how to convert sym to double?

Question

Physiker192 2014-9-13

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https://ww2.mathworks.cn/matlabcentral/answers/154605-how-to-convert-sym-to-double

评论： Mischa Kim 2014-9-15

hello, i have a system of n equations which i need to differentiate, so the variables must be symbolic variables, and i stored those equations in an array, my problem is that i want to solve the system using the fsolve but i am not able to convert my symbolic variables to double, so let my code be like that:

function dif = mydifffunction2 (~)
n=input('n=');
h=3;
dif=sym(zeros(1,n));
F=0;
x=sym(zeros(1,n));
for k=1:n
  x(k) = sym(sprintf('x%d',k));
end
for i=1:n
  D= 0.5;
%     V=0;
%     b=0;
  for j=1:n
      if (mod(j,2)==0)
         D = D + cos(h*x(j));
      else
         D = D - cos(h*x(j)); 
      end 
  end
  D=(2*sqrt(2)/(pi*h))*abs(D);
  F = F + 1/h*(D^2)
  h=h+2;
end
for j=1:n
  dif(j)=diff(F,x(j),1);
end

how can i convert each of these variables to double??? Thanks in advance.

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Answer 1

Mischa Kim 2014-9-13

2
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/154605-how-to-convert-sym-to-double#answer_151577

编辑：Mischa Kim 2014-9-15

在 MATLAB Online 中打开

Physiker, one approach is to convert the symbolic equation into a MATLAB function (handle) and then use fsolve. E.g.

my_dif = mydifffunction2;

and then use

g = matlabFunction(my_dif);

to create the MATLAB function handle, which in turn needs to be converted

h = my_vectorize(g);

to be used in

fsolve(h,[1,1,1]) % e.g. with n = 3

my_vectorize could look something like:

 function f_vec = my_vectorize(fun)
    f_vec = @fun_vectorize;
    function out = fun_vectorize(x)
        x   = num2cell(x);
        out = fun(x{:});
    end
 end

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Physiker192 2014-9-14

well i am indeed sorry to bother you but i didn't get it. i'm not used to work with that type of functions so i am a little bit lost. i used what you gave me but it was useless (i mean i couldn't get any answer)

Mischa Kim 2014-9-15

To start with I'd just run the whole thing from the command window:

Execute mydifffunction2, e.g. with n=3
Run the matlabFunction command
Run my_vectorize. For this you need to have the function (see code above) saved in a dir which is in the MATLAB path
Finally, run fsolve