#define COL_COUNT 3
#define ROW_COUNT 4

const int colPins[] = {21, 22, 23};
const int rowPins[] = {12, 13, 14, 15};
const char symbols[4][3] = {
  {'1', '2', '3'},
  {'4', '5', '6'},
  {'7', '8', '9'},
  {'*', '0', '#'}
};

void setup() {
  Serial.begin(115200);

  for (int i = 0; i < COL_COUNT; i++) {
    pinMode(colPins[i], INPUT_PULLUP);
  }

  for (int i = 0; i < ROW_COUNT; i++) {
    pinMode(rowPins[i], OUTPUT);
  }
}

void loop() {
  for (int i = 0; i < COL_COUNT; i++) {
    if(!digitalRead(colPins[i])) {
      for (int j = 0; j < ROW_COUNT; j++) {
        digitalWrite(rowPins[j], HIGH);
        if (digitalRead(colPins[i])) Serial.printf("%c\n", symbols[j][i]);
        digitalWrite(rowPins[j], LOW);
      }
    }
  }
}