diff(f) saying "Undefined function 'diff' for input arguments of type 'function_handle'." and diff(f(x)) giving"Undefined function or variable 'x'."

I am trying to differentiate an equation f(x) which is input as as function_handle (e.g: @(x) x^2) and evaluate at different points

When I call the function this snippet is in I make sure to first call syms x;

df=str2func(['@(x)' char(diff(f(x)))]);
X=repelem(Xlist,2);
%Creating the list of Y values and derivative values from the function and
%the points
Ylist=zeros(1,numel(X));
for i=1:numel(X)
    Ylist(1,i)=vpa(subs(f,x,X(1,i)));
end
Dlist=zeros(1,numel(Xlist));
for i=1:numel(Xlist)
    Dlist(1,i)=vpa(subs(df,x,Xlist(1,i)));
end

Whenever I use this version, I get the error "Undefined function or variable 'x'."

However, when I remove the '(x)' from f(x) in the first line and write:

df=str2func(['@(x)' char(diff(f))]);
X=repelem(Xlist,2);
%Creating the list of Y values and derivative values from the function and
%the points
Ylist=zeros(1,numel(X));
for i=1:numel(X)
    Ylist(1,i)=vpa(subs(f,x,X(1,i)));
end
Dlist=zeros(1,numel(Xlist));
for i=1:numel(Xlist)
    Dlist(1,i)=vpa(subs(df,x,Xlist(1,i)));
end

I get the error: "Undefined function 'diff' for input arguments of type 'function_handle'."

Are either of these syntaxes right? If so, why I am getting the errors?

[Walter Roberson]

Why not leave df as a symbolic expression instead making it a function handle?

Note by the way that you should be using matlabFunction to convert to function handle. There are some subtle bugs in using char() the way you do.

[Konrad Brine]

If I can leave it as a symbolic expression and still create Ylist and Dlist that would be fine too. How would I do that?

Changing from char to matlabFunction:

df=str2func(['@(x)' matlabFunction(diff(f))]);
X=repelem(Xlist,2);
%Creating the list of Y values and derivative values from the function and
%the points
Ylist=zeros(1,numel(X));
for i=1:numel(X)
    Ylist(1,i)=vpa(subs(f,x,X(1,i)));
end
Dlist=zeros(1,numel(Xlist));
for i=1:numel(Xlist)
    Dlist(1,i)=vpa(subs(df,x,Xlist(1,i)));
end

Edit: this gives the same errors.

[Walter Roberson]

If given a function handle then in most cases (but not all) you can pass a symbolic variable to the function to get out the symbolic expression, which you probably do not need to convert back to function handle.

[Konrad Brine]

Assuming I don't turn it back into a function handle, how do I pass the symbolic variable (x) into f to make it a symbolic expressions which diff(f(x)) can differentiate?

[Walter Roberson]

syms x
Fx = f(x) ;
df = diff(Fx, x);

Answer 1

[Konrad Brine]

Figured it out entirely thanks to Walter Robinson.

syms x;
df=matlabFunction(diff(f(x)));
X=repelem(Xlist,2);
%Creating the list of Y values and derivative values from the function and
%the points
    Ylist=zeros(1,numel(X));
    for i=1:numel(X)
        Ylist(1,i)=f(i);
    end
    Dlist=zeros(1,numel(Xlist));
    for i=1:numel(Xlist)
        Dlist(1,i)=df(i);
    end

I added syms x; above the df line, and changed the df line to matlabFunction(diff(f(x)));