Variable persisting between function calls - matlab

Question

Suppose I want to call a function twice, but I need the function to remember variables it initialized the first time it is called so execution can be changed in subsequent calls.
For example if I have a piece of code like this:
function random
if exist('a','var') == 0
fprintf('hello\n');
a = 1;
else
disp('goodbye\n');
end
end
How could I get MATLAB to remember that a equals 1 when when I call the code again? Specifically, I'm hoping to use this for a push button callback function in a program I'm writing.

Answer 1

MATLAB supports the keyword persistent, which you would use as follows:
function toggleval_persist
% Set up the persistent variable and initialize it.
persistent a;
if isempty(a)
a = 0;
end
if ( a == 0 )
disp('hello');
a = 1;
else
a = 0;
disp('goodbye');
end
end
Also, I wouldn't recommend using a persistent variable for toggling a button state. The button's state is usually available in the object structure for the GUI if you're using MATLAB's UI system.

Answer 2

What you could do (in the optics of using this code in a GUI) is set up a flag telling whether a has been initialized or not and pass it as an argument to the function random. Storing the flag (let's call it a_flag) in the handles structure of the GUI, for instance, would allow you to keep track of its value (actually stored in handles.a_flag) for example.
So in other words,you could set the flag to 0 during the creation of the GUI (or in its Opening_Fcn if you are using GUIDE) as follows:
handles.a_flag = false;
and then in the function called random, which you could call with: random(handles.a_flag):
function random(a_flag)
if ~a_flag
%// Update the flag
a_flag = true;
fprintf('hello\n');
a = 1;
else
fprintf('goodbye\n');
end
end
Even simpler would be to use global variables...but I like that idea better :)
EDIT
The point of my code is to demonstrate that we can use the handles structure of a GUI (as asked by the OP) to store the value of the flag. Since the structure is accessible from every callback pressing the pushbutton will update it the same way a persistent variable would.
Please try this code to see what I mean:
function TestGUI
clear
clc
hFigure = figure('Position',[200 200 200 150]);
handles.Disp_a_title = uicontrol('Style','text','String','a','Position',[20 100 60 20]);
handles.Disp_aflag = uicontrol('Style','text','String','0','Position',[100 100 60 20]);
handles.Button = uicontrol('Style','Push','Position',[50 50 60 20],'String','Update a','Callback',#(s,e) PushCb);
a = 0;
handles.a_flag = false;
guidata(hFigure,handles)
function PushCb(~,~)
handles = guidata(hFigure);
fprintf('Flag is %i\n',handles.a_flag)
if handles.a_flag == false;
disp('hello\n');
a = 1;
handles.a_flag = true;
else
disp('goodbye\n');
end
guidata(hFigure,handles)
end
end
Pressing the button twice results in the following output in the Command Window:
Flag is 0
hello\n
Flag is 1
goodbye\n
Which from what I understood is the expected behaviour the OP is looking for.