const int NUM_BTNS = 4;   // how many buttons
const int BTN_PINS[NUM_BTNS] = {5, 4, 3, 2};

// initialize global variables
int btnState[NUM_BTNS];
int oldBtnState[NUM_BTNS];

// function returns which button was pressed, or 0 if none
int checkButtons()  {
  int btnPressed = 0;

  for (int i = 0; i < NUM_BTNS; i++) {
    // check each button
    btnState[i] = digitalRead(BTN_PINS[i]);
    if (btnState[i] != oldBtnState[i])  { // if it changed
      oldBtnState[i] = btnState[i];       // remember state for next time
      if (btnState[i] == LOW)  {          // was just pressed
        btnPressed = i + 1;
        //Serial.print("Button ");
        //Serial.print(btnPressed);
        //Serial.println(" pressed");
      } else {                            // was just released
        //Serial.print("Button ");
        //Serial.print(btnPressed);
        //Serial.println(" released");
      }
      delay(20);  // debounce
    }
  }
  return btnPressed;
}

void setup() {
  Serial.begin(115200);
  for (int i = 0; i < NUM_BTNS; i++) {
    pinMode(BTN_PINS[i], INPUT_PULLUP);
  }
  Serial.println("Button Demo\n");
}

void loop() {
  int btnNumber = checkButtons();
  if (btnNumber != 0) {
    Serial.print("Button number: ");
    Serial.println(btnNumber);
  }
}