#include <LiquidCrystal_I2C.h>
#include <Keypad.h>
#define I2C_ADDR    0x27
#define LCD_COLUMNS 20
#define LCD_LINES   4
const uint8_t ROWS = 4;
const uint8_t COLS = 4;
uint8_t colPins[COLS] = { 5, 4, 3, 2 }; // Pins connected to C1, C2, C3, C4
uint8_t rowPins[ROWS] = { 9, 8, 7, 6 }; // Pins connected to R1, R2, R3, R4
char keys[ROWS][COLS] = {
  { '1', '2', '3', '+' },
  { '4', '5', '6', '-' },
  { '7', '8', '9', '*' },
  { '=', '0', 'C', '/' }
};

LiquidCrystal_I2C lcd(I2C_ADDR, LCD_COLUMNS, LCD_LINES);
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);
String n1 = "", n2 = "";
char o = ' ';

void setup() {
  lcd.init();
  Serial.begin(9600);
  lcd.backlight();
}

double cal(double num1, char ope, double num2) {
  switch (ope) {
    case '+':
      return num1 + num2;
      break;
    case '-':
      return num1 - num2;
      break;
    case '*':
      return num1 * num2;
      break;
    case '/':
      return num1 / num2;
      break;
  }
}

void updateDisplay() {
  lcd.setCursor(1, 0);
  lcd.print(n1);
  if (o != ' ') {
    lcd.setCursor(0, 1);
    lcd.print(String(o));
  }
  lcd.setCursor(1, 1);
  lcd.print(n2);
}

void loop() {
  char key = keypad.getKey();
  switch (key) {
    case '+':
    case '-':
    case '*':
    case '/':
      o = key;
      break;
    case 'C':
      n1 = "";
      n2 = "";
      o = ' ';
      break;
    case '=':
      lcd.clear();
      n1 = cal(n1.toDouble(), o, n2.toDouble());
      n2 = "";
      o = ' ';
      break;
    default:
      if (o == ' ') n1 += String(key);
      else n2 += String(key);
  }
  updateDisplay();
}