Custom m-file function in fitting tool

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

[bym]

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

Answer 1

[Jon Cherrie]

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.