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

const int ROW_NUM = 4; //four rows
const int COLUMN_NUM = 4; //four columns
const int rs = 13, en = 12, d4 = 11, d5 = 10, d6 = 9, d7 = 8;

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] = {7, 6, 5, 4}; //connect to the row pinouts of the keypad
byte pin_column[COLUMN_NUM] = {3, 2, A1, A2}; //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, num2;
char op;

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

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

  if (key) {
    if (key >= '0' && key <= '9' || key == '.') {
      input += key;
      lcd.print(key);
    } else if (key == '+' || key == '-' || key == '*' || key == '/') {
      num1 = input.toFloat();
      op = key;
      input = "";
      lcd.print(key);
    } else if (key == '=') {
      num2 = input.toFloat();
      float result;
      switch (op) {
        case '+': result = num1 + num2; break;
        case '-': result = num1 - num2; break;
        case '*': result = num1 * num2; break;
        case '/': 
          if (num2 != 0) {
            result = num1 / num2;
          } else {
            lcd.clear();
            lcd.print("Error: Div by 0");
            return;
          }
          break;
      }
      lcd.clear();
      lcd.print(result);
      input = "";
    }
  }
}