Re: proposal for new m-file function

[Ben Abbott]

On Mar 22, 2012, at 10:37 AM, Ben Abbott wrote:

> On Mar 22, 2012, at 10:12 AM, Martin Helm wrote:
> 
>> Am 22.03.2012 13:17, schrieb Ben Abbott:
>>> On Mar 22, 2012, at 6:31 AM, Martin Helm wrote:
>>> 
>>>> Am 22.03.2012 10:43, schrieb Carlo de Falco:
>>>>> 
>>>>> 2012/3/22 <address@hidden
>>>>> <mailto:address@hidden>>
>>>>> 
>>>>>  Message: 2
>>>>>  Date: Wed, 21 Mar 2012 20:33:01 -0400
>>>>>  From: Ben Abbott <address@hidden <mailto:address@hidden>>
>>>>>  To: octave maintainers mailing list <address@hidden
>>>>>  <mailto:address@hidden>>
>>>>>  Subject: proposal for new m-file function
>>>>>  Message-ID: <address@hidden
>>>>>  <mailto:address@hidden>>
>>>>>  Content-Type: text/plain; charset="us-ascii"
>>>>> 
>>>>>  A user's question about fixed points piecewise-linear fitting led
>>>>>  to the development of a function looks to be a good fit for
>>>>>  Octave's core.
>>>>> 
>>>>> 
>>>>>   
>>>>> https://mailman.cae.wisc.edu/pipermail/help-octave/2012-March/050900.html
>>>>> 
>>>>>  The idea was to fit a piece-wise polynomial to a set of data.
>>>>>  After some discussion between myself and Martin Helm, the attached
>>>>>  ppfit.m was produced. The name was chosen to match the existing
>>>>>  ppval().
>>>>> 
>>>>>  The function provides a least-squares fit of a 1D interpolation
>>>>>  with specified break positions to a set of data.
>>>>> 
>>>>>  Demos and tests are included.
>>>>> 
>>>>>  Any concern about adding this to Octave's core ?
>>>>> 
>>>>>  Ben
>>>>> 
>>>>> 
>>>>> googling for the name 'ppfit' I found this function:
>>>>> 
>>>>> https://www.assembla.com/code/zaxxon_scripts/subversion/nodes/trunk/project/scripts/ppfit.m
>>>>> 
>>>>> which seems to do the same as yours with the additional option of
>>>>> returning a spline with N continuous derivatives
>>>>> Would it be possible to add that option to your code?
>>>>> The license of the linked function looks like BSD so it should be no
>>>>> problem to get the code from there.
>>>>> c.
>>>> Wouldn't it be much cleaner to add that additional fitting options
>>>> (quadratics and higher order splines) to interp1 (without breaking
>>>> matlab compatibility of course)?
>>>> I think that is the place where such functionality should naturally
>>>> live. I could look at it over the weekend and propose a patch for it.
>>> Ok. I'll wait on your changeset.
>>> 
>>> Is it possible for interp1 to return the available methods and whether the 
>>> underlying interpolants are linear functions ?
>>> 
>>> Ben
>> 
>> What do you have in mind, is something like interp1("methods") which
>> returns some cellarray for example with the method names and a flag
>> indicating linear/nonlinear an option?
> 
> Something like below?
> 
>       [methods, linear] = interp1 ("methods")

I just realized I can easily check if the interpolant is linear by ...

        yi = interp1 (xi, eye (numel (xi)), x, "method");
        s = sum (yi, 2);
        s = s / mean (s);
        if (sqrt (mean (abs (s).^2)) < sqrt (eps))
                islinear = true;
        endif

So, I'll only need to have access to the different methods.

        methods =  interp1 ("methods");

Ben