Undefined function 'eval' for input arguments of type 'double'.

When using the code below:

function [Df]=difc(f,u,h)
syms x
x=u+h; lp=eval(f);
x=u-h; ll=eval(f);

Df=(lp-ll)/(2*h);
end

, the same error keeps appearing ''Undefined function 'eval' for input arguments of type 'double'.''

I would like to know what´s the problem and what can I do to solve it, in order to have a working code.

Thankyou very much

[Walter Roberson]

What are you passing in to the function?

Note: your assignments to "x" are undoing the effect of "syms x".

[Ana Luisa]

I want to pass the values lp=u+h and ll=u-h, being that u and h are real numbers given by the user

Answer 1

[C.J. Harris]

You are getting that error because you are trying to evaluate a double. A double is just a double - there is nothing there to evaluate.

What are you expecting the 'eval' function to do?

[Ana Luisa]

I'm trying to calculate the numerical first derivative of a function f using the central differences method with an error of O(h^2).

[Walter Roberson]

What are you passing in to your routine?