how to get index of selected item in listbox of appdesginer

Hi:

I want to get the index of selected item in listbox of appdesigner.

I found a solution here: https://www.mathworks.com/matlabcentral/answers/376091-index-of-the-items-in-a-listbox

The weakness of the solution provided here using 'ismember' function is that, if I have two items that are totally the same, the returned index is not unique. so, is there any other solutions?

Thanks!

Yu

[Adam Danz]

Why have duplicate options in the listbox? What's the difference between apples and apples? How could it possibly matter which "apples" is selected?

Maybe with some additional background info we could suggest an alternative but there doesn't seem be much sense in duplicate listbox values.

[Yu Li]

I understand your concern-having two variables with totally the same value in a list box may not be necessary.

but I want know how to get the actual selected item's location, in case this situation occurs.

[Adam Danz]

The problem isn't that duplicate values are unnecessary. The problem is that there should be no difference between "apples" and "apples" so selecting either of them should result in the same outputs. Otherwise, how would the user know which "apples" to select?

In any case, I added an answer that suggests a workaround.

Answer 1

[Adam Danz]

Continuing from comments under the question.

There is unfortunately no property of a listbox that returns the index of the selected item. The only way to get the index is by finding the selection within a list of options by using strcmp() or ismember() etc. If your list of options has duplicates and one of the duplicates was selected, there is no way to match which of the duplicates was selected.

One potential workaround in app designer is to assign unique values to the ItemsData property. When the ItemsData property is set, the Value property of a listbox returns the ItemsData selection rather than the Items selection.

Example:

fig = uifigure();
lbx = uilistbox(fig,'Items',{'Apples','Apples','Pears','Bananas'}, ...
    'ItemsData',{'Apples1','Apples2','Pears1','Bananas1'});

Now when the 2nd "Apples" is selected,

lbx.Value
ans =
    'Apples2'

[Yu Li]

Thanks for your reply and answer.

Bests,

Yu

[Adam Danz]

Glad I could help!

Answer 2

[Marvin Seifert]

You cant get the index directly from the listbox class but you can get the index from the mousclick event. In the callback function for the listbox you can do

indices = event.Value;

This works for multiple selections as well.

[Adam Danz]

The ValueChangedData structure (mouseclick event) does not contain the selection index.

event.Value provides the same information as the app.ListBoxHandle.Value data described in the other answer here.

The app.ListBoxHandle.Value data is also available in any callback function within the app, not just within the ValueChange function.

Here's a functional demo that can be run from an m-file that demonstrates this. The ValueChangedFcn function displays the results of event.Value and hObj.Value (r2019b).

fig = uifigure();
lbx = uilistbox(fig,'Items',{'Apples','Apples','Pears','Bananas'}, ...
    'ItemsData',{'Apples1','Apples2','Pears1','Bananas1'});
lbx.ValueChangedFcn = @listboxValChgFcn; 

function listboxValChgFcn(hObj, event)
disp(['From event: ', event.Value])
disp(['From Object: ', hObj.Value])  
%within app designer the obj handle would be something like app.ListBox.Value
end

Results when items are selected from listbox

From event: Apples2
From Object: Apples2

From event: Pears1
From Object: Pears1

From event: Bananas1
From Object: Bananas1

[Marvin Seifert]

Yes you are right, what I had in mind was this, to get the index rather than a unique string:

Items = {'Apples','Apples','Pears','Bananas'};
nr_Items = numel(Items);
fig = uifigure();
lbx = uilistbox(fig,'Items',Items, ...
    'ItemsData',(1:nr_Items));
lbx.ValueChangedFcn = @listboxValChgFcn; 
function listboxValChgFcn(hObj, event)
idx = event.Value %This is the index as a double
disp(['From event: ', num2str(event.Value)])
disp(['From Object: ', num2str(hObj.Value)])  
%within app designer the obj handle would be something like app.ListBox.Value
end

[Adam Danz]

Yes, with the ItemsData you can output the pre-established index. But if there is an error in the numbering of the the ItemsData, the true index will not be returned. But your method of indexing the number of list items is a good approach.