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Re: Comparing 2 arrays [message #55658 is a reply to message #55564]

Tue, 28 August 2007 11:43

Paul Van Delst[1]
Messages: 1157
Registered: April 2002

Senior Member

David Fanning wrote:
> Jean H. writes:
>
>> to get back to a previous discussion we had a few month ago about being
>> "sufficiently close to zero", shouldn't it be (data1.A - data2.B) LT
>> epsilon * data1.A , with epsilon=(machar()).eps?
>
> OK, I found that discussion and read it eight or ten times until
> I finally understood it. (Probably why I forgot it before.)
>
> I've put a significantly edited discussion of this
> problem here:
>
> http://www.dfanning.com/code_tips/comparearray.html
>
> In my preferred solution now, I choose a number that
> is "sufficiently close" to zero like this:
>
> epsilon = (MACHAR()).eps
> NUMBER = (array_1 > array_2) * epsilon
>
> Then, the comparison between arrays is done like this:
>
> IF Total(Abs(array_1 - array_2) LT NUMBER) EQ N_Elements(array_1) $
> THEN RETURN, 1 ELSE RETURN, 0
>
> Additional comments welcome if you want to argue further. :-)

Sure! :o)

I think you should also pass a scaling factor, ala,

FUNCTION FLTARRAYS_EQUAL, array_1, array_2, ULP=ulp
....
IF ( N_ELEMENTS(ulp) EQ 0 ) THEN ulp=1.0
....
NUMBER = (array_1 > array_2) * epsilon * ulp
....
END

Also, there needs to be differentiation for singel and double precision so you can
determine epsilon correctly and set ulp to a suitable default (1.0 or 1.0d0).

cheers,

paulv

>
> Cheers,
>
> David

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[Message index]

		Comparing 2 arrays By: teich on Sat, 25 August 2007 21:18
		Re: Comparing 2 arrays By: ph le sager on Wed, 29 August 2007 10:32
		Re: Comparing 2 arrays By: David Fanning on Wed, 29 August 2007 06:17
		Re: Comparing 2 arrays By: Carsten Lechte on Wed, 29 August 2007 03:31
		Re: Comparing 2 arrays By: Allan Whiteford on Wed, 29 August 2007 01:04
		Re: Comparing 2 arrays By: James Kuyper on Tue, 28 August 2007 13:02
		Re: Comparing 2 arrays By: Jean H. on Tue, 28 August 2007 12:18
		Re: Comparing 2 arrays By: Paul Van Delst[1] on Tue, 28 August 2007 12:33
		Re: Comparing 2 arrays By: David Fanning on Tue, 28 August 2007 11:56
		Re: Comparing 2 arrays By: Paul Van Delst[1] on Tue, 28 August 2007 11:43
		Re: Comparing 2 arrays By: David Fanning on Tue, 28 August 2007 09:52
		Re: Comparing 2 arrays By: Jean H. on Tue, 28 August 2007 09:20
		Re: Comparing 2 arrays By: David Fanning on Tue, 28 August 2007 09:18
		Re: Comparing 2 arrays By: Allan Whiteford on Tue, 28 August 2007 09:10

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