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

2019 8 26
1 个回答
更新时间：2019 8 26
5 次查看（30 天）

0 个投票

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?

5 个评论
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Walter Roberson
Walter Roberson 2019-8-26
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
Konrad Brine 2019-8-26
编辑：Konrad Brine 2019-8-26
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
Walter Roberson 2019-8-26
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
Konrad Brine 2019-8-26
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?

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回答（1 个）

Konrad Brine
Konrad Brine 2019-8-26

0 个投票

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)));

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