You cannot do that. You cannot differentiate character vectors (usefully) or string objects, or function handles.
Given a function handle, you could evaluate it at several locations and use the results to estimate numeric gradient.
Given a character vector, if you have the Symbolic Toolbox, you could potentially use str2sym() to create a symbolic expression that you could then differentiate with diff() . Be sure to specify the variable to differentiate with respect to. You can assign the result to a variable and char() the expression to get something that you can set() as the string property of a uicontrol.
Note: if you have R2017a or earlier, you need to use sym() instead of str2sym() . When you use sym() then the language is not exactly the same as MATLAB and it is not exactly the same as MuPAD either.
for 
and 
for 
?
Best Answer