Persistent variables don't persist outside an executed callback function

Hi everyone guys, i would submit a question that's maybe can be trivial but i never encountered this problem before: I'm programming a programmatic GUI, in this GUI i have used a persistent variable that's will reassigned in many step inside a nested function in a callback

function main_GUI()
persistent V
V = [] ; % initialize the variable
  function callback(...)
  V = test (...,V)
  end
end
___________________
function V = test(...,V)
V = 0 ;
...some commands
V = 1 ;
...some commands
V = 'exit'
end

my problem is that the variable V after exit the function callback seems to be cleared and don't persist the value V = 'exit'!? have someone an idea? many thanks in advance :-) Luca

[karandeep singh seehra]

One thing you should consider is that persistent variables are only accessible in different function, but can only be changed in the function it is defined

Answer 1

[Geoff Hayes]

Luca - why are you declaring the variable V as persistent? You would only do this if you want to retain the value of this variable between function calls. As your main_GUI will only be called once, declaring V as persistent is unnecessary.

So at what point does V get cleared? It will always be empty (it's initial value) until the callback is called. Just because your function is nested within main_GUI, this doesn't mean that it will be called in the order of

• Initialize V to empty matrix
• declare function callback with body
• etc.

All you are doing in the above code is declaring the nested callback function. It won't get evaluated until it is called by whatever control is using this callback.

Also, there is no need to pass V into the callback function. Since V is a local variable in the main_GUI workspace, and callback is a nested function within this workspace, then the function has access to the variable (and any other) by default.

 function main_GUI()

     V = [];
     function nestedFunction()
         if isempty(V)
             V = 42;
         else
            V = V + 1;
         end
     end

     nestedFunction;
     fprintf('V is %d\n', V);
 end

It isn't until nestedFunction is called that V is assigned a value.

[Luca Di Leta]

Hi Geoff, thank you very much for your answer. In my Gui i need the variable V as persistent why she must be readable in any moment for usage in other callback.

function main_GUI()
persistent V
V = [] ; % initialize the variable
  function callback(...)
  V = test (...,V)
  % here V seems to be cleared (strange)
  end
end

i have tested in debug mode that the variable will be cleared(???) right after the callback function it's executed...

- (Annotation) In the function test(...,V) the variable V will be readed und reassigned many times in others sub-function like V = subtest (...,V)

function V = subtest(...,V)
V = 0;
% some commands
V = 1;
end

[Luca Di Leta]

Hi Geoff i have added an example GUI to explain what i mean

function TESTPER()
close
clear
clc

persistent psv

psv = 0 ;

fig = figure('MenuBar','figure',...
    'Name','TEST',...
    'NumberTitle','off',...
    'Color',[1 1 1],...
    'MenuBar','None',...
    'ToolBar', 'figure',...
    'Resize','Off',...
    'Position',[200,100,400,250]);

Start_Btn = uicontrol('Style', 'pushbutton',...
    'String', 'Start',...
    'Position', [25 125 125 25],...
    'FontSize',9,...
    'BackgroundColor',[0.85 0.85 0.85],...
    'Callback',@Start_Btn_Callback);

DISP_Btn = uicontrol('Style', 'pushbutton',...
    'String', 'DISP',...
    'Position', [175 125 125 25],...
    'FontSize',9,...
    'BackgroundColor',[0.85 0.85 0.85],...
    'Callback',@DISP_Btn_Callback);

function Start_Btn_Callback(hObject,handle)
psv = testfunction(psv) ;
end

function DISP_Btn_Callback(hObject,handle)          
info_message = num2str(psv) ;
msgbox(info_message);
end

end

the testfunction (psv) is simply:

function psv = testfunction(psv)
psv = 2 ;
pause(3)
psv = 3;
pause(3)
psv = 7;
end

the problem that i have it's that DISP button don't display the current value of psv but only the output value when the testfunction(psv) is executed though i need an instant access to the current psv value (in these example the psv value don't persist in memory and i don't understand why...).

[Geoff Hayes]

Luca - please re-read the documentation for persistent. Declaring a variable as persistent is not necessary for nested variables. You would only declare it as persistent if you wish the variable to retain its value between repeated function calls. But as your main_GUI or TESTPR are only called once, declaring as such would have no effect.

As for why psv isn't being updated, look closely at your code.

 function Start_Btn_Callback(hObject,handle)
    psv = testfunction(psv) ;
 end

psv, the local variable defined in TESTPR is only updated when the testfunction completes.

 function psv = testfunction(psv)
    psv = 2 ;
    pause(3)
    psv = 3;
    pause(3)
    psv = 7;
 end

The psv that is updated in the above function is a local variable to testfunction. It isn't the psv defined in TESTPR. And so that psv will only change after six seconds.

What I might do is change your two functions to

 function Start_Btn_Callback(hObject,handle)
     testfunction;
 end

 function testfunction
     psv = 2 ;
     pause(3)
     psv = 3;
     pause(3)
     psv = 7;
 end

The above assumes that both functions are nested in TESTPR. If testfunction is not nested, then you will have to come up with an alternative means to update psv.