| Re: WHERE problems (longish) [message #35907] |
Tue, 22 July 2003 09:45 |
![Paul Van Delst[1] is currently offline](theme/default/images/xoffline.png.pagespeed.ic.XRkd1fkXye.png "Paul Van Delst[1] is currently offline")
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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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
Hmm. Reading that razor's edge article made me dig out a little f90 program I
wrote to determine the real number spacing.
Given some number, A, one can do:
exponent=ceil(alog(abs(A))/alog(2.0d))
radix=2.0d ; (machar(/double)).ibeta ??
epsilon=(machar(/double)).eps
spacing=epsilon * (radix^(exponent-1))
So for, say, A = 1.234568d+16
EXPONENT LONG = 54
SPACING DOUBLE = 2.0000000
For A = 1.234568d+01
EXPONENT LONG = 4
SPACING DOUBLE = 1.7763568e-15
For A = 1.234568d-01
EXPONENT LONG = -3
SPACING DOUBLE = 1.3877788e-17
and for A = 1.234568d-16
EXPONENT LONG = -52
SPACING DOUBLE = 2.4651903e-32
which agree with the outputs of my f90 code using the intrinsic functions
EXPONENT and SPACING. The problem is, of course, what to do when A = 0.0. I
would just use spacing = epsilon/2.0. I think that, in this case, doing
something like
two=2.0d
radix=two
IF ( A .eq. 0.0 ) THEN $
spacing = epsilon/two $
ELSE BEGIN
exponent=ceil(alog(abs(A))/alog(two))
spacing=epsilon * (radix^(exponent-1))
ENDELSE
where you are counting on the equality check .EQ. *not* to work for numbers that
aren't represented as exactly zero (since exact zero can be represented). But
I'm not sure. You may also need a check to limit the calculated exponent to the
range allowed (the minexp and maxexp fields of the output from machar).
ANYway.... back to work....
paulv
--
Paul van Delst
CIMSS @ NOAA/NCEP/EMC
Ph: (301)763-8000 x7748
Fax:(301)763-8545
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| Re: WHERE problems (longish) [message #35909 is a reply to message #35907] |
Tue, 22 July 2003 09:07  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Benjamin Panter writes:
> 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.
And that would be the right approach. With floats
you can't (generally) ask if one float *equals*
another. You can ask if one float is within some
very tiny delta of another. That's what you ended
up doing here, and it works spectacularly. :-)
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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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
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Junior Member |
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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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| Re: WHERE problems (longish) [message #35911 is a reply to message #35910] |
Tue, 22 July 2003 08:49 |
Justin[2]
Messages: 16
Registered: October 2002
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Junior Member |
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This could possibly be a rounding problem with floating points. Try
using integers instead of floats for your wavelengths:
print, where(3000 eq ROUND(reform(dust_lookup[*,0])))
or look for values between 2999.9 and 3000.1 say...
Justin
Benjamin Panter <See@the_end.not> wrote in
news:3F1D5706.7050901@the_end.not:
> print, where(2900. eq reform(dust_lookup[*,0]))
> which works absolutly fine for most values: unfortunatly not for all:
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| Re: WHERE problems (longish) [message #35912 is a reply to message #35911] |
Tue, 22 July 2003 08:47 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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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
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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