| Re: POLY_FIT gives wrong answer ! [message #20074 is a reply to message #19941] |
Wed, 10 May 2000 00:00   |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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Henk Schets (Henk.Schets@oma.be) writes:
> David Fanning wrote:
>
>> Henk Schets (Henk.Schets@oma.be) writes:
>>
>>> Maybe all of you already know this, but I didn't and I could not find
>>> any documentation about it.
>>> The problem : I have two long arrays (x- and y-values) and I made
>>> subarrays out of them by using an array as subscript, like
>>> x = fltarray(l)
>>> y = fltarray(l)
>>> indexarr = longarray(l)
>>> (you know, the way to sort different arrays together)
>>> However, when I do something like out =
>>> poly_fit(x[indexarr],y[indexarr]), the outcome is simply wrong ! I know
>>> this because other programs gave another solution (all the same).
>>> The only way to do it right is by making other arrays like x2 and y2 and
>>> doing a poly_fit on it.
>>>
>>> Is this a known issue ?
>>
>> I don't know. What is known is that things are not always
>> the way they seem to be. I'm guessing this is one of those
>> times. :-)
>>
>> Cheers,
>>
>> David
>>
>> --
>> David Fanning, Ph.D.
>
> Could you be a littlebit more cryptic, please ?
I'm just saying that about 5 times out of 10 when users
describe problems to you, they are offering red herrings.
I think this is one of those times. There is no explanation
that I can think of why a subscripted array is not identical
to a variable made from that subscripted array. Subscripted
arrays are, of course, passed into the program by value, while
variables are passed in by reference. But since the input
variables don't change when they are passed into POLY_FIT,
I think we can rule this out as a suspect.
And from what I learned from my Sherlock Holmes, if you
eliminate everyone who can't possibly have committed the
crime, who you are left with--no matter how improbable--
is the murderer. My list of suspects is pretty much narrowed
down to programmer error in the absence of a working
(or mis-working, in this case) example. :-)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@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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