Why is empty string test failing?

.

This GUI's source code is giving me error message:

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

 % show open file dialog
 [filename, pathname] = uigetfile({ '*.jpg'; '*.png';'*.bmp';'*.jpeg'; }, ...
                                'Open image', ...
                                '' ...
                                );

 % obtain image-file's path
 imagePath = strcat(pathname, filename);

% this test is failing

 if (imagePath ~= '')    
    image = imread(imagePath);

    % digging out image related info
    [pathstr,name,ext] = fileparts(filename) ;
    fileinfo = imfinfo(imagePath);
    FileSize1 = fileinfo.FileSize(1,1);
    width = fileinfo.Width;
    height = fileinfo.Height;

    % 
    axes(handles.imagesPictureBox);
    imshow(image);

 else
    h = msgbox('Invalid Value', 'Error','error');
 end

Error message

 Error using  ~= 
 Matrix dimensions must agree.

 Error in OpenFileDialogBoxTest>browsePushButton_Callback (line 91)
 if (imagePath ~= '')

 Error in gui_mainfcn (line 95)
        feval(varargin{:});

 Error in OpenFileDialogBoxTest (line 42)
    gui_mainfcn(gui_State, varargin{:});

 Error in @(hObject,eventdata)OpenFileDialogBoxTest('browsePushButton_Callback',
           hObject,eventdata,guidata(hObject))

 Error while evaluating UIControl Callback

Answer 1

[dpb]

>> p=cd
p =
C:\ML_R2014b\work
>> whos p
Name      Size            Bytes  Class    Attributes
p         1x17               34  char               

>> if p~=''
   ,disp('not empty'),end
Error using  ~= 
Matrix dimensions must agree. 
>>

imagePath is an array of characters just as the variable p above; Matlab relational operators are vectorized and do a comparison element-by-element. Above you're trying to compare an array of length(imagePath) to an empty (zero-length) array so if there is anything in the variable the LH and RH lengths are incompatible; hence the error.

Use isempty instead, that's what it's for...

Example using above illustration...

>> p='';
>> isempty(p)
ans =
   1
>> p=cd;
>> isempty(p)
ans =
   0
>>

[Walter Roberson]

If you have a new enough version (R2016b or later) you could also consider using the string data type. Especially from R2017a onward when you can start coding using ""

if imagePath ~= ""

[dpb]

Is the promotion between types built into the operators, Walter? That would seem a performance hit, indeed...surely not every function that previously returned character arrays now returns a string so wouldn't above need to be

if string(imagePath) ~= ""

(unless were already cast previously, of course)?

[Walter Roberson]

Yes, what I posted does work.

[dpb]

OK, so the comparison must have new smarts internally to match up multiple classes on either side of the logical operators...