#include <LiquidCrystal_I2C.h>
#include <Keypad.h>

LiquidCrystal_I2C LCD = LiquidCrystal_I2C(0x27, 16, 2);

const byte KEYPAD_ROWS = 4;
const byte KEYPAD_COLS = 4;

byte rowPins[KEYPAD_ROWS] = {5, 4, 3, 2};
byte colPins[KEYPAD_COLS] = {A3, A2, A1, A0};

char keys[KEYPAD_ROWS][KEYPAD_COLS] = {
  {'1', '2', '3', '+'},
  {'4', '5', '6', '-'},
  {'7', '8', '9', '*'},
  {'.', '0', '=', '/'}
};

Keypad keypad = Keypad(makeKeymap(keys), 
                rowPins, 
                colPins, 
                KEYPAD_ROWS, 
                KEYPAD_COLS);

String txt;
float mem;
char op;

void setup() {
  Serial.begin(9600);
  LCD.init();
  LCD.backlight();
  txt = "0";
  LCD.print(txt);
  mem = 0;
  op = 'x';
}

String clean_str(String txt_in) {
  boolean proc = true; 
  while(proc) {
    proc = false;
    if(txt_in.length() > 1)
      if(txt_in.charAt(0) == '0') {
        proc = true; 
        txt_in = txt_in.substring(1);
      }
  }
  if((txt_in.length()>0) && (txt_in.charAt(0)=='.'))
    txt_in = "0" + txt_in;
  if(txt_in.indexOf("..") >= 0) {
    txt_in = "0";
    op = 'x';
    mem = 0;
  }
  return txt_in;
}

void loop() {

  // put your main code here, to run repeatedly:
  char key = keypad.getKey();
  if ((key >= '0' && key <= '9') || 
      key == '.') {
    txt = txt + key;
    txt = clean_str(txt);
    LCD.setCursor(0, 0);
    LCD.print("                ");
    LCD.setCursor(0, 0);
    LCD.print(txt);
  } else if(key == '+' || key == '-' || key == '*' || key == '/' || key == '='){
    if(op == 'x') {
      mem = txt.toFloat();
      op = key;
      txt = "0";
      if(key == '=') {
        op = 'x';
        mem = 0;
      }
    }
    else if (op == '+' || op == '-' || op == '*' || op == '/'){
      float v = txt.toFloat();
      if(op == '+')
        mem = mem + v;
      else if (op == '-')
        mem = mem - v;
      else if (op == '*')
        mem = mem * v;
      else if (op == '/')
        mem = mem / v;
      LCD.setCursor(0, 0);
      LCD.print(String(mem));
      
      op = key;
      if(key == '=') {
        mem = 0;
        op = 'x';
      }
    }
    txt = "0";
  }
  Serial.println(String(mem)+"---"+op+"---"+txt);
}