// C++ code

int ledpin=13; //define a variable with digital pin interface 13 as an initial value

void setup()
{

  pinMode(ledpin, OUTPUT);  //initialize digital pin 13 as output.  When using I/O ports on an Arduino, this kind of setup is 
                                                //alwaayss needed..
}

void loop()
{
    digitalWrite(ledpin, HIGH);
    delay(500);
    digitalWrite(ledpin, LOW);
     delay(500);
}