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

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Tam Ho
Tam Ho on 29 Dec 2017
Answered: Walter Roberson on 29 Dec 2017
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);
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!
  1 Comment
Tam Ho
Tam Ho on 29 Dec 2017
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?

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Accepted Answer

Walter Roberson
Walter Roberson on 29 Dec 2017
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())

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