cant assign the value to 'cmd_msg.data' in runtime.

edit

I am doing a stepper motor programming.what i want is "when i enter an angle on command line,then stepper should rotate that angle". but i cant assign the value in runtime. help me please.

#include <ros.h>
#include <std_msgs/Empty.h>
#include <std_msgs/UInt16.h>
signed int angle;
int enable2 = 13;
const int stepPin = 9; 
const int dirPin = 8;
int x;

ros::NodeHandle  nh;
void pwm( const std_msgs::UInt16& cmd_msg)
{

 analogWrite(enable2, cmd_msg.data);
angle=analogRead(enable2);


}


ros::Subscriber<std_msgs::UInt16> sub("servo", pwm);

void setup(){
 // pinMode(angle, OUTPUT); 
 pinMode(enable2, OUTPUT);   
 pinMode(stepPin,OUTPUT); 
  pinMode(dirPin,OUTPUT);
  nh.initNode();
  nh.subscribe(sub);

 // servo.attach(9); //attach it to pin 9
}

void loop()
{

   if (angle<0)
     {
       cw();
       delay(100);
      }
 else
   {
    ccw();
    delay(100);
   }
delay(10000);
nh.spinOnce();
  delay(1);
}
void cw()
{
  digitalWrite(dirPin,LOW); 
  for(x = 0; x < 17.77*(angle*-1); x++) {
    digitalWrite(stepPin,HIGH); 
    delayMicroseconds(500); 
    digitalWrite(stepPin,LOW); 
    delayMicroseconds(500); 
  }}
  void ccw()
{
  digitalWrite(dirPin,HIGH); 
  for(x = 0; x < 17.77*angle; x++) {
    digitalWrite(stepPin,HIGH); 
    delayMicroseconds(500); 
    digitalWrite(stepPin,LOW); 
    delayMicroseconds(500); 
  }}