#include <Keypad.h>
#include <LiquidCrystal.h>

const byte ROWS = 4;
const byte COLS = 4;

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

byte rowPins[ROWS] = {9, 8, 7, 6};
byte colPins[COLS] = {5, 4, 3, 2};

Keypad kpd = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

const int rs = 8, en = 9, d4 = 10, d5 = 11, d6 = 12, d7 = 13;

LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

long num1, num2, number;
char key, op;
boolean result = false;

void setup() {
  lcd.begin(16, 2);
  lcd.print("Arduino Calculator");
  lcd.setCursor(0, 1);
  lcd.print("by Rahul");
  delay(2000);
  lcd.clear();
}

void loop() {
  key = kpd.getKey();

 if (key != NO_KEY && (key == '1' || key == '2' || key == '3' || key == '4' || key == '5' || key == '6' || key == '7' || key == '8' || key == '9' || key == '0')) {
    if (result == false) {
      number = number * 10 + (key - '0');
      lcd.print(key);
    }
    else {
      num2 = num2 * 10 + (key - '0');
      lcd.print(key);
    }
  }

  // Check if the key is an operation
   if (key == 'A' || key == 'B' || key == 'C' || key == 'D') {
    num1 = number;
    number = 0;
    if (key == 'A') {
      op = '+';
    }
    if (key == 'B') {
      op = '-';
    }
    if (key == 'C') {
      op = '*';
    }
    if (key == 'D') {
      op = '/';
    }
    lcd.print(op);
  }

  if (key == '#') {
    num2 = number;
    result = true;
    lcd.print("=");
  }

  if (key == '*') {
    num1 = num2 = number = 0;
    result = false;
    lcd.clear();
  }


  if (result == true) {
    switch (op) {
      case '+':
        number = num1 + num2;
        break;
      case '-':
        number = num1 - num2;
        break;
      case '*':
        number = num1 * num2;
        break;
      case '/':
        number = num1 / num2;
        break;
    }
    lcd.print(number);
  }
}