#include <LiquidCrystal_I2C.h>
#include <Keypad.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);
const byte ROWS = 4;
const byte COLS = 4;
char keys[ROWS][COLS] = {
  {'1','2','3','+'},
  {'4','5','6','-'},
  {'7','8','9','*'},
  {'C','0','=','/'}
};
byte rowsPins[ROWS] = {9, 8, 7, 6};
byte colsPins[COLS] = {5, 4, 3, 2};
Keypad myKeypad = Keypad(makeKeymap(keys), rowsPins, colsPins, ROWS, COLS);
boolean presentValue = false;
boolean final = false;
String num1, num2;
int answer;
char op;

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("Calculator");
  delay(2000);
  lcd.clear();
}

void loop() {
  char key = myKeypad.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 (!presentValue) {
      num1 += key;
      lcd.setCursor(15 - num1.length(), 0);
      lcd.print(num1);
    } else {
      num2 += key;
      lcd.setCursor(15 - num2.length(), 1);
      lcd.print(num2);
    }
  } else if (!presentValue && key != NO_KEY && (key == '/' || key == '*' || key == '-' || key == '+')) {
    presentValue = true;
    op = key;
    lcd.setCursor(15, 0);
    lcd.print(op);
  } else if (final && key != NO_KEY && key == 'A') {
    if (num2.toInt() != 0 || op != '/') {
      if (op == '+') {
        answer = num1.toInt() + num2.toInt();
      } else if (op == '-') {
        answer = num1.toInt() - num2.toInt();
      } else if (op == '*') {
        answer = num1.toInt() * num2.toInt();
      } else if (op == '/') {
        answer = num1.toInt() / num2.toInt();
      }
      lcd.clear();
      lcd.setCursor(0, 0);
      lcd.print(answer);
    } else {
      lcd.clear();
      lcd.print("Error: Division by zero");
    }
    presentValue = false;
    final = false;
    num1 = "";
    num2 = "";
    answer = 0;
    op = ' ';
  } else if (key != NO_KEY && key == 'C') {
    lcd.clear();
    presentValue = false;
    final = false;
    num1 = "";
    num2 = "";
    answer = 0;
    op = ' ';
  }
}