const int NUM_BTNS = 10;
const int BTN_PINS[] = {
  12, 11, 10, 9, 8, 7, 6, 5, 4, 3
};

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++) {
    btnState[i] = digitalRead(BTN_PINS[i]); // check each button
    if (btnState[i] != oldBtnState[i])  {   // if it changed
      oldBtnState[i] = btnState[i];         // remember state for next time
      if (btnState[i] == 0)  {              // was just pressed
        btnPressed = i + 1;
      }
      delay(20);  // debounce
    }
  }
  return btnPressed;
}

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

void loop() {
  int btnNumber = checkButtons();
  if (btnNumber)  {
    Serial.print("Button ");
    Serial.print(btnNumber);
    Serial.println(" pressed.");
  }
}