#include <LiquidCrystal.h>
#include <Keypad.h>
int rs=12, en=11, d4=5, d5=4, d6=3, d7=2;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);
const byte ROWS = 4; 
const byte COLS = 4; 
char hexaKeys[ROWS][COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};
String num1 = "";
String num2 = "";
int x = 1;
int i = 0;
char op='x';
byte rowPins[ROWS] = {21, 20, 19, 18}; 
byte colPins[COLS] = {17, 16, 15, 14}; 
Keypad keys = Keypad(makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS); 

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

void loop() {
  char key = keys.getKey();
  if(key)
  {
    lcd.setCursor(i,0);
    if(key=='A')
    {
      lcd.print('+');
    }
    else if(key=='B')
    {
      lcd.print('-');
    }
    else if(key=='C')
    {
      lcd.print('*');
    }
    else if(key=='D')
    {
      lcd.print('/');
    }
    else
    {
      lcd.print(key);
    }
    i++;
    if(key=='*')
    {
      lcd.clear();
      num1="";
      num2="";
      x=1;
      i=0;
    }
    if(key!='A'&&key!='B'&&key!='C'&&key!='D'&&key!='#'&&key!='*'&&x==1)
    {
      num1=num1+key;
    }
    if(key=='A'||key=='B'||key=='C'||key=='D'||key=='*')
    {
      x=2;
      op=key;
    }
    if(key!='A'&&key!='B'&&key!='C'&&key!='D'&&key!='*'&&x==2)
    {
      num2=num2+key;
    }
    if(key=='#')
    {
      x=3;
      lcd.clear();
    }
  }
  if(x==3)
  {
    long int n1 = num1.toInt();
    long int n2 = num2.toInt();
    if(op=='A')
    {
      lcd.setCursor(0,0);
      long sum = n1+n2;
      String s = String(sum);
      lcd.print(s);
      char newkey=keys.waitForKey();
      lcd.clear();
      num1="";  
      num2="";
      x=1;
      i=0;
      lcd.setCursor(i,0);
      if(newkey!='*'&&newkey!='#')
      {
        num1=num1+newkey;
        lcd.print(newkey);
        i++;
      }
      else
      {
        lcd.clear();
        lcd.setCursor(0,0);
      } 
    }
    if(op=='B')
    {
      lcd.setCursor(0,0);
      long sum = n1-n2;
      String s = String(sum);
      lcd.print(s);
      char newkey=keys.waitForKey();
      lcd.clear();
      num1="";  
      num2="";
      x=1;
      i=0;
      lcd.setCursor(i,0);
      if(newkey!='*'&&newkey!='#')
      {
        num1=num1+newkey;
        lcd.print(newkey);
        i++;
      }
      else
      {
        lcd.clear();
        lcd.setCursor(0,0);
      } 
    }
    if(op=='C')
    {
      lcd.setCursor(0,0);
      long sum = n1*n2;
      String s = String(sum);
      lcd.print(s);
      char newkey=keys.waitForKey();
      lcd.clear();
      num1="";  
      num2="";
      x=1;
      i=0;
      lcd.setCursor(i,0);
      if(newkey!='*'&&newkey!='#')
      {
        num1=num1+newkey;
        lcd.print(newkey);
        i++;
      }
      else
      {
        lcd.clear();
        lcd.setCursor(0,0);
      } 
    }
    if(op=='D')
    {
      lcd.setCursor(0,0);
      if(n2!=0)
      {
        long sum = n1/n2;
        String s = String(sum);
        lcd.print(s);
      }
      else
      {
        lcd.print("ERROR-div/0");
      }
      char newkey=keys.waitForKey();
      lcd.clear();
      num1="";  
      num2="";
      x=1;
      i=0;
      lcd.setCursor(i,0);
      if(newkey!='*'&&newkey!='#')
      {
        num1=num1+newkey;
        lcd.print(newkey);
        i++;
      }
      else
      {
        lcd.clear();
        lcd.setCursor(0,0);
      } 
    }
  }
}