machine precision By: David Fanning on Mon, 18 May 2009 04:53

Previous Topic:	Re: Simultaneous fitting in IDL
Next Topic:	Simultaneous fitting in IDL

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Sun Oct 12 19:26:18 PDT 2025

Total time taken to generate the page: 1.04185 seconds