#include <Keypad.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27,16,2);

 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'}
};

byte rowPins[ROWS] = {9, 8, 7, 6}; // - detail arduino - pin detail - 
byte colPins[COLS] = {5, 4, 3, 2}; 

Keypad customKeypad = Keypad(makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS); 

void setup(){
  Serial.begin(9600);
  lcd.init();
  lcd.backlight();
}
  
void loop(){
  
   static int a=0;

   static int c=0;
   static int r=0;
   lcd.setCursor(c,r);
   static char ch;
   static int b=0;
   static int x=0;
  
  char customKey = customKeypad.getKey();
    
 if(customKey) // get the display on serial monitor
 {
   if(customKey=='A') // * symbol is used as enter key -- 
   {
     ch='+';
     b=a;
     lcd.init();
     a=0;
     c=0;
     lcd.setCursor(0,0);
   
   } 
   else
   if(customKey=='B') // * symbol is used as enter key -- 
   {
     ch='-';
     b=a;
     lcd.init();
     a=0;
     c=0;
     lcd.setCursor(0,0);
   
   }
   else
   if(customKey=='*')
   {
     if(ch=='+')
     {
        x=a+b;
        lcd.setCursor(0,0);
        lcd.print("sum - ");
        lcd.setCursor(0,1);
        lcd.print(x);
     }
     
      if(ch=='-')
     {
        x=b-a;
        lcd.setCursor(0,0);
        lcd.print("subtraction - ");
        lcd.setCursor(0,1);
        lcd.print(x);
     }
     

   }
   else
   if(customKey=='#')
   {
     lcd.init();
     a=0;
     b=0;
     c=0;
     r=0;
     x=0;

   }
   else
   {
     a=a*10+customKey-'0'; // -'0' method is char to int conversion
     lcd.init();
     lcd.print(a);
     c++;
   }
   
 }
 
  

}