[MATLAB GUI] GUI中计算器的程序编写

[huo951]

我打入的数字怎么在显示框里显示不了呢，显示框是用什么工具拉出来的呢？要怎么定义呢？希望高手解答一下，谢谢啊！
% --- Executes on button press in pushbutton4.
function pushbutton4_Callback(hObject, eventdata, handles)
% hObject handle to pushbutton4 (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
if handles.flag
handles.s_Exp(handles.i)=')';
handles.flag=0;
handles.i=handles.i+1;
end
handles.s_Exp(handles.i)='-';
handles.j=1;
handles.s_Num='-';
handles.i=handles.i+1;
guidata(handles.mainWnd,handles);
set(handles.pExp,'string',handles.s_Num)
运行的时候，说没定义mainWnd和pExp，怎么定义呢