Matlab: Convert function handle to string preserve parameters

Question

Any variables and their values originally stored in a function handle when it was created are lost if you convert the function handle to a string and back again using the func2str and str2func functions.

Is there a way to overcome this ?

Like

y = 1;
fun = @(x) x + y
fun_str = func2str(fun);

clear y

fun = eval(fun_str);
result = fun(12); % This doesn't work

Concrete I have a measurement method creating some data which then gets fitted (with the help of fminsearch) against some custom functions for an alignment procedure. I would now like to store all the data for future analysis.

Since Data normally survives longer than code, I would like to make it independent from any source file. That's why I also convert the function handle to string. In the string only matlab builtin functions are allowed to be called.

Now what I can do of course is to also store y in the example. But I was wondering if there is a more elegant solution to this where I would not need to do this.

That's not quite true. It's stored with the anonymous function, and then gets lost when you convert it to a string. You can clear y and still call the anonymous function. — Kim
– Kim, Commented Jul 14, 2015 at 18:58

TroyHaskin · Accepted Answer · 2015-07-14 19:02:36Z

1

The problem stems from Matlab storing the value of y within a workspace local to the anonymous function while keeping the symbol y part of the function definition. Then, as you indicated, the func2str function converts the definition without replacing the symbol with its value. Since your example indicates that you want to function handle to work even after clearing the variable y (both value and symbol) from the current workspace, I'd suggest two paths forward.

The first is to create the function handle with str2func using the value of y instead of letting the anonymous function hold it:

fun = str2func(['@(x) x + ',num2str(y)]);

The other is to write a variant of func2str that does the value replacement after converting the function to a string. The functions function can be used to acquire the anonymous function's workspace, and some regular expressions can be used to replace the symbols with literals. Here is a minimal working example:

function str = func2strrep(fun)

    %   Grab workspace information
    info      = functions(fun);
    workspace = info.workspace{1};
    symbols = fieldnames(workspace);

    %   Initialize string
    str = func2str(fun);

    %   Replace workspace variables with numeric literals
    if not(isempty(symbols))

        seps = '[\s\+\-\*\/\\\^\:\=\>\<]';
        expr = @(s) ['(',seps,'{1})',s,'(',seps,'{1})'];

        for k = 1:numel(symbols)

            symbol = symbols{k};
            value  = num2str(workspace.(symbol));

            %   Check end of string
            str = regexprep(str,['(',seps,'{1})',symbol,'$'],['$1',value]);

            %   Check bulk
            stro = regexprep(str,expr(symbol),['$1',value,'$2']);
            %
            % Put in loop for repeated replacement since 
            % 'all' is not working as I thought it would.
            while not(strcmp(stro,str))
                str  = stro;
                stro = regexprep(str,expr(symbol),['$1',value,'$2']);
            end

        end
    end
end

I'm not that talented with regular expressions, so the replacement method may need some tweaking (particularly since regexprep didn't replace what I thought would be all matches for repeated symbols). However, this first go produces

>> y = 1;
>> z = 2;
>> yy = 3;
>> f = @(x) x + y + y + z + yy;
>> func2strrep(f)
ans =
@(x)x+1+1+2+3

Both approaches rely on num2str and, therefore, are only good for scalar values. For vector values, a num2str variant that produces a valid array literal in string form is required for the method to work.

The second approach also only works for simple variable storage as in the example. Nested handles and closures and the-like will require a more complicated algorithm for symbol-value determination and possibly replacement.

answered Jul 14, 2015 at 19:02

TroyHaskin

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2 Comments

magu_ Over a year ago

Thank you for your answer. I was actually looking to avoid strrep and regex, since I would like to give the user the possibility to give a user defined function (of course not linking to a file (matlab builtins are fine)). As such I cannot be sure about the internal structure and I'm afraid I would replace too much, but maybe with a very smart regex this could be done. I'll wait a little longer to if somebody else comes up with an other solution. Thanks again for taking the time.

TroyHaskin Over a year ago

Could you explain that use case a little more and possibly provide an additional example in your question? I think the answer to your question is highly dependent on what exactly you are attempting to do.

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