MATLAB: How to determine the number of elements that can be displayed at one time in a List Box in MATLAB 7.9 (R2009b)

listboxlistboxlengthMATLABuicontrol

I am building a GUI and I wish to determine the number of items that can fit on the visible portion of the List Box. This GUI will be used on different operating systems which may have different font sizes, screen resolutions, etc. Thus, the number of lines of text displayed in the List Box is not necessarily predictable.

The 'ListBoxTop' property can be used to set or query the index of the top-most item displayed in the listbox. I am looking for something like a 'ListBoxBottom' property to access the index of the last item displayed in the List Box.

Best Answer

There is no direct way to query either the number of items displayed in the List Box or the index of the last item displayed currently displayed.

The following code illustrates one workaround that can be used to determine the number of lines of text that can be viewed at one time in the List Box. Note that this code assumes that the List Box contains more items than can be displayed at one time.

strs = {'one','two','three','four','five','six','seven','eight'};
num_strs = numel(strs); % total number of items to be displayed in the List Box
h = uicontrol('Style','listbox', 'Units','inches', 'Position',[1 1 2 1], ...
      'String',strs);
drawnow
% First set the 'ListBoxTop' property to the total number of items in the
% List Box. This has the effect of scrolling to the bottom of the List Box
set(h, 'ListBoxTop',num_strs);
drawnow
% Next query the value of 'ListBoxTop'
top_index = get(h, 'ListBoxTop');
% The difference between the total number of items in the List Box and the
% index of the top item when we have scrolled to the bottom of the list
% gives the number of items that can be displayed at one time
fprintf('%d items can be completely displayed at once\n', num_strs-top_index)

Note that the above example makes use of DRAWNOW a few times due the following bug which prevents the 'ListBoxTop' property from being updated:

 <http://www.mathworks.com/support/bugreports/249434>

Related Solutions

MATLAB: How to create a listbox with a scrollbar starting at the bottom of the listbox, when using GUIDE in MATLAB

One way to have the scroll bar begin at the bottom of the Listbox is to set the 'Value' property of the Listbox to the number of elements in the list through the Property Inspector. The Property Inspector is a separate user interface that can be found as one of the menu items when right-clicking on the Listbox when designing the GUI. For example, if you have 6 elements choices in a list, setting the 'Value' property of the Listbox to '6' displays the starting point of the scroll bar at the bottom of the Listbox.

An alternative way to set the position of the scroll bar is to use the commands set(handles.listbox1,'Value',x) or set(handles.listbox1,'ListboxTop',x) in the Callback function, where x is the index of the last list item in the Listbox. For example,

set(handles.listbox1,'String',data); %populate the list with data

index = size(get(handles.listbox1,'string'), 1); %get how many items are in the list box

set(handles.listbox1,'ListboxTop',index); %set the index of last item to be the index of the top-most string displayed in list box.

set(handles.listbox1,'Value',index);

When setting the 'Value' parameter, the last element in the list gets highlighted. On the other hand, setting the 'ListboxTop' parameter does not highlight the last element in the Listbox.

MATLAB: Do I receive a warning when I repopulate the “listbox” uicontrol in MATLAB

The ability to automatically reset the "Value" property to "1" when repopulating a listbox is not available in MATLAB.

To work around this issue, set the active item in the list box to the first item before repopulating the listbox:

set(hListBox,'Value',1);

The following code provides an example where this workaround is used:

function [] = ChangeList()
% Example to demonstrate changing the items in a list
% Create two different lenght lists
list1 = {'Option 1', 'Option 2'};
list2 = {'Only Option'};
% Create a list box with the first list, set to second time.
hListBox = uicontrol('Style','listbox',...
                       'String',list1,...
                       'Position',[20 20 100 200],...
                       'Value',2);
% Create a button to set to list 1
hList1Button = uicontrol('Style','pushbutton',...
                           'String','List 1',...
                           'Position',[130,100,100,30],...
                           'Callback',@(eventObject,eventData)ChangeList(eventObject,eventData,hListBox,list1),...
                           'Tag','btn1');
% Create a button to set to list 2
hList2Button = uicontrol('Style','pushbutton',...
                           'String','List 2',...
                           'Position',[130,30,100,30],...
                           'Callback',@(eventObject,eventData)ChangeList(eventObject,eventData,hListBox,list2),...
                           'Tag','btn1');
end
% Create the callback
function [] = ChangeList(hObject,eventData,hListBox,list)
      set(hListBox,'Value',1); % Add this line so that the list can be changed
      set(hListBox,'String',list);
end

Best Answer

Related Solutions

MATLAB: How to create a listbox with a scrollbar starting at the bottom of the listbox, when using GUIDE in MATLAB

MATLAB: Do I receive a warning when I repopulate the “listbox” uicontrol in MATLAB

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