Getting an error using servo function in GUI MATLAB

What I'm doing is that I'm trying to control a servo motor using arduino and GUI, the gui has a two push buttons when to make the servo 0 degree and one to make it 90 ... when a press the push buttons it gives me this massage

"Undefined function or variable 's'."

Here is my coding in the opening function

clear a;
global a;
clear a;
a = arduino('com4','uno','Libraries','servo');
s = servo(a,5);

And here is my coding in the two push buttons

writePosition(s , 0);  % for 0 degree
writePosition(s , 0.5); % for 90 degree

If anyone can help I would appreciate it

[hamzah alamoodi]

clear a;
global a;
clear a;
a=arduino('com4','uno','Libraries','servo');
s=servo(a,5);
writePosition(s,0)

Answer 1

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.3F