comp.lang.idl-pvwave archive: archive » Re: Newbie's question

Home » Public Forums » archive » Re: Newbie's question

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: Newbie's question [message #45920 is a reply to message #45918]

Thu, 20 October 2005 00:33

Paolo Grigis
Messages: 171
Registered: December 2003

Senior Member

ChiChiRuiz@gmail.com wrote:
> Hi there,
>
> I have a scatter plot which has the shape of a parabola, like y=x^2.
> I want to find the best curve fit to the scatter plot, so I used the
> function "curvefit" with no weights and with initial guesses (1.0, 2.0)
> i.e. y = 1.*x^(2.). So, here's the problem...when I use only the right
> half of the data points (i.e. x and y values are positive), I get the
> curvefit returns parameter (0.5, 1.5), which means, the best fit curse
> is y=.5*x^(1.75). I know the fit should be symmetric, so the same curve
> SHOULD fit the other half. Now unto the left half side of the data
> set, curvefit does not work anymore, and here's why, x^(1.5)=x^(3/2)
> and when x is a negative number, IDL returns "NaN" because it can't
> take the square root of a negative number, hence the entire procedure
> will not work. I ended up having to throw away half of my data points,
> and I'm not very comfortable with that. Any idea how to go around it
> or suggest another function to do the same thing?
What about fitting the data to the function a*abs(x)^b ?

Paolo

>
> Besides, I've thought about using "polyfit", but if I remember
> correctly, polyfit only takes in one x value vs. one y value. Scatter
> plot has one x value vs. several y values. I don't think it'll
> work in my case, but I may be wrong...
>
> TIA (thanks in advance)
>
> Angie
>

Report message to a moderator

[Message index]

		Re: Newbie's question By: JD Smith on Thu, 20 October 2005 17:07
		Re: Newbie's question By: James Kuyper on Thu, 20 October 2005 15:08
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Thu, 20 October 2005 13:18
		Re: Newbie's question By: James Kuyper on Thu, 20 October 2005 11:41
		Re: Newbie's question By: Paolo Grigis on Thu, 20 October 2005 08:10
		Re: Newbie's question By: Paul Van Delst[1] on Thu, 20 October 2005 07:10
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Thu, 20 October 2005 06:53
		Re: Newbie's question By: Paul Van Delst[1] on Thu, 20 October 2005 06:44
		Re: Newbie's question By: peter.albert@gmx.de on Thu, 20 October 2005 04:02
		Re: Newbie's question By: Paolo Grigis on Thu, 20 October 2005 00:33
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Fri, 21 October 2005 09:58
		Re: Newbie's question By: James Kuyper on Fri, 21 October 2005 05:55

Previous Topic:	map_set miracle II
Next Topic:	Re: middle mouse button emulation in widget_draw

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Sun Oct 12 09:41:32 PDT 2025

Total time taken to generate the page: 1.51953 seconds