#include <Arduino.h>

// Pin where the push button is connected
const int buttonPin = 4;

// Pin where the LED is connected
const int ledPin = 2;

void setup() {

  // Set button pin as INPUT with INTERNAL PULL-UP resistor
  // The internal resistor connects the pin to VCC (HIGH) by default.
  // This prevents the pin from "floating" (random readings).
  pinMode(buttonPin, INPUT_PULLUP);

  // Set LED pin as OUTPUT
  pinMode(ledPin, OUTPUT);
}

void loop() {

  // Read the current state of the button
  int buttonState = digitalRead(buttonPin);

  // IMPORTANT:
  // Because we use INPUT_PULLUP:
  // - NOT pressed  -> HIGH (connected internally to VCC)
  // - Pressed      -> LOW  (button connects pin to GND)

  if (buttonState == LOW) {   // Button is pressed
    digitalWrite(ledPin, HIGH);  // Turn LED ON
  } else {                     // Button is not pressed
    digitalWrite(ledPin, LOW);   // Turn LED OFF
  }
}