FIT function output structure

I am using the fit function for the first time, and I notice that all the examples pass the returned object into the PLOT function. However I am not interested in plotting the results, but rather I need to get the fitted curve into an array. I don't see any documentation that show how to do this.

Answer 1

[Sreeja Banerjee]

Hi Bob,

The function FIT returns a cfit object for curves or a sfit object for surfaces. You can learn more about the objects in the documentation:

1. CFIT Object: http://www.mathworks.com/help/curvefit/cfit.html 2. SFIT object: http://www.mathworks.com/help/curvefit/sfit.html

In short, the CFIT object will return the model formula and the coefficients. For example,

>> load census;
>> f=fit(cdate,pop,'poly2')

f = 

     Linear model Poly2:
     f(x) = p1*x^2 + p2*x + p3
     Coefficients (with 95% confidence bounds):
       p1 =    0.006541  (0.006124, 0.006958)
       p2 =      -23.51  (-25.09, -21.93)
       p3 =   2.113e+04  (1.964e+04, 2.262e+04)

I assume from your question that you want to extract the coefficients into an array. You can do that by using the following syntax:

>> coeff = coeffvalues(f)

coeff =

     1.0e+04 *

      0.0000   -0.0024    2.1130

You will need use the COEFFVALUES function, details of which you can find in the following link: http://www.mathworks.com/help/curvefit/coeffvalues.html

If you also need to extract the formula and the coefficient names then you may use the following functions:

>> formula(f)

ans =

p1*x^2 + p2*x + p3

>> coeffnames(f)

ans = 

      'p1'
      'p2'
      'p3'

[Yanis Arencibia]

If I may add to Sreeja's answer, a fast and nice way to access the coefficients is as follows:

>> load census;

>> f=fit(cdate,pop,'poly2');

Now you can call (or reuse in another function) the coeffs just typing:

>> f.p1

>> f.p2

>> f.p3

I hope this helps.