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Re: WHERE problems (longish) [message #35910 is a reply to message #35909]

Tue, 22 July 2003 08:59

Benjamin Panter
Messages: 6
Registered: August 2002

Junior Member

David Fanning wrote:
> Benjamin Panter writes:
>
>
>> The values which have -1 certainly exist - and were generated in exactly
>> the same way as the others. I've put the array online if anyone fancies
>> looking at it - http://www.roe.ac.uk/~bdp/where_problem.idl
>>
>> Am I being stupid again? What is special about 2980,3000 and 3020??
>
>
> There is nothing special about *those* numbers, but those
> are not the numbers you are using in your WHERE statement.
> You are using 2980.0, 3000.0, and 3020.0. While there isn't
> a big difference between integers and floats to you, there
> is a HUGE difference to a computer. Better read these articles:
>
> http://www.dfanning.com/math_tips/sky_is_falling.html
> http://www.dfanning.com/math_tips/razoredge.html

Yeup, thanks for that David - I'm still a bit confused though, as the
values I give in the test program (3000.) are floats and the values in
the look up table are also floats! I nievely thought that would avoid
the razors and ints. Is it just by luck that where found the majority
and lost those three?

After reading your pages I've come up with a solution though... if I
replace the where lines with

print, where(abs(3000. - reform(dust_lookup[*,0])) lt 0.1)

I come out with the right answer.

thanks for you help,

Ben

--
Ben Panter, Edinburgh
My name (no spaces)@bigfoot which is a com.

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[Message index]

		Re: WHERE problems (longish) By: Paul Van Delst[1] on Tue, 22 July 2003 09:45
		Re: WHERE problems (longish) By: David Fanning on Tue, 22 July 2003 09:07
		Re: WHERE problems (longish) By: Benjamin Panter on Tue, 22 July 2003 08:59
		Re: WHERE problems (longish) By: Justin[2] on Tue, 22 July 2003 08:49
		Re: WHERE problems (longish) By: David Fanning on Tue, 22 July 2003 08:47

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