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

Question

Tam Ho 2017-12-29

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/374854-differentiation-error-conversion-of-variable-type-problem-struct-to-function

回答： Walter Roberson 2017-12-29

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!

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Tam Ho 2017-12-29

在 MATLAB Online 中打开

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?

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Walter Roberson 2017-12-29

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/374854-differentiation-error-conversion-of-variable-type-problem-struct-to-function#answer_298012

在 MATLAB Online 中打开

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())