Get selection index for DropDown object using AppDesigner

Using AppDesigner, how do I access the selected index of a DropDown menu. The Value is the selected string but I can't find anywhere that indicates the selected index. Instead I seem to have to do:

I = find(strcmp(h.Items,h.Value));

Is this really the suggested approach?

Answer 1

[Matt J]

The h.Value property has different behavior depending on how h.Items and h.ItemsData are set up. If you define the ItemsData property to be some sequence of numeric indices i=1:N, then h.Value will be the numeric value of i corresponding to the selection.

[Ahmed]

I've tried alot but it's not working. Could you please elaborate with an example?

[Jim Hokanson]

In the AppDesigner, in the Drop-Down editor, there are 3 entries: Value, Items, and ItemsData. By default, ItemsData is not populated, which means that Value takes on the Items value. However, if you manually enter ItemsData, then the selection takes on the value in ItemsData.

So by default, in my version of Matlab, I get Items = {'Option 1'} {'Option 2'} {'Option 3'} {'Option 4'}

So when selecting the second option, the value becomes 'Option 2', which in some cases may not be ideal. Now if you enter in 1,2,3,4 (EDIT: on the same line, 1 entry per line makes them strings) in the ItemData field, now the Value for the 2nd selection will be 2.

Alternatively you can enter whatever you want so that when a selection is made, you get that particular ItemData entry. So for example 12,34,59,36 when the 2nd element is selection (Option 2), then you'll get 34 as the value.

[Ahmed]

Thanks alot. Now it works.

[Darren Akehurst]

This really needs to be added to the documentation. Documentation is not at all clear about this

[Kristoffer Walker]

Designers,

I like the ability to have Value point to the string in the Items array. However, there are many times in code where it is useful to have Value point to "Items" and several lines later having Value point to an Index array. Why not simply take the three Value, Items, and ItemsData and replace them with Value and ValueIndex, both of which refer to the Items array? I miss the older functionality, which seems more logical and easier to use.

Kris

[Erik]

@Jim Hokanson

You wrote

" Now if you enter in 1,2,3,4 (each on its own line) in the ItemData field, now the Value for the 2nd selection will be 2."

Unfortunately when entered this way, itemdata will become a cell array of strings

 {'1','2','3','4'}

if 'Option 1' is selected then

disp(app.DropDown.Value*1) 

will display 49 since char(49) = '1'

However, if we first set it to a numerical array

app.DropDown.ItemsData = 1:numel(app.DropDown.Items);

then disp(app.DropDown.Value*1) will display 1

[Jim Hokanson]

@Erik

I think this is the first time I've been @ mentioned on MATLAB answers :)

Weird, I wonder if MATLAB changed its behavior since I last tested. If you enter the values on one line it works but indeed it seems that multiline entry forces the entries to be strings. Regardles, at some point typing 1,2,3,etc. may get annoying and simply setting ItemsData as you suggest at run time may be more desirable.

[Erik]

@Jim Hokanson

Hmm, (R2019b) Update 5

If I fill in 1,2,3,4 for ItemData on the first line, then ItemsData = { '1,2,3,4'}

So only one item and hence only the first option is selectable. So the Callback gets never triggered.

Strangely, when you edit the Value, it give the numbers 1 2 3 4 on separate lines.

Seems like a bug to me

[Jim Hokanson]

@Erik

I just tested in 2020b and 1,2,3,4 produces an array of numbers, not a single ellement cellstr

For what it is worth, I found the app designer to be too much. It may be getting better over time and I can't remember what the tipping point was (different figure type?), but I find GUIDE to be sufficient. I only use GUIDE for layout, not code generation. I create a handle class that loads the GUI and the handles on construction and go from there ...

[Mario Malic]

What is the purpose of setting up your cell array of numeric data to be cell array of character data? It will only cause confusion.

Value can take only one argument, not more, and it has to be one of Items.

Regarding this comment:

if 'Option 1' is selected  then  
disp(app.DropDown.Value*1) 
will display 49 since char(49) = '1'

I think this was recently mentioned on MATLAB frustrations question, how does one even multiply character '1' with 1? There are also other examples related to it, but if you know you will be multiplying the DropDown Value, you shouldn't be using characters.

[Erik]

@Mario Malic Maybe you missed the point.

This string array versus numerical array salad was not intended, but I got into it by the unexpected and confusing behaviour of the appdesigner dropbox editor in version 2019b. @Jim HokansonShowed that it is fixed in version 2020b.

[Erik]

@Jim Hokanson

How did you actually make the screen shot with the command window query of app?

Are you in debug mode? I do not see the K> prompt.

Or is the app object public or is it somehow in the global workspace?

[Mario Malic]

TLDR on the bottom.

This behaviour is standard I guess, snippet below is ran in 2020b

a = 'b'
a*2
ans =
   196

Referring to your picture from the component inspector @Erik:

• If your expand your Items, they will appear only in one row which means that it has only one element
• Can you check if in your ItemsData, each entry is in its own row? If yes, that will cause the data to be treated as cell array of character arrays. Even if it consists only of numeric values, those will be treated as characters

An example, how you'd write things in the App Designer:

   Items = opt1, opt2, opt3
   ItemsData = 1 2 3
   % Items will be 1x1 cell array, whereas items will be 1x3 numeric array
   
   Items = opt1, opt2,
           opt3
            % NOTE <-- Sometimes users like to finish the entry with 'Enter' button so we have an empty row like here
   ItemsData = 1
               2
               3
   % Items and ItemsData will both be 3x1 cell array of character arrays, 3rd Item will be empty
   
   Items = opt1, opt2
   ItemsData = 1, 2, 3
           % same note as above, an empty row       
   % Items will be 1x1 cell array, whereas ItemsData will be 2x1 cell array of character values,
   % Since Items(1) refers to ItemsData(1), ItemsData(1) will appear in Value

On the last case, which you showed on the picture, if I expand the Value box, I see the same, except in 2020b other rows are a shifted by space like this.

   1
    2
    3

TLDR: Check for empty rows in your Items and ItemsData.

To get the access to app in your command window, you can call your app directly

app = NameOfTheApp;

If you have opened the app already and you want to get the access, there's property called RunningAppInstance in App Designer figure. If you have more figures open, then use app tags to differentiate your app from other figures.

app = get(findall(0,'type', 'figure'), 'RunningAppInstance');

[Erik]

Thanks on the info on how to get access to a running app.

There is no way in my version to enter the ItemData so that it becomes a numerical array. What ever I do in de compent inspector, the end result is the same (see below). And yes the (single) Value is shown in the inspector as numbers under each other while the ItemData is shown as 1,2,3,4

But, like I said, in the 2020b version, this behaviour fortunately changed apparently

>> app.DropDown
ans = 
  DropDown (Option 1) with properties:

              Value: '1 2 3 4'
              Items: {'Option 1'  'Option 2'  'Option 3'  'Option 4'}
          ItemsData: {'1 2 3 4'}
           Editable: 'off'
    ValueChangedFcn: ''
           Position: [131 192 100 22]

[Mario Malic]

Oh, I see what you mean now.