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

char colPins[cols] = { 5, 4, 3, 2 }; // Pins connected to C1, C2, C3, C4
char rowPins[rows] = { 9, 8, 7, 6 }; // Pins connected to R1, R2, R3, R4
void setup () {
      Serial.begin(9600);
      
   for(char r = 0; r < rows; r++){
         pinMode(rowPins[r], INPUT);    //set the row pins as input
         digitalWrite(rowPins[r], HIGH);    //turn on the pullups
   }
   
   for(char c = 0; c < cols; c++){
         pinMode(colPins[c], OUTPUT);   //set the column pins as output
   }
   
}

void loop() {
      char key = getKey();
      
      if(key != 0){
   Serial.println(key);
      }
}

char getKey(){
      char k = 0;
      
      for(char c = 0; c < cols; c++){
        digitalWrite(colPins[c], LOW);
         for(char r = 0; r < rows; r++){
            if(digitalRead(rowPins[r]) == LOW){
            delay(20);    //20ms debounce time
            while(digitalRead(rowPins[r])== LOW);
            k = keys[r][c];
            }
         }
   digitalWrite(colPins[c], HIGH); 
      }
      return k;
}