Switch statement not recognizing a valid input

Question

James Dossett 2016-2-26

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/270201-switch-statement-not-recognizing-a-valid-input

评论： Jan 2016-2-28

I'm making a GUI and it has this code:

% --- Executes on selection change in AverageItem.
function AverageItem_Callback(hObject, eventdata, handles)
% hObject    handle to AverageItem (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 AverageItem contents as cell array
%        contents{get(hObject,'Value')} returns selected item from AverageItem
% Determine the selected data set.
str = get(hObject, 'String');
val = get(hObject,'Value');
% Set current data to the selected data set.
switch str{val};
case 'Phase (Degrees)' % User selects Phase (Degrees)
   AverageItem = 1;
case 'Magnitude (V/m)' % User selects Magnitude (V/m)
   AverageItem = 2;
case 'RCS (dBsm)' % User selects RCS (dBsm)
   AverageItem = 3;
otherwise
    error(['Unknown Choice: ' str{val}])
end
% Save the data to the workspace
assignin('base','AverageItem',AverageItem);

The problem is, whenever I choose 'RCS (dBsm)' in the GUI, it does not set AverageItem = 3. It just displays the 'Unknown Choice:' error. Any ideas? Phase and Magnitude work just fine. Thanks!

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Answer 1

Titus Edelhofer 2016-2-26

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Hi,

it must be then that the two strings are not equal. Put a breakpoint before the switch and compare str{val} with 'RCS (dBsm)'. Sometimes it helps to convert to doubles (i.e. ascii numbers) to see the difference.

double('RCS (dBsm)')
ans =
  82    67    83    32    40   100    66   115   109    41

A tab might look like one space for example.

Titus

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Titus Edelhofer 2016-2-28

:)

Jan 2016-2-28

在 MATLAB Online 中打开

To catch similar problems I add delimiters to the error messages:

error('Unknown Choice: [%s]', str{val})

This shows trailing spaces and does not fail in case of escape characters inside the string. The output of ['Unknown Choice: ' str] is undefined, if str contains e.g. '%s' and the output is unexpected, when control characters like \t, \n or the backspace \b are included.

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