Using a arduino keypad module from scratch, without using extra libraries. In this case, one columns is targeted. This gist is a complement to https://youtu.be/1iPnFEWHnqo, where this is explained

arduino_keypad_using_1_column_from_scratch.ino

// Using a a arduino keypad module from scratch, without using extra libraries. 
// In this case, one columns is targeted.
// This gist is a complement to https://youtu.be/1iPnFEWHnqo, where this is explained

int pinSource = 3;
int pinReceiver1 = 9;
int pinReceiver2 = 8;
int pinReceiver3 = 7;
int pinReceiver4 = 6;

bool isPressDetected = false;

void setup() {
  pinMode(pinSource, OUTPUT);
  pinMode(pinReceiver1, INPUT_PULLUP);
  pinMode(pinReceiver2, INPUT_PULLUP);
  pinMode(pinReceiver3, INPUT_PULLUP);
  pinMode(pinReceiver4, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(pinSource, LOW);

  if (isPressDetected == true && (digitalRead(pinReceiver1) == LOW || digitalRead(pinReceiver2) == LOW || digitalRead(pinReceiver3) == LOW || digitalRead(pinReceiver4) == LOW)) {
    return;
  } else {
    isPressDetected = false;
  }

  if (digitalRead(pinReceiver1) == LOW) {
    isPressDetected = true;
    Serial.println("3 was pressed");
  } else if (digitalRead(pinReceiver2) == LOW) {
    isPressDetected = true;
    Serial.println("6 was pressed");
  } else if (digitalRead(pinReceiver3) == LOW) {
    isPressDetected = true;
    Serial.println("9 was pressed");
  } else if (digitalRead(pinReceiver4) == LOW) {
    isPressDetected = true;
    Serial.println("# was pressed");
  }

  digitalWrite(pinSource, HIGH);
}