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Re: Comparing 2 arrays [message #55546 is a reply to message #55540]

Mon, 27 August 2007 06:48

Conor
Messages: 138
Registered: February 2007

Senior Member

On Aug 26, 12:43 pm, David Fanning <n...@dfanning.com> 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?
>
> Humm, I don't recall that discussion. But I can see how
> this number might meet the criteria of "sufficiently close".
> On the other hand, I can also envision situations where
> the number could be orders of magnitude larger and still
> work for a particular application. I'm probably mistaken,
> but it seems to me "sufficiently close" is an arbitrary
> value that must be picked empirically to match the data
> and what you are trying to do with it.
>
> Cheers,
>
> David
>
> P.S. I'm just thinking that "sufficiently close" to a
> black hole, for example, might be a completely different
> number than "sufficiently close" to my house.
>
> --
> 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.")

Hmm... I think Jean might be on to something. After all, the error in
question hear is the rounding error of the computer, and that rounding
error is always an error on the last 'bit' of a floating point
number. So for instance if you had two floating point numbers:

1.1123453e15
and
1.1123454e15

These might be the same number (to within the rounding error) but the
difference between them is about 6.7e07. That's assuming of course
that I'm properly understanding floating point representation (I'm an
astronomer, not a computer engineer).

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[Message index]

		Re: Comparing 2 arrays By: David Fanning on Mon, 27 August 2007 08:06
		Re: Comparing 2 arrays By: Conor on Mon, 27 August 2007 06:48
		Re: Comparing 2 arrays By: David Fanning on Sun, 26 August 2007 09:43
		Re: Comparing 2 arrays By: Jean H. on Sun, 26 August 2007 08:40
		Re: Comparing 2 arrays By: David Fanning on Sun, 26 August 2007 07:32
		Re: Comparing 2 arrays By: airy.jiang on Sun, 26 August 2007 02:39
		Re: Comparing 2 arrays By: teich on Sat, 25 August 2007 22:25
		Re: Comparing 2 arrays By: David Fanning on Sat, 25 August 2007 21:47
		Re: Comparing 2 arrays By: David Fanning on Sat, 25 August 2007 21:45
		Re: Comparing 2 arrays By: David Fanning on Tue, 28 August 2007 08:15
		Re: Comparing 2 arrays By: James Kuyper on Mon, 27 August 2007 10:40
		Re: Comparing 2 arrays By: Paul Van Delst[1] on Mon, 27 August 2007 10:52

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