int PB_1 = 4;
int LED_1 = 7;

void setup()
{
  //initializing pushbutton pin as an input:
  pinMode(PB_1, INPUT_PULLUP);

  //initializing led as an output:
  pinMode(LED_1, OUTPUT);
}

void loop()
{
  //turn led on:
  if (digitalRead(PB_1) == LOW)
  {
    digitalWrite(LED_1, HIGH);
  }
  else
  {
    digitalWrite(LED_1, LOW);
  }
}