How to insert a function into a push button

Hello, I'm trying to make a GUI for solving integral using trapezoidal rule. So far, I've done until the calculate push button which gives an error of

??? Undefined function or variable 'x'.

Error in ==> Trapezoidal_rule>calculate_Callback at 81
calculate = trapz(x,equation)

Error in ==> gui_mainfcn at 96
      feval(varargin{:});

Error in ==> Trapezoidal_rule at 42
  gui_mainfcn(gui_State, varargin{:});

Error in ==> @(hObject,eventdata)Trapezoidal_rule('calculate_Callback',hObject,eventdata,guidata(hObject))

??? Error while evaluating uicontrol Callback

However, in the m.file I've specified the x value as

function value_n_Callback(hObject, eventdata, handles)
% hObject    handle to value_n (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of value_n as text
%        str2double(get(hObject,'String')) returns contents of value_n as a double
value_n = get(hObject, 'string');
h = (value_b - value_a)/ (value_n);
x = value_a:h:value_b;

How do I overcome this?

Your help is greatly appreciated

Answer 1

[Mahdi]

One of your problems might be that you are defining x in two different sections of the GUI. What you can do is pass the handles by using these lines and then calling x again into the other function, so you should write

handles.x=value_a:h:value_b;
guidata(hObject, handles)

Then, right before you run the calculate=trapz(x,equation), insert this line

x=handles.x;