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

const byte ROW_NUM = 4; // vier Reihen
const byte COL_NUM = 4; // vier Spalten

char keys[ROW_NUM][COL_NUM] = {
  {'1','2','3','+'},
  {'4','5','6','-'},
  {'7','8','9','*'},
  {'#','0','=','/'}
};

byte pin_rows[ROW_NUM] = {9, 8, 7, 6}; //connect to the row pinouts of the keypad
byte pin_column[COL_NUM] = {5, 4, 3, 2}; //connect to the column pinouts of the keypad

Keypad keypad = Keypad( makeKeymap(keys), pin_rows, pin_column, ROW_NUM, COL_NUM );

LiquidCrystal_I2C lcd(0x27, 16, 2);

double num1, num2;
char op, key;
bool num1Complete = false;

void setup(){
  lcd.init();
}

void loop(){
  key = keypad.getKey();
  if (key){
    if ('0' <= key && key <= '9') {
      if (!num1Complete) {
        num1 = num1 * 10 + (key - '0');
        lcd.print(key);
      } else {
        num2 = num2 * 10 + (key - '0');
        lcd.print(key);
      }
    } else if (key == '+' || key == '-' || key == '*' || key == '/') {
      num1Complete = true;
      op = key;
      lcd.print(key);
    } else if (key == '=') {
      lcd.clear();
      switch(op) {
        case '+': lcd.print(num1 + num2); break;
        case '-': lcd.print(num1 - num2); break;
        case '*': lcd.print(num1 * num2); break;
        case '/': lcd.print(num1 / num2); break;
      }
      num1 = num2 = 0;
      num1Complete = false;
    }
  }
}