MATLAB: How to get a symbolic differentiation of cfit/sfit functions

Question

Hi everyone,

I am trying to get the symbolic partial derivative of a sfit function. Best case would be having the differentition as an sfit function, too.

Thanks in advance!

EDIT:

Here's an example:

%data
x = [0.5850;0.0734;0.8223;0.7229;0.9259;0.4926];
y = [0.6549;0.8901;0.5385;0.2822;0.9760;0.0364];
z = [0.3262;0.9730;0.3650;0.3091;0.1209;0.9158];
%surface fit
sf = fit([x, y],z,'poly22')
plot(sf)
%      Linear model Poly22:
%      sf(x,y) = p00 + p10*x + p01*y + p20*x^2 + p11*x*y + p02*y^2
%      Coefficients:
%        p00 =       5.048
%        p10 =      -11.04
%        p01 =      -1.326
%        p20 =       5.179
%        p11 =       5.682
%        p02 =      -3.135

Now I could use the differentiate method to get the derivative at specific points but I would like to have the derived function, like this (witthout having to derive it manually):

%      sf_x(x,y) = p10 + 2*p20*x + p11*y

Answer 1

When sf is the fit object,

f = subs(str2sym(formula(sf)),coeffnames(sf),num2cell(coeffvalues(sf).'));
vars = sym(indepnames(sf));
diff(f, vars(1))