Simple stepper arduino sketch

int stp = 13;  //connect pin 13 to step
int dir = 12;  // connect pin 12 to dir
int a = 0;     //  gen counter

void setup() 
{                
  pinMode(stp, OUTPUT);
  pinMode(dir, OUTPUT);       
}


void loop() 
{
  if (a <  200)  //sweep 200 step in dir 1
   {
    a++;
    digitalWrite(stp, HIGH);   
    delay(10);               
    digitalWrite(stp, LOW);  
    delay(10);              
   }
  else 
   {
    digitalWrite(dir, HIGH);
    a++;
    digitalWrite(stp, HIGH);  
    delay(10);               
    digitalWrite(stp, LOW);  
    delay(10);
    
    if (a>400)    //sweep 200 in dir 2
     {
      a = 0;
      digitalWrite(dir, LOW);
     }
    }
}