What is the cause of this weird phenomenon with symbolic variables and nested function?

This makes no sense to me.

This script returns the symbolic variable 'symb' as expected

funct1

function funct1

syms symb
disp(symb)

end

Adding a nested function causes syms to return an error, even if the second function isn't called

funct1

function funct1

syms symb
disp(symb)

    function funct2
        %nothing
    end

end

Error using assignin

Attempt to add "symb" to a static workspace.

See Variables in Nested and Anonymous Functions.

Error in syms (line 257)

assignin('caller', x, xsym);

Error in weird>funct1 (line 5)

syms symb

Error in weird (line 1)

funct1

However, if you assign a value to symb anywhere within the parent function, the error is avoided. Even if the assignment of symb is never reached by the function.

funct1

function funct1

syms symb
disp(symb)

    function funct2
        %nothing
    end

return
symb=0;

end

Why? What is going on here? Why does adding a nested function to a function prevent you from creating symbolic variables, and why does defining the variable in an unreachable part of the parent function make it possible again?

Answer 1

[Walter Roberson]

https://www.mathworks.com/help/matlab/matlab_prog/variables-in-nested-and-anonymous-functions.html

Normally if you have an end matching a function statement then the workspace is static and nothing can be added to it dynamically by assignin('caller') such as is used by syms. syms is not a keyword: it is an ordinary function that uses assignin to create the variables.

It appears that the case of a function inside a script might be an exception that is not considered static, unless the function has a nested function.

The assignment to the variable provides enough transparency that the static workspace mechanism is able to decide that the variable should exist.

However you should not count on this continuing to work exactly the same way: it is likely that at some point MATLAB will be changed so that only variables plainly assigned before the function call are considered to be known, or perhaps only variables plainly assigned to before the sub-function definition. Mathworks has been actively making the rules stricter recently.

[Philip M]

Dude, you rock. MATLAB Answers would be just about worthless without you. Thanks so much for all your help.

[Steven Lord]

If you're defining symbolic variables inside a function like this consider using the sym function instead of syms.

symb = sym('symb');

This avoids "poofing" variables into the workspace like syms does and so should be safe regardless of whether or not you're in the context of a nested function.

[Walter Roberson]

Steven Lord's suggestion is a good one for working with syms()

[James Morrison]

@Steven Lord Once again, helped me make better functions. Thank you!