load folder's images names to a Matlab listbox - matlab

Question

I'm trying to load all the images' names that exist in a particular folder that i selected using matlab GUI into a listbox.
the problem is that when i select the folder:
if it's empty, I can see the list empty with a white background color (which is the right thing).
But when i select a folder that contains images, the listbox disappears from the GUI. and i get a warning saying:
Warning: single-selection listbox control requires a scalar Value
Control will not be rendered until all of its parameter values are valid
i'm stuck in this issue for a long time, and i couldn't find a way to solve it.
here's the code i tried:
% --- Load up the listbox with tif files in folder handles.handles.ImageFolder
function handles=LoadImageList(handles)
ListOfImageNames = {};
folder = handles.ImageFolder;
if ~isempty(handles.ImageFolder)
if exist(folder,'dir') == false
warningMessage = sprintf('Note: the folder used when this program was last run:\n%s\ndoes not exist on this computer.\nPlease run Step 1 to select an image folder.', handles.ImageFolder);
msgboxw(warningMessage);
return;
end
else
msgboxw('No folder specified as input for function LoadImageList.');
return;
end
% If it gets to here, the folder is good.
ImageFiles = dir([handles.ImageFolder '/*.*']);
for Index = 1:length(ImageFiles)
baseFileName = ImageFiles(Index).name;
[folder, name, extension] = fileparts(baseFileName);
extension = upper(extension);
switch lower(extension)
case {'.png', '.bmp', '.jpg', '.tif', '.avi'}
% Allow only PNG, TIF, JPG, or BMP images
ListOfImageNames = [ListOfImageNames baseFileName];
otherwise
end
end
set(handles.lstImageList,'string',ListOfImageNames);
return

Answer 1

This happen to a Matlab uicontrol when some of it's parameters are set to an invalid value. Matlab choose to not display them.
In your case, the error message tells you the Value parameter is incorrect.
Your listbox is defined as a "single-selection", which means you can only have one line selected at a time. The Value parameter represent the line number which is selected. This parameter has to be a "scalar", a single number, not a vector which would represent several line selected (possible with "multi-selection" listboxes).
Note that Matlab doesn't support an empty Value either, so if your Value was set to [], it will also error when you populate the lisbox. (at least not on my R2009a version)
The simple way to overcome that, is to set the Value at the same time you populate the listbox:
set(handles.lstImageList,'string',ListOfImageNames,'Value',1);

Answer 2

The problem is with the way you create ListOfImageNames. The result of your code is a long string containing all the image names without any space or delimiter.
The String property of MATLAB's listbox expect a cell array with the strip of each line. You can try this:
ListOfImageNames =[];
NumOfImages = 0;
ImageFiles = dir([handles.ImageFolder '/*.*']);
for Index = 1:length(ImageFiles)
baseFileName = ImageFiles(Index).name;
[folder, name, extension] = fileparts(baseFileName);
extension = upper(extension);
switch lower(extension)
case {'.png', '.bmp', '.jpg', '.tif', '.avi'}
% Allow only PNG, TIF, JPG, or BMP images
NumOfImages = NumOfImages + 1;
ListOfImageNames{NumOfImages} = baseFileName;
otherwise
end
end
set(handles.lstImageList,'string',ListOfImageNames);