Differentiation error - conversion of variable type problem - 'struct' to function

I ran this code:

   syms theta %create symbolic variable theta
assume(theta,'real') %theta is real
f = fit_v2; %calling our fitted polynomials

g = diff(f,theta);
g0=solve(g,theta);
double(g0);

I get this error:

Error using sym>tomupad (line 1251)
Unable to convert 'struct' to 'sym'.

Error in sym (line 215)
              S.s = tomupad(x);

Error in sym/privResolveArgs (line 988)
                  argout{k} = sym(arg);

Error in sym/diff (line 21)
args = privResolveArgs(S,varargin{:});

Error in code (line 54)
g = diff(f,theta);

please help! Thanks!

[Tam Ho]

I think I know why.

fit_v2 is a 'struct' variable type:

f = 

struct with fields:

     type: 'polynomial degree 10'
    coeff: [0.0012 4.2344e-04 -0.0101 -0.0033 0.0303 0.0097 -0.0373 -0.0126 0.#### 0.#### #.####]

how can I conveniently convert fit_v2 to get f in function form and ready to be differentiated?

Answer 1

[Walter Roberson]

poly = poly2sym(f.coeff, theta);
g = diff(poly, theta)

However, you do not need the symbolic toolbox to differentiate a polynomial with numeric terms, and you do not need the symbolic toolbox to get the roots of a polynomial with numeric terms (you can use roots())