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

String rzeichen = "";
String ergebnisz = "";
float zahl1 = 0;
float zahl2 = 0;
float ergebnis = 0;

LiquidCrystal_I2C LCD = LiquidCrystal_I2C(0x27,16,2);

const uint8_t ROWS = 4;
const uint8_t COLS = 4;
char keys[ROWS][COLS] ={
  { '1', '2', '3', '+' },
  { '4', '5', '6', '-' },
  { '7', '8', '9', '*' },
  { '.', '0', '=', '/' }
};

uint8_t colPins[COLS] = { 17, 5, 18, 19 }; // Pins connected to C1, C2, C3, C4
uint8_t rowPins[ROWS] = { 15, 2, 4, 16 }; // Pins connected to R1, R2, R3, R4

Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

void setup(){
  Serial.begin(9600);
  LCD.init();
  LCD.backlight();
  LCD.setCursor(1,0);
  LCD.print("Calculator_v2");
  delay(1000);
  LCD.init();
  LCD.setCursor(0,0);
}

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

  if(key != NO_KEY){
    Serial.println(key);
    zahl1 = zahl1 * 10 + (key-'0');
    LCD.setCursor(0,0);
    LCD.print(zahl1);
  }
}