MATLAB: How to calculate gradients of a symbolic function

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Hi,
I have a symbolic function of the form f = 2*y*z*sin(x) + 3*x*sin(z)*cos(y) and want to calcuate gradients with respect to x, y and z. I cannot define syms x y z before creating the function as its an input. Therefore, I defined syms x y z afterwards but its not working. The follwing error appeared:
Unable to convert expression into double array.
How do I calcuate gradients of this symbolic function?

Best Answer

%construct the input
syms x y z
f(x,y,z) = 2*y*z*sin(x) + 3*x*sin(z)*cos(y);
%and assume you are passing f into a different function so x y z are not still around
g = yadayada(f);
function g = yadayada(f)
  %so we have an input symbolic function but for some reason
  %we are not in the context where the variables came from. 
  %it turns out to be as simple as
  g = gradient(f);
  %or for greater certainty you could
  g = gradient(f, symvar(f));  %which is what the simple call does
  %or perhaps you have a reason to do
  syms x y z
  g = gradient(f(x, y, z), [x y z])
  %for example it might be a symbolic function in three other
  %variables that you want to re-label as x, y, z.
end
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