how to replace the variable value permanently

Question

I am trying to update a variable value permanently in Matlab. I want to make a GUI where there will be a button which would have variable's value (e.g. k = 3), I will add that with 4 and show the value (e.g. 7) in a text box. Now the value (i.e. 7) will be the new value of k. So if again I click on the button it would show the updated value in text box (e.g. k=7, in text box: 7+4 =11). I am new in matlab and tried many ways to solve it. The simplest way was:

  function addition_Callback(hObject, eventdata, handles)
  k =3;
  k = 4+k;
  set(handles.value,'String', ...   %here value is the name of the text box
  [ k ]);

but each time I am clicking the button, it is starting from very beginning as assumed. How can I declare the variable so that it will work the way I just mentioned?

Answer 1

Why not simply use the currently displayed string as starting point?

function addition_Callback(hObject, eventdata, handles)
  % get the currently displayed value and convert it to a number
  current = str2double(get(handles.value, 'String'));
  % current will be nan if the string is empty or not a valid number
  if isnan(current)
      current = 3; % start or fallback-value
  end
  new = 4+current;
  set(handles.value,'String', new)   %here value is the name of the text box