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

arduino_keypad_using_2_columns_from_scratch.ino

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

int pinSource1 = 3;
int pinSource2 = 4;
int pinReceiver1 = 9;
int pinReceiver2 = 8;
int pinReceiver3 = 7;
int pinReceiver4 = 6;

bool isPress1Detected = false;
bool isPress2Detected = false;

void setup() {
  pinMode(pinSource1, OUTPUT);
  pinMode(pinSource2, OUTPUT);
  pinMode(pinReceiver1, INPUT_PULLUP);
  pinMode(pinReceiver2, INPUT_PULLUP);
  pinMode(pinReceiver3, INPUT_PULLUP);
  pinMode(pinReceiver4, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {

  if (isPress2Detected == false) {
    digitalWrite(pinSource1, LOW);

    if (isPress1Detected == true && (digitalRead(pinReceiver1) == LOW || digitalRead(pinReceiver2) == LOW || digitalRead(pinReceiver3) == LOW || digitalRead(pinReceiver4) == LOW)) {
      return;
    } else {
      isPress1Detected = false;
    }

    if (digitalRead(pinReceiver1) == LOW) {
      isPress1Detected = true;
      Serial.println("3 was pressed");
    } else if (digitalRead(pinReceiver2) == LOW) {
      isPress1Detected = true;
      Serial.println("6 was pressed");
    } else if (digitalRead(pinReceiver3) == LOW) {
      isPress1Detected = true;
      Serial.println("9 was pressed");
    } else if (digitalRead(pinReceiver4) == LOW) {
      isPress1Detected = true;
      Serial.println("# was pressed");
    }

    digitalWrite(pinSource1, HIGH);
  }

  if (isPress1Detected == false) {
    digitalWrite(pinSource2, LOW);

    if (isPress2Detected == true && (digitalRead(pinReceiver1) == LOW || digitalRead(pinReceiver2) == LOW || digitalRead(pinReceiver3) == LOW || digitalRead(pinReceiver4) == LOW)) {
      return;
    } else {
      isPress2Detected = false;
    }

    if (digitalRead(pinReceiver1) == LOW) {
      isPress2Detected = true;
      Serial.println("2 was pressed");
    } else if (digitalRead(pinReceiver2) == LOW) {
      isPress2Detected = true;
      Serial.println("5 was pressed");
    } else if (digitalRead(pinReceiver3) == LOW) {
      isPress2Detected = true;
      Serial.println("8 was pressed");
    } else if (digitalRead(pinReceiver4) == LOW) {
      isPress2Detected = true;
      Serial.println("0 was pressed");
    }

    digitalWrite(pinSource2, HIGH);
  }
}