operands to the and && operators must be convertible to logical scalar values. This error appears in app designer. Why does this happen?

function NextButtonPushed(app, event)
            global DD MM Y
            opts = 'non-modal';
            year = num2str(Y);
            ycheck1 = floor(Y/4);
            ycheck2 = floor(Y/100);
            ycheck3 = floor(Y/400);
            if ((DD > 29) && (MM == 2))
                errordlg('February has 29 days max', 'Error!', opts);
                error('Error occurred!');
            end
            if ((ycheck2 == Y/100) && (ycheck3 ~= Y/400) && (DD == 29) && (MM == 2))
                errordlg(['Year ' year ' is not bissextile therefore February does not have 29 days'], 'Error!', opts);
                error('Error occurred!');
            end
            if ((DD == 29) && (MM == 2) && (ycheck1 ~= Y/4))
                errordlg(['Year ' year ' is not bissextile therefore February does not have 29 days'], 'Error!', opts);
                error('Error occurred!');
            else
                RightAscensionDate;   % this is a function I need to run.
                run('ApogeePerigeeApp.mlapp');
            end
        end

[Riccardo Basile]

It's a code to check if the user's input (a date) is correct

Answer 1

[Rik]

The use of globals makes this an impossible task to debug. How are you making sure they are non-empty scalar values? If they aren't, the logical comparisons will yield either an empty logical array (not checked if that's true), or they will return a logical array, on which you can't use the short circuit operands.

The source of your problem is probably there.

[Riccardo Basile]

The global variables are defined in the callback functions of three numeric edit fields the user can input in the UI figure of the app, so they are scalars. Basically they are all three defined like this:

% Value changed function: YearYYYYEditField
        function YearYYYYEditFieldValueChanged(app, event)
            global Y
            value = app.YearYYYYEditField.Value;
            Y = value;

The same goes for DD and MM. They are defined as global because I need them in other functions that are involved in other apps linked to this one.

[Rik]

If you use breakpoints just before the line where the error occurs, what are the classes and sizes of those variables?

Also, sharing data between GUIs is still possible without globals: you can use setappdata with groot, or use getpref. In any of these cases it is a bad idea to assume your data is present and is in the form you expect. You should have some check in place.

[Riccardo Basile]

After some debugging I found out that those variables were always empty for some reason... I had to define those variables as properties of the app instead of using globals. And yes, apparently data is not always in the expected form! Also it looks like globals are a bad idea more often than not... Thank you for your help!

[Rik]

You're very welcome, glad to be of help.

And yes, globals can be very tricky to use and almost always be avoided.