clear Arduino as a global variable in Matlab

Question

I'm trying to use Arduino in Matlab. cause I used Arduino in some function I had to define Arduino as a global variable.

clc,clear,close all;
global a
a = arduino();
.
some functions
.
clear a

at the end of my code I clear the variable but as I run the code again this error appear:

Failed to open serial port (user COM) to communicate with board Uno. ...
 Make sure there is no other MATLAB arduino object for this board.

I guess the global function can't be clear by clear command so I tried clear all instead clear in the last line. It works perfect as I run it again and again but as you know this is Matlab tips for clear all command

Using clear all causes MATLAB® to recompile previously cached code, 
and can decrease performance.

I've tried delete(a) so the result was:

Error using arduino/delete
Cannot access method 'delete' in class 'arduino'.

any help would be appreciated.

• J. Piquard: What does contain the global variable a = arduino(); ?
• Amin Qassemi: cause it's recall in all sub functions. like writePWMDutyCycle(a,E1, 0);
• Amin Qassemi: sub functions use local value by default and save in separated location in matlab. one way is use a as an input argument for all of function. but i'm looking for another solution.

Answer 1

In the MatLab Help Center they had nice description of how to clear a variable from memory. Check it here: https://www.mathworks.com/help/matlab/ref/clearvars.html

In your case should be: clearvars -global a;