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Re: Comparing 2 arrays [message #55647 is a reply to message #55564]

Wed, 29 August 2007 01:04

Allan Whiteford
Messages: 117
Registered: June 2006

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. :-)
>
> Cheers,
>
> David

David,

Not sure about this point but wouldn't:

NUMBER = (abs(array_1) > abs(array_2)) * epsilon

make more sense?

Certainly it would make more sense to me in terms of both dealing with
the comparison of two negative numbers (in this case NUMBER will end up
being negative which can't be good!) or even when comparing a small
positive number with a large negative number.

Thanks,

Allan

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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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