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

LiquidCrystal lcd(13, 12, 11, 10, 9, 8);

const int rows = 4;
const int cols = 4;

char keys[rows][cols] = 
{
  {'1', '2', '3', '+'},
  {'4', '5', '6', '-'},
  {'7', '8', '9', '*'},
  {'C', '0', 'E', '/'}
};

byte rowPins[rows] = {0, 1, 2, 3};
byte colPins[cols] = {4, 5, 6, 7};

Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, rows, cols);

long num1 = 0;
long num2 = 0;
double res = 0;

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

void loop() {
  char key = keypad.getKey();

  if(key >= '0' && key <= '9') {
    num1 = num1 * 10 + (key - '0');
    lcd.print(key);
  }

  else if(key == '+' || key == '-' || key == '*' || key == '/') {
    lcd.print(key);
    num2 = secondNumber();
    
    if(key == '+') {
      res = num1 + num2;
    }

    else if(key == '-') {
      res = num1 - num2;
    }

    else if(key == '*') {
      res = num1 * num2;
    }

    else if(key == '/') {
      res = (float)num1 / (float)num2;
    }
    lcd.setCursor(0, 1);
    lcd.print(res);
  }

  else if(key == 'C'){
    lcd.clear();
    num1 = num2 = res = 0;
  }
}

long secondNumber() {
  while(true) {
    char key = keypad.getKey();
    if(key >= '0' && key <= '9') {
      num2 = num2 * 10 + (key - '0');
      lcd.print(key);
    }

    if(key == 'E'){
      lcd.print('=');
      break;
    }
  }
  return num2;
}