#include <Keypad.h>     //include keypad library
#include <Wire.h>       //include I2C library
#include <LiquidCrystal_I2C.h>      //include liquidcrystal with i2c communication

LiquidCrystal_I2C lcd(0x27, 16, 2); //I2C address is 0x27,and the liquidcrystal with 16-column and 2-rows

const byte numRows = 4;     //prototype row-number
const byte numCols = 4;     //prototype column-number
String pad;

byte rowPins[numRows] = {2,3,4,5};     //prototype
byte colPins[numCols] = {6,7,8,9};

char keymap[numRows][numCols] =
{
  {'1', '2', '3', '+'},
  {'4', '5', '6', '-'},
  {'7', '8', '9', '*'},
  {'C', '0', '=', '/'}
};          

//keymap the keypad and the prototype

Keypad keypad = Keypad(makeKeymap(keymap), rowPins, colPins, numRows, numCols);

int num1 = 0;
int num2 = 0;
char op;
String currentInput = "";
bool inputReady = true, initialPos = true, clear = false;

void setup() {
  
  lcd.init();                      // Initialize the LCD
  lcd.backlight();
  lcd.setCursor(0,0);
}

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

  if (key) {

    if (key == '+' || key == '-' || key == '*' || key == '/') {
      if (inputReady) {
        num1 = currentInput.toInt();
        currentInput = "";
        lcd.setCursor(15,0);
        lcd.print(key);
        initialPos = false;
        op = key;

      }
    } else if (key == '=') {
      if (inputReady) {
        num2 = currentInput.toInt();
        int result = 0;

        switch (op) {
          case '+':
            result = num1 + num2;
            break;
          case '-':
            result = num1 - num2;
            break;
          case '*':
            result = num1 * num2;
            break;
          case '/':
            if (num2 != 0) {
              result = num1 / num2;
            } else {
              lcd.setCursor(0,1);
              lcd.print("Error");
              delay(2000);
            }
            break;
        }
        lcd.clear();
        lcd.setCursor(0,0);
        lcd.print(result); // Display result with 2 decimal places
        num1 = result;
        num2 = 0;
        currentInput = "";
        inputReady = false;
        initialPos = true;
        clear = true;
      }
    } else if (key == 'C') {
      num1 = 0;
      num2 = 0;
      op = '\0';
      currentInput = "";
      lcd.clear();
      initialPos = true;
    } else {
      if (clear){
        lcd.clear();
        clear = false;
      }
      if (initialPos){
        lcd.setCursor(0,0);
        currentInput += key;
        lcd.print(currentInput);
        inputReady = true;
      }
      else
        {
        lcd.setCursor(0,1);
        currentInput += key;
        lcd.print(currentInput);
        inputReady = true;}
      
    }
    
  }
  
}