How to use str2func with a function handle in the string

Hi,

I want to insert a function handle in the input string of str2func but I get an error.

string1 = '@(x)sin(x)';
A = str2func(string1);
string2 = '@(x,y)(A(x) + sin(y))'
B  = str2func(string2);

B(pi/2,pi/2)

It says: Undefined function or variable 'A'.

How can I fix it?

Thanks

Answer 1

[Tommy]

From the documentation for str2func,

"Workspace variables are not available to the str2func function."

See this answer. You could do something like this:

string1 = '@(x)sin(x)';
A = str2func(string1);
string2 = ['@(y)(' num2str(feval(A,pi/2)) ' + sin(y))'];
B = str2func(string2);

B(pi/2)

[dario]

Thanks Tommy,

yes, I was having a look about the workspace issue.

But how to keep both variables in B: B(x,y)?

[dario]

string1 = '@(x)sin(x)';
A = str2func(string1);
A = func2str(A)
A = A(5:end);
string2 = ['@(x,y)(', A, '+ sin(y))']
B  = str2func(string2);

B(pi/2,pi/2)

Not sure if there is anything neater and it still does not allow me to use a function handle defined in the main worksapce.

[Tommy]

That was going to be my suggestion, although using strfind to find the relevant part of A rather than assuming there is only a single character of input arguments:

string1 = '@(x)sin(x)';
idx = strfind(string1, ')');
string2 = ['@(x,y)(' string1(idx+1:end) ' + sin(y))'];
B = str2func(string2);

B(pi/2,pi/2)

Either way, I think you might only have the two options: (1) evaluate the function before constructing B (like in my original answer) or (2) trim A to just the executable part (like in our comments). There may be some other way, but I can't think of it off the top of my head.

Can you explain what you mean by "it still does not allow me to use a function handle defined in the main workspace"?

[dario]

Thanks.

I mean that I'd prefer to work directly on the handle instead of retreiving the original function.

I'll leave it as not answered for now in case someone has a better idea.

[Tommy]

Gotcha. There's always eval...

string1 = '@(x)sin(x)';
A = str2func(string1);
string2 = '@(x,y)(A(x) + sin(y))';
B = eval(string2);

B(pi/2,pi/2)

I guess I ignore eval by default, but it definitely works here. I would still recommend either of the previous answers.

Or here's a somewhat interesting possibility:

string1 = '@(x)sin(x)';
A = str2func(string1);
string2 = @(x) (['@(y)(' num2str(feval(A,x)) ' + sin(y))']);
B = @(x,y) feval(str2func(string2(x)), y);

B(pi/2, pi/2)

Maybe it can be simplified.

Good idea, I'm curious!

[dario]

Thanks for your support