undefined variable out of nowhere

Question

I have MALTAB application with GUIDE interface, in OpeningFcn function which is executed before GUI is made visible i have defined such global variable:

global P1;
P1 = [];

Then I have timer function executing every 2 sec, at the very beginning of this function is:

handles=guidata(hObj);
global P1;

After that two lines function makes some calculations using P1 variable. everything was working fine for couple of hours and then suddenly I occurred this message:

??? Error while evaluating TimerFcn for timer 'timer-1'

Undefined function or variable "P1".

and application stopped. I'd understand index out of range error or something similar but undefined variable? How is that even possible?

Answer 1

Per the documentation for global:

To use a global within a callback, declare the global, use it, then clear the global link from the workspace. This avoids declaring the global after it has been referenced. For example,

cbstr = sprintf('%s, %s, %s, %s, %s', ...
   'global MY_GLOBAL',                ...
   'MY_GLOBAL = 100',                 ...
   'disp(MY_GLOBAL)',                 ...
   'MY_GLOBAL = MY_GLOBAL+1',         ...
   'clear MY_GLOBAL');

uicontrol('style', 'pushbutton', 'CallBack', cbstr, ...
   'string', 'count')

That being said, there typically is never a need to use a global variable. For GUIs built with GUIDE, consider using setappdata to access your variables across functions.