char keypad [4][4] = {
  {'1', '2', '3','A'},
  {'4', '5', '6','B'},
  {'7', '8', '9','C'},
  {'*', '0', '#','D'}
};


char getKey() {
  // set key value to null
  char keyValue = '\0';
  // a variable to read the row state
  int rowState;
  // for each pin 0-2 connected to COL1-3 do the following
  for (int col = 0; col < 3; col++) {
    // set the column HIGH
    digitalWrite(col, HIGH);
    // for each pin 3-6 connected to ROW1-4 do the following
    for (int row = 3; row < 7; row++) {
      // read the state of the row
      rowState = digitalRead(row);
      // if the state of the row is HIGH the do the following
      if (rowState == HIGH) {
        // set key value to the character
        // corresponding to the pressed button
        keyValue = (keypad[row - 3][col]);
      }
    }
    // set the column back to LOW
    digitalWrite(col, LOW);
  }
  // return the key value
  return (keyValue);
}

// get key value
char key = getKey();

void setup() {
  Serial.begin(9600);
  // set pins 0-2 as outputs to control COL1-3 of the keypad
  for (int col = 0; col < 3; col++) {
    pinMode(col, OUTPUT);
  }
  // set pins 3-6 as inputs to read ROW1-4 of the keypad
  for (int row = 3; row < 7; row++) {
    pinMode(row, INPUT);
  }
}

void loop() {


  // if the key value is not NULL then
  if (key != '\0') {
    // display the key value on the serial monitor
    Serial.println(key);
  }


}