How can I check existence of a variable in matlab app designer?

I'm creating an app designer UI. Every time I run below code in app designer, the answer is zero:

if true
  % code
% Button pushed function: PreviewButton
      function PreviewButtonPushed(app, event)
          app.z = videoinput('winvideo',1,'RGB24_960x720');
          app.x = getsnapshot(app.z);
          a = exist('app.x','var');
end

On the other hand while I run this code in editor the is true:

if true
  % code
z = videoinput('winvideo',1,'RGB24_960x720');
x = getsnapshot(z);
a = exist('x','var')
end

How shoud I use existence function?

Answer 1

[Steven Lord]

The expression "app.x" is not a variable name. In this context it is an expression referring to the property x of an object stored in the variable app. [There are other potential meanings, but that's the one that's relevant here.]

If you want to check if the object stored in the variable app has a property x, use isprop.

isprop(app, 'x')

If you want to check if the property app.x exists and is non-empty:

isprop(app, 'x') && ~isempty(app.x)

If you know that the object app must have a property x (because it appears in the class definition file rather than being a dynamic property added using addprop on a class that subclasses dynamicprops) you can omit the first part of that test.

~isempty(app.x)

[MC]

Hi Steven,

Many thanks for your prompt and full explanation. ~isempty() works perfectly for me.

This is my first time using appdesigner (only used guide previously).

Off to read up on properties now...

Best regards, Mark.

[reaujahn]

Hello Steven,

I have a similar question regarding ~isempty().

link to the question

Would you mind having a look? I appreciate it.

Answer 2

[MC]

I have the same problem.

I can evaluate the variable I am looking for while in debug mode, but 'exist' returns 0 nonetheless.

Edit: I got around it by using

try
    app.x;
    % continue ...
catch
    % if x doesn't exist
    return
end