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37 
#include <Keypad.h>

const int ROW_NUM = 4; //four rows
const int COLUMN_NUM = 4; //four columns
const int Ctrl = 13;
int currentState;
int answ;
char oper = ' ';
boolean firstNumState = true;
String firstNum = "";
String secondNum = "";
float answer = 0.0;

char keys[ROW_NUM][COLUMN_NUM] = {
  {'1', '2', '3', '+'},
  {'4', '5', '6', '-'},
  {'7', '8', '9', '*'},
  {'C', '0', '=', '/'}
};
char Ctrlkeys[ROW_NUM][COLUMN_NUM] = {
  {'(', ')', 's', '+'},
  {'4', '5', 'x', '-'},
  {'^', 'c', 't', '*'},
  {'C', '0', '=', '/'}
};
byte pin_rows[ROW_NUM] = {9, 8, 7, 6}; //connect to the row pinouts of the keypad
byte pin_column[COLUMN_NUM] = {5, 4, 3, 2}; //connect to the column pinouts of the keypad

Keypad keypad = Keypad( makeKeymap(keys), pin_rows, pin_column, ROW_NUM, COLUMN_NUM );
Keypad Ctrlkeypad = Keypad( makeKeymap(Ctrlkeys), pin_rows, pin_column, ROW_NUM, COLUMN_NUM );
#include <Wire.h> // Wire Bibliothek einbinden
#include <LiquidCrystal_I2C.h> // Vorher hinzugefügte LiquidCrystal_I2C Bibliothek einbinden
LiquidCrystal_I2C lcd(0x27, 16, 2); //Hier wird festgelegt um was für einen Display es sich handelt. In diesem Fall eines mit 16 Zeichen in 2 Zeilen und der HEX-Adresse 0x27. Für ein vierzeiliges I2C-LCD verwendet man den Code "LiquidCrystal_I2C lcd(0x27, 20, 4)"

void setup() {
 Serial.begin(115200);
 lcd.init(); // initialize the lcd
 lcd.backlight();
}

void loop() {
  char key;
  currentState = digitalRead(Ctrl);
  if (currentState == LOW) { //Wenn Ctrl nicht gedrückt -> primäres Keypad
    key = keypad.getKey();
  }
  else  { //Wenn Ctrl gedrückt -> sekundäres Keypad
    key = Ctrlkeypad.getKey();
  }
  if (key) {
    Serial.println(key);
  }
  char newKey = key;
  // Was tun, wenn er eine Nummer bekommt?
  if ((newKey != NO_KEY) && (newKey == '1' || newKey == '2' || newKey == '3' || newKey == '4' || newKey == '5' || newKey == '6' || newKey == '7' || newKey == '8' || newKey == '9' || newKey == '0')) {
    if (firstNumState == true) {
      firstNum = firstNum + newKey;
      lcd.print (newKey);
    }
    else {
      secondNum = secondNum + newKey;
      lcd.print (newKey);
    }
  }
  // Was tun, wenn er einen Operator bekommt?
  if (newKey != NO_KEY && (newKey == '+' || newKey == '-' || newKey == '*' || newKey == '/' || newKey == '^' || newKey == 'x' || newKey == 's' || newKey == 'c' || newKey == 't')) {
    // code to execute if the condition is true
  }
  if (firstNumState == true) {
    oper = newKey;
    firstNumState = false;
    lcd.print (oper);
  }
  if (newKey != NO_KEY && newKey == '=') {

  if (oper == '+') {
    answ = firstNum.toFloat () + secondNum.toFloat ();
  }
  if (oper == '-') {
    answ = firstNum.toFloat () - secondNum.toFloat ();
  }
  if (oper == '*') {
    answ = firstNum.toFloat () * secondNum.toFloat ();
  }
  if (oper == '/') {
    answ = firstNum.toFloat () / secondNum.toFloat ();
  }
  if (oper == '^') {
    answ = pow(firstNum.toFloat (),secondNum.toFloat () );
  }
  if (oper == 'x') {
    answ = sqrt(firstNum.toFloat() );
  }
  if (oper == 's') {
    answ = sin(firstNum.toFloat() );
  }
  if (oper == 'c') {
    answ = cos(firstNum.toFloat() ) ;
  }
  if (oper == 't') {
    answ = tan(firstNum.toFloat() );
  }
  }

  lcd.setCursor (0, 0);
  lcd.print (firstNum);
  lcd.setCursor (0,1);
  lcd.print (oper); 
  lcd.setCursor (0,2);
  lcd.print (secondNum);
  lcd.setCursor (0,3);
  lcd.print ("=");
  lcd.setCursor (1,1);
  lcd.print (answ);
  firstNumState = true;
  delay(100);
}