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Re: CURVEFIT [message #39642 is a reply to message #9041]

Wed, 02 June 2004 11:40

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

Andreas Ernst <aernst@ari.nis> writes:

> Hi,
>
> (1) I am using CURVEFIT to fit a straight line
> through my data points. My user-supplied
> function looks like this:
>
> PRO gerade, X, A, F, PDER
> F=A[0]+A[1]*X
> IF N_PARAMS() GE 4 THEN $
> PDER = [[REPLICATE(1.0, N_ELEMENTS(X))], [X]]
> RETURN
> END
>
> Anyway, even though it defines a straight line,
> the routine CURVEFIT seems to fit some other
> curve through the data points, which is something

A linear fit is trivial, and CURVEFIT shouldn't produce problems like
you found. My guess is that it is more likely that you have a data
handling problem. For example, if you perform a data selection on the
X values but forget to do the same for the Y values.

As the other poster said, you can use LINFIT/POLYFIT* to do the simple
fits you desire.

For more complicated fits, or as a cross check, you can use my own
fitting program, MPFIT + MPCURVEFIT, which have a wide acknowledgement
in the IDL community. As a combination, they are a drop-in
replacement for CURVEFIT (although if you don't have the requirement
to keep CURVEFIT compatibility, then I recommend using MPFITFUN).

Happy fitting!

Craig

--
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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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[Message index]

		Curvefit By: zanotti on Wed, 21 May 1997 00:00
		Re: Curvefit By: thompson on Wed, 21 May 1997 00:00
		Re: curvefit By: David Fanning on Tue, 07 January 2003 20:13
		Re: CURVEFIT By: Craig Markwardt on Wed, 02 June 2004 11:40
		Re: CURVEFIT By: K. Bowman on Wed, 02 June 2004 08:56
		Re: curvefit By: Paolo Grigis on Tue, 14 June 2005 09:16
		Re: curvefit By: nolan.smith1 on Tue, 14 June 2005 08:20
		Re: curvefit By: nolan.smith1 on Mon, 13 June 2005 21:02
		Re: curvefit By: nolan.smith1 on Mon, 13 June 2005 20:07
		Re: curvefit By: Craig Markwardt on Mon, 13 June 2005 15:24
		Re: curvefit By: kashyap on Mon, 13 June 2005 14:50
		Re: curvefit By: nolan.smith1 on Mon, 13 June 2005 12:59
		Re: curvefit By: Benjamin Hornberger on Mon, 13 June 2005 12:46

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