int LED = 7;
int BUTTON = 3;

void setup() 
{
  pinMode(LED,OUTPUT);
  pinMode(BUTTON,INPUT);
}

void loop() 
{
  if(digitalRead(BUTTON) == LOW)
  blink();
  else 
  {
    digitalWrite(LED,LOW);
  }
}
void blink(void)
 {
  digitalWrite(LED,HIGH);
  delay (500);
  digitalWrite(LED,LOW);
  delay (500);
 }