| Re: Yet again, The Sky is Falling! [message #52863 is a reply to message #52862] |
Thu, 08 March 2007 11:11   |
yp
Messages: 42
Registered: February 2005
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Member |
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On Mar 8, 6:22 pm, David Fanning <n...@dfanning.com> wrote:
> yp writes:
>> Why is such discrepancy? In my problem the accuracy after 3rd decimal
>> point is not so important, however, after seeing the results I lose
>> confidence on IDL's capability on Real number arithmetic!
>
>> May be I am missing something?
>
> Well, maybe because I can't see it, but I'm immediately
> suspicious of what is going on in OPERATION. If you
> perform these two calls in the opposite order do you
> get the same result? That is, do you know for a fact
> that A, B, and F are not changing? (You have compared
> them before and after?)
>
> If it was some other number, perhaps, but zero!? It seems
> to me all computers can represent 0 accurately. :-)
>
> Cheers,
>
> David
> --
> 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.")
Thanks David, for your suggestion. I am pretty sure that none of the
argunet values change before or after the "Operation". And yes, the
discrepancy occurs both ways...
Here is the section from my running script.
;-------------
PRO test_brdf
wave = [412.5, 442.5, 490., 510., 560., 620., 660.] ;A (static)
nwave = n_elements(wave) ;B (static)
sza = 45.0D ;C (static)
vza = 1.078D ;D (static)
dphi = 0.0D ;E (static)
chl = 0.03D ;F (static)
null = 0.0D
print,'BEFORE: ', wave, nwave, sza, vza, dphi, chl
foq = (foq0 = (dblarr(nwave)))
for i=0, n_elements(chl)-1 do begin
int_LUT, wave, nwave, 0.0D, 0.0D, 0.0D, chl[i], foq0 ;Case1
; int_LUT, wave, nwave, null, null, null, chl[i], foq0 ;Case2
int_LUT, wave, nwave, sza[i], vza[i], dphi[i], chl[i], foq
print,'AFTER: ',wave, nwave, sza, vza, dphi, chl
help,BRDF
print,'BRDF: ',double(foq0[*]) / double(foq[*])
endfor
END
;----------
#1
-----------------------------------------
IDL> test_brdf
BEFORE: 412.500 442.500 490.000 510.000
560.000 620.000 660.000
7 45.000000 1.0780000 0.00000000
0.030000000
Loading f/Q table
AFTER: 412.500 442.500 490.000 510.000
560.000 620.000 660.000
7 45.000000 1.0780000 0.00000000
0.030000000
f/Q: 0.087899996 0.092399998 0.10349999
0.10879999 0.11449999 0.11319999 0.11339999
BRDF: 1.0250284 1.0281385 1.0367150
1.0450368 1.0480349 1.0547704 1.0573193
FOQ DOUBLE = Array[7]
-----------------------------------------
#2
-----------------------------------------
IDL> test_brdf
BEFORE: 412.500 442.500 490.000 510.000
560.000 620.000 660.000
7 45.000000 1.0780000 0.00000000
0.030000000
Loading f/Q table
AFTER: 412.500 442.500 490.000 510.000
560.000 620.000 660.000
7 45.000000 1.0780000 0.00000000
0.030000000
f/Q: 0.087899996 0.092399998 0.10349999
0.10879999 0.11449999 0.11319999 0.11339999
BRDF: 1.0247013 1.0279051 1.0365066
1.0447065 1.0477210 1.0543894 1.0569390
FOQ DOUBLE = Array[7]
-----------------------------------------
In my previous example, "Operation" = int_LUT and it does not change
any of the variables during execution or after. I don't suspect that
anything wrong happening inside "int_LUT". For any one case and for
same combination of the arguments:- if I run the code for several
times, I get same and consistent result each time. But when I switch
between passing the argument by value and by variable, I see the
discrepancy. Weird!
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