how to extract the formula from fit function?

I want to use fit function to get the formula with its coefficient for my data. so i have used: [curve, goodness, output] = fit(fk,Rfft,'linearinterp'); in case of 'poly1' or 'poly2' the curve variable (which is in cfit format) shows the formula with its coefficients however when i use linearinterp the curve does not show the formula, it just says "piecewise polynomial". i need to get the formula with its coefficients. does anyone have any idea how to get the formula?

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Answer 1

[Vishwas]

There really isn't a formula for the surface. It's a piecewise linear surface so it's a single formula for a plane at each location of your data making it many formulas altogether.

Answer 2

[Bowen Song]

Treat it as a string... lol check this out

[fitresult{1}, gof(1)] = fit( [xData, yData], zData, ft );

... from above you get your sfit or what not

f = string(formula(fitresult{1})); // get the formular in string format
names = coeffnames(fitresult{1}); // get the coeff names
vals = coeffvalues(fitresult{1}); // get those values
for i = 1:length(names)
    f = strrep(f,string(names(i)),string(vals(i))); // replace the coeff names with values
end

eval(f) // muhaha

[Walter Roberson]

I don't think this will work for piecewise polynomials. (At least not in MATLAB; you appear to be using Octave.)

[Xiyan Li]

I wonder how to get the equation out after piecewise fitting? It makes no sense to me without gettting the equation out.

[Walter Roberson]

@Xiyan Li

The output is a piecewise polynomial. It does not have a single equation. It is effectively a list of equations that are each valid over a specific range.