const int buttonPin = 4;
int oldValue = HIGH; // default/idle value for pin 4 is high.

void setup() 
{
  Serial.begin(115200);
  Serial.println("Press the button.");

  // Initialize the pin for reading the button.
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(2, OUTPUT);
}

void loop() 
{
  // Read the value of pin 4.
  int newValue = digitalRead(buttonPin);

  // Check if the value was changed,
  // by comparing it with the previous value.
  if(newValue != oldValue)
  {
    if(newValue == HIGH)
    {
      Serial.println("The button is pressed.");
      digitalWrite(2, HIGH);
    }
    else
    {
      Serial.println("The button is released.");
    }
    // Remember the value for the next time.
    oldValue = newValue;
  }

  // Slow down the sketch.
  // Also for debouncing the button.
  delay(100);
}