Button and LED on an Arduino Uno

// Button: a pushbutton on pin 2 lights the LED on pin 13.
//
// INPUT_PULLUP switches on the ATmega328P's 35 k pull-up, so the pin sits at
// 5 V until the button pulls it to GND. That is why the button reads LOW when
// it is pressed: wire one side to pin 2 and the other to GND, no resistor.

const int BUTTON_PIN = 2;

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  bool pressed = digitalRead(BUTTON_PIN) == LOW;
  digitalWrite(LED_BUILTIN, pressed ? HIGH : LOW);
  delay(5);
}