error Attempt to add (variable) to a static workspace, when it is not in workspace?

Why does this function generate

   function bug
   clear all;
   syms y(idx)  
   foo()

      function r = foo()
          r=y(1);
      end  
   end

error is

Error using assignin
Attempt to add "y" to a static workspace.
 See Variables in Nested and Anonymous Functions.

Error in syms (line 106)
        assignin('caller', name, symfun(xsym, [vars{:}]));

Error in bug (line 3)
syms y(idx)

I do not have `y` in workspace. It is defined inside function, and I want to access it from inside an inner function. So why is Matlab saying the above? If I make the inner function another separate, but inside the same m file. It works.

function bug
clear all;
syms y(idx)
foo(y)
end

function r = foo(y)
        r=y(1);
end

I am using Matlab 2015a, windows 7.

Answer 1

[Steven Lord]

I responded to this question in the newsgroup.

[Stephen23]

Newsgroup is no longer provided on MATLAB Central, this text copied from mathforum.org:

It's the way you're adding y to the workspace that's the problem. A simpler reproduction step for this behavior:

function NOTbug
eval('y = 42;')
foo()

function r = foo()
r = y(1);
end
end

This is everyone's favorite "poofing" problem combined with the restrictions on what you're allowed to do in the workspace of a nested function.

> It is defined inside function, and I want to access it from inside an
> inner function. So why
> is Matlab saying the above? If I make the inner function
> another separate, but inside the same m file. It works.

Yes, because the rules are different for subfunctions than they are for nested functions. The documentation page to which the error message linked is:

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

Why is this relevant, you ask, since you're not using LOAD, EVAL, EVALIN, ASSIGNIN, calling a script, or defining variables in the debugger? YOU aren't doing any of those things ... but SYMS is. Specifically, SYMS uses ASSIGNIN to create the symbolic variable in its caller's workspace. [That's the reason one of the tips on its documentation page says not to use it to create variables with the same name as a MATLAB function. That would be "poofing", just like if you used EVAL to create a variable with that name or LOADed such a variable from a MAT-file.] Normally this use of ASSIGNIN isn't a problem, but in a nested function it is.

The solution is to define the variables in your symbolic expression explicitly in the code. Changing the variables isn't a problem; "poofing" them into the workspace is.

function NOTbug
y = [];
idx = [];
syms y(idx)
foo()

function r = foo()
r = y(1);
end
end

--

Steve Lord

[e-mail]

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