// Debounc validates a button press only when longer than the debounce timeout.

byte buttonpin[] = { 2, 3, 4 };                             // declare button pins in an array
byte element;                                               // indicates pressed button/pin
byte arraySize = sizeof(buttonpin) / sizeof(buttonpin[0]);  // get number of elements in button array

unsigned long timer;          // timer keeps track of length of button press (or noise/ringing).
unsigned long timeout = 100;  // timeout (in milliseconds) is the minimum for "a button is pressed".

void setup() {
  Serial.begin(115200);                   // start serial monitor
  for (int i = 0; i < arraySize; i++)     // count to number of elements in button array
    pinMode(buttonpin[i], INPUT_PULLUP);  // configure button pins for reading (LOW when pressed)
}

void loop() {
  if (element > arraySize)                    // count through array... do not use for() loops. they blocking program flow.
    element = 0;                              // if beyond the array size, start again from the first element
  if (!digitalRead(buttonpin[element])) {     // button may have been pressed, or just noise/ringing
    if (millis() - timer > timeout) {         // measure time from last "press" to this "press. replaces delay();
      if (digitalRead(buttonpin[element])) {  // valid button was released after debounce timeout
        timer = millis();                     // reset timer after valid button press
        myFunction(element);                  // call the function associated with the button press
      }
    }
  }
  element++;  // check next array element
}

void myFunction(int i) {
  Serial.print(buttonpin[i]);  // print the pin number
}