/*Wap to interface LCD and keypad (4 X 4) , to exhibit the functionality of
  a basic calculator.
  1 2 3 4
  5 6 7 8
  9 0 + -
  / * Clear enter         */
#include <Keypad.h>
#include <LiquidCrystal.h>
LiquidCrystal lcd(0, 1, 2, 3, 4, 5);
const byte ROWS = 4;
const byte COLS = 4;
char keys[ROWS][COLS] = {
  {'1', '2', '3', '4'},
  {'5', '6', '7', '8'},
  {'9', '0', '+', '-'},
  {'/', '*', 'C', 'E'}
};
byte rowPins[ROWS] = {13, 12, 11, 10};
byte colPins[COLS] = {9, 8, 7, 6};
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);
long num1 = 0;
long num2 = 0;
double total = 0;
void setup() {
  lcd.begin(16, 2);
  lcd.print("Enter a Number");
  delay(1000);
  lcd.clear();
}
void loop() {
  char value = keypad.getKey();
  switch (value) {
    case '0' ... '9':
      lcd.setCursor(0, 0);
      num1 = num1 * 10 + (value - '0');
      lcd.print(num1);
      break;
    case '+':
      num1 = (total != 0 ? total : num1);
      lcd.print('+');
      num2 = secondNumber();
      total = num1 + num2;
      lcd.setCursor(0, 2);
      lcd.print(total);
      num1 = num2 = 0;
      break;
    case '-':
      num1 = (total != 0 ? total : num1);
      lcd.print('-');
      num2 = secondNumber();
      total = num1 - num2;
      lcd.setCursor(0, 2);
      lcd.print(total);
      num1 = num2 = 0;
      break;
    case '*':
      num1 = (total != 0 ? total : num1);
      lcd.print('*');
      num2 = secondNumber();
      total = num1 * num2;
      lcd.setCursor(0, 2);
      lcd.print(total);
      num1 = num2 = 0;
      break;
    case '/':
      num1 = (total != 0 ? total : num1);
      lcd.print('/');
      num2 = secondNumber();
      lcd.setCursor(0, 2);
      num2 == 0 ? lcd.print("Invalid") :
      total = (float)num1 / (float)num2;
      lcd.print(total);
      num1 = num2 = 0;
      break;
    case 'C':
      total = num1 = num2 = 0;
      lcd.clear();
      break;
  }
}
long secondNumber() {
  while (1) {
    char value = keypad.getKey();
    if (value >= '0' && value <= '9') {
      num2 = num2 * 10 + (value - '0');
      lcd.setCursor(7, 0);
      lcd.print(num2);
    }
    if (value == 'E') break;
  }
  return num2;
}