String function derivative

I'm trying to do

    F_diff_x = matlabFunction(diff(sym(f), 'x'));

when f is

    f = get(handles.equation,'String');

And sometimes I can calculate the derivative, sometimes not Becouse I get:

    ??? Error using ==> sym.sym>convertExpression at 2515
  Error: 'expression' expected [line 1, col 12]

Error in ==> sym.sym>convertChar at 2426
    s = convertExpression(x);

Error in ==> sym.sym>convertCharWithOption at 2410
        s = convertChar(x);

Error in ==> sym.sym>tomupad at 2164
        S = convertCharWithOption(x,a);

Error in ==> sym.sym>sym.sym at 111
                S.s = tomupad(x,'');

Error in ==> moo>bc_Callback at 383
F_diff_x = matlabFunction(diff(sym(f), 'x'));

How can I do this in a different way?

[Walter Roberson]

Could you give us an example of a string that you get the error for?

[InterNeo]

'sin(x)+cos(2*y)' - works

'sin(x)+x.^2+cos(2*y)' - doesn't work

Answer 1

[Walter Roberson]

The dot operators are not valid MuPAD code, and it is MuPAD code that must be sym()'d, not MATLAB code.

Not 'sin(x)+x.^2+cos(2*y)' but 'sin(x)+x^2+cos(2*y)'

[InterNeo]

Ok, but now I have another problem.

Because I'm using inline...

iF = inline(f)

contourf(X,Y,iF(X,Y),80,':');

How can I execute function without dots?

[Walter Roberson]

There is no good reason to mix inline and matlabFunction. Just use

iF = matlabfunction(sym(f),'X','Y');

If you feel you must use inline, then use a vectorize() call:

iF = inline(vectorize(f));

[InterNeo]

It works great. Thank you very much!

Answer 2

[Walter Roberson]

Before using f, check ischar(f) . I suspect you will find that in the cases you are getting errors for, that f will be a cell array of strings rather than a string itself.

The String property of a uicontrol style 'edit' will be a string or char array if the property was initialized with a string or with a char array, but the property will be a cell array of strings if the property was initialized with a cell array.

[InterNeo]

ischar(f) is true in both the above examples