#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27,20,4);

const int R1=12, R2=11, R3=10, R4=9;
const int C1=7, C2=6, C3=5, C4=4;

char key;

void setup() {
  Serial.begin(115200);
  lcd.init();
  lcd.backlight();
  pinMode(R1, OUTPUT);
  pinMode(R2, OUTPUT);
  pinMode(R3, OUTPUT);
  pinMode(R4, OUTPUT);
  pinMode(C1, INPUT_PULLUP);
  pinMode(C2, INPUT_PULLUP);
  pinMode(C3, INPUT_PULLUP);
  pinMode(C4, INPUT_PULLUP);
  digitalWrite(C1, HIGH);
  digitalWrite(C2, HIGH);
  digitalWrite(C3, HIGH);
  digitalWrite(C4, HIGH);
};

void loop() {
  ROW1();
  ROW2();
  ROW3();
  ROW4();
  lcd.setCursor(0,0);
  lcd.print("Your Pressed : ");
  lcd.print(key);
};

void ROW1() {
  // ------------- ROW1 ---------------
  digitalWrite(R1, LOW);
  digitalWrite(R2, HIGH);
  digitalWrite(R3, HIGH);
  digitalWrite(R4, HIGH);
  if (digitalRead(C1) == LOW) {
    key = '1';
    delay(200);
  }
  else if (digitalRead(C2) == LOW) {
    key = '2';
    delay(200);
  }
  else if (digitalRead(C3) == LOW) {
    key = '3';
    delay(200);
  }
  else if (digitalRead(C4) == LOW) {
    key = 'A';
    delay(200);
  }
}

void ROW2() {
  // ------------- ROW2 ---------------
  digitalWrite(R1, HIGH);
  digitalWrite(R2, LOW);
  digitalWrite(R3, HIGH);
  digitalWrite(R4, HIGH);
  if (digitalRead(C1) == LOW) {
    key = '4';
    delay(200);
  }
  else if (digitalRead(C2) == LOW) {
    key = '5';
    delay(200);
  }
  else if (digitalRead(C3) == LOW) {
    key = '6';
    delay(200);
  }
  else if (digitalRead(C4) == LOW) {
    key = 'B';
    delay(200);
  }
}

void ROW3() {
  // ------------- ROW3 ---------------
  digitalWrite(R1, HIGH);
  digitalWrite(R2, HIGH);
  digitalWrite(R3, LOW);
  digitalWrite(R4, HIGH);
  if (digitalRead(C1) == LOW) {
    key = '7';
    delay(200);
  }
  else if (digitalRead(C2) == LOW) {
    key = '8';
    delay(200);
  }
  else if (digitalRead(C3) == LOW) {
    key = '9';
    delay(200);
  }
  else if (digitalRead(C4) == LOW) {
    key = 'C';
    delay(200);
  }
}

void ROW4() {
  // ------------- ROW4 ---------------
  digitalWrite(R1, HIGH);
  digitalWrite(R2, HIGH);
  digitalWrite(R3, HIGH);
  digitalWrite(R4, LOW);
  if (digitalRead(C1) == LOW) {
    key = '*';
    delay(200);
  }
  else if (digitalRead(C2) == LOW) {
    key = '0';
    delay(200);
  }
  else if (digitalRead(C3) == LOW) {
    key = '#';
    delay(200);
  }
  else if (digitalRead(C4) == LOW) {
    key = 'D';
    delay(200);
  }
}