#include <LiquidCrystal_I2C.h>
const int K = 20; 
const char keymap[4][4] = {
 { '1', '2', '3' ,'A'} ,
 { '4', '5', '6','B' } ,
 { '7', '8', '9','C' } ,
 { '*', '0', '#','D' }
};


const int rP[4] = {9, 8, 7, 6}; 
const int cP[4] = {5, 4, 3, 2}; 

void setup() {
  
  Serial.begin(9600);
  Serial.println("Ready to input:");

  
  for (int r = 0; r < 4; r++) {
    pinMode(rP[r], INPUT_PULLUP); 
  }
  for (int c = 0; c <4; c++) {
    pinMode(cP[c], OUTPUT);
    digitalWrite(cP[c], HIGH); 
  }
}

void loop() {
  char key = getKey();
  if (key != 0) {
    Serial.print("Key pressed: ");
    Serial.println(key);
  }
}


char getKey() {
  char key = 0; 
  for (int c = 0; c < 4; c++) {
    digitalWrite(cP[c], LOW); 
    for (int r= 0; r <4 ; r++) { 
      if (digitalRead(rP[r]) == LOW) { 
        delay(K); 
        while (digitalRead(rP[r]) == LOW)
          ; 
        key = keymap[r][c]; 
      }
    }
    digitalWrite(cP[c], HIGH); 
  }
  return key; 
}