switch function in GUI

I am learning GUI through this youtube Video which is uploaded by matlab.

http://www.youtube.com/watch?v=czArO6PhdHM&src_vid=D_hmws6dwgg&feature=iv&annotation_id=annotation_896325

I tried to follow and write down everything but I got error message while using the pop up button. The related error is the usage of switch function.

here are parts of the code of the GUI.

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% --- Executes on selection change in popup1.
function popup1_Callback(hObject, eventdata, handles)
% hObject    handle to popup1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: contents = cellstr(get(hObject,'String')) returns popup1 contents as cell array
%        contents{get(hObject,'Value')} returns selected item from popup1
val = get (hObject,'Value');
str = get (hObject,'String');
switch str(val)
    case 'peaks'
        handles.current_data = handles.peaks;
    case 'membrane'
        handles.current_data = handles.membrane;
    case 'sinc'
        handles.current_data = handles.sinc;
end
guidata(hObject, handles);

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the error messages are:

??? SWITCH expression must be a scalar or string constant.

Error in ==> test2>popup1_Callback at 204 switch str(val)

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

Error in ==> test2 at 42 gui_mainfcn(gui_State, varargin{:});

Error in ==> @(hObject,eventdata)test2('popup1_Callback',hObject,eventdata,guidata(hObject))

??? Error while evaluating uicontrol Callback

- - - - - - - - - - - - Thank you.

[bhawesh pathak]

Write Switch str{Val}; Instead of str(Val)