Check if a function handle is valid

I use str2fun(...) to create a function handle from a user-inputted string. I want to be able to run a check to make sure the function created is valid and no typos were made. I could not find a way to do this. Any suggestions?

Thanks, Ryan

MATLAB R2014a

[Steven Lord]

What is a "valid" function handle?

f = @quit; % NOTE: Don't execute f if you have data in your MATLAB session you need!

f is a "valid" function handle in that it won't error when called. But you may not want users to enter 'quit' as the user-inputted string and blindly evaluate that function handle -- the "Bobby Tables" scenario.

g = @() zeros(1:12); % MAY error or MAY succeed and take a LONG time doing so

This may successfully execute (though it may take a while) or may throw an error depending on your computer's memory availability and your MATLAB settings. Is it "valid"?

Answer 1

[Kurt Schmutz]

Hi Ryan

To CHECK whether a function handle is valid or not (without trial-and-error) you can do the following:

    f = str2func('nameofthefunction')   

    fh = functions(f)

    if isempty(fh.file)
        % invalid :(
    else
        % valid :)
    end

Note: file "nameofthefunction.m" must be in the current folder

Cheers, Kurt

MATLAB R2013b

[Sean de Wolski]

This could fail if the function is an anonymous function which doesn't have a file.

x = @(x)x^2;

[Kurt Schmutz]

Hi Sean,

obviously it will fail in this case!

Thanks for the hint :)

[Ryan]

Thanks Kurt. This worked for me since the function being used will never be anonymous.

Ryan

[David Sinex]

This fails if the function is a class member.

Answer 2

[Sean de Wolski]

try
  fun()
catch
  disp 'didn''t work'
end

Try it and catch failures. Or provide a listbox with known functions to remove the human typing element.

Answer 3

[Ryan]

I thought about the try, catch method but I want to be more descriptive in the error returned. I do not know all the known functions as it is a user created function that they are using instead of a default one.

[Sean de Wolski]

Well if you catch the MException, you could look at the message of it.

...
catch ME
  ME.message
end

Answer 4

[Jos (10584)]

You might be interested in the function ISFUNCTION .