#include <Keypad.h>
#include <LiquidCrystal.h>

const int ROW_NUM = 4; //four rows
const int COLUMN_NUM = 4; //four columns

const int rs = A5, en = A4, d4 = 13, d5 = 12, d6 = 11, d7 = 10;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

char keys[ROW_NUM][COLUMN_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[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 );

String input = "";
float num1 = 0;
float num2 = 0;
char op = 0;
boolean newInput = false;

void setup() {
  lcd.begin(16, 2);
  lcd.setCursor(0, 0);
  Serial.begin(9600);
}

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

  if (key) {
    if (key >= '0' && key <= '9' || key == '.') {
      input += key;
      Serial.print(key);
      lcd.print(key);
    } else if (key == '+' || key == '-' || key == '*' || key == '/') {
      num1 = input.toFloat();
      op = key;
      input = "";
      newInput = true;
      // lcd.setCursor(0, 1);
      Serial.print(key);
      lcd.print(key);
    } else if (key == '=') {
      if (newInput) {
        num2 = input.toFloat();
        float result;
        switch (op) {
          case '+':
            result = num1 + num2;
            break;
          case '-':
            result = num1 - num2;
            break;
          case '*':
            result = num1 * num2;
            break;
          case '/':
            result = num1 / num2;
            break;
        }
        lcd.setCursor(0, 1);
        Serial.print(" = " + String(result) + "\n");
        lcd.print("= " + String(result));
        newInput = false;
        input = "";
      }
    }
  }
}