Problem in interfacing Arduino with Matlab GUI.

hello! i am interfacing Arduino Mega 2560 with matlab, i have to control DC Motor speed using Matlab GUI by PWM technique, but its not working when i use analogRead/Write command. this shows error of "Undefined analogRead/Write..." and it is defined already... (while using digitalRead/Write command, it works. only problem in analog) Here is the Code.

clear a;
global a;
a= arduino('COM1')
a.pinMode(7,'output');
a.pinMode(6,'output');
a.pinMode(5,'output');

double av;

av=a.analogRead(7);
av=(av/1023)*255;
a.analogWrite(7,av);

a.digitalWrite(6,1);
a.digitalWrite(5,0);

Can anyone help me please. Thanks.