float maths;
float leftNum = NAN;
float rightNum = NAN;
char operater = '\0';
String leftString;
String rightString;
#include <Keypad.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(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] = { 5, 4, 3, 2 };
uint8_t rowPins[ROWS] = { 9, 8, 7, 6 };
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.clear();
  Serial.println(String(leftNum));
}

void processKey(char thisKey) {
  if (thisKey == '=') {
    doMaths(leftNum,operater,rightNum);
    leftNum = maths;

  }
  if (thisKey == '^') {
    operater = '\0';
    lcd.setCursor(1,0);
    lcd.clear();
    lcd.clear();
    rightNum = NAN;
    leftNum = NAN;
    leftString = '0';
    rightString = '0';
  }
  switch (thisKey) {
    case '0':
    case '1':   
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':
    if (operater == '\0') {
      leftString = leftString+String(thisKey);
      leftNum = leftString.toDouble();
      lcd.setCursor(1,0);
      lcd.print(leftNum);
    } else {
      rightString = rightString+String (thisKey);
      rightNum = rightString.toDouble();
      lcd.setCursor(1,1);
      lcd.print(rightNum);
    }
  }
  switch (thisKey) {
    case '+':
    case '-':
    case '*':
    case '/':
    rightNum = 0;
    rightString = '0';
    Serial.println(String(leftNum));
    if (leftNum == NAN) {
      lcd.clear();
      lcd.setCursor(0,0);
      lcd.print("Error, Please Pr");
      lcd.setCursor(0,1);
      lcd.print("ess Number First");
      delay(1000);
      lcd.clear();
    } else {
      operater = thisKey;
      lcd.setCursor(0,1);
      lcd.print(operater);
    }
  }
}

void doMaths(float left, char doThis, float right) {
  switch (doThis) {
    case '+':
    maths = left+right;
    break;
    case '-':
    maths = left-right;
    break;
    case '*':
    maths = left*right;
    break;
    case '/':
    maths = left/right;
  }
  if (maths == int(maths)) {
    lcd.clear();
    lcd.setCursor(1,0);
    lcd.print(int(maths));
    lcd.setCursor(0,1);
    lcd.print('=');
  } else {
    lcd.clear();
    lcd.setCursor(1,0);
    lcd.print(maths);
    lcd.setCursor(0,1);
    lcd.print('=');
  }
}

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