Custom m-file function in fitting tool

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Thomas
Thomas on 23 Jan 2011
Dear Sir/Madam, I have a question regarding the fitting tool. I was wondering if it is possible to use a custom function from a m-file in an equation used for fitting.
Thanks in advance, Thomas van der Vliet
  1 Comment
bym
bym on 23 Jan 2011
the basic tool does not provide for custom functions. If you could provide more information about you data and custom function, perhaps there is a work around

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Answers (1)

Jon Cherrie
Jon Cherrie on 30 Mar 2011
Yes, it is possible to use a Custom Equation in CFTOOL defined from a MATLAB file. Here is an example. Start by creating a file like this:
function y = my_curve( x, a, b, c )
y = zeros( size( x ) );
isLeft = x < 0.5;
y(isLeft) = a + b*x(isLeft);
y(~isLeft) = a + c*x(~isLeft);
end
The key things about this file are
  1. it has an input for the x-data, x
  2. it has one input for each coefficient, a, b and c.
  3. the y-data, y, that is returned is the same size as x
  4. it will work when the x-data, x, is a vector.
In CFTOOL you can then follow the usual steps to create a Custom Equation. Choose a General rather than Linear equation. For the equation, enter my_curve( x, a, b, c ).
If you need a surface, then you can do all the same stuff in SFTOOL, just add an input argument for y-data and make sure that your function works when the x- and y-data are vectors.

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