Arduino Calculator Code

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

LiquidCrystal lcd(0, 1, 2, 3, 4, 5);
const byte ROWS = 4;
const byte COLS = 4;

char keys [ROWS] [COLS] = {
  {'7', '8', '9', '/'},
  {'4', '5', '6', '*'},
  {'1', '2', '3', '-'},
  {'C', '0', '=', '+'}
};
byte rowPins[ROWS] = {13 ,12 ,11 ,10};
byte colPins[COLS] = {9, 8, 7, 6};

Keypad myKeypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );


boolean presentValue = false;
boolean next = false;
boolean final = false;
String num1, num2;
int answer;
char op;

void setup()
{
  lcd.begin(16,2);
  lcd.setCursor(0,0);
  lcd.print("Electronics Hub ");
  lcd.setCursor(0,1);
  lcd.print("    Presents    ");
  delay(5000);
  lcd.setCursor(0,0);
  lcd.print(" Arduino based  ");
  lcd.setCursor(0,1);
  lcd.print("  Calculator"    );
  delay(5000);
  lcd.clear();
}

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

  if (key != NO_KEY && (key=='1'||key=='2'||key=='3'||key=='4'||key=='5'||key=='6'||key=='7'||key=='8'||key=='9'||key=='0'))
  {
    if (presentValue != true)
    {
      num1 = num1 + key;
      int numLength = num1.length();
      lcd.setCursor(15 - numLength, 0); //to adjust one whitespace for operator
      lcd.print(num1);
    }
    else 
    {
      num2 = num2 + key;
      int numLength = num2.length();
      lcd.setCursor(15 - numLength, 1);
      lcd.print(num2);
      final = true;
    }
  }

  else if (presentValue == false && key != NO_KEY && (key == '/' || key == '*' || key == '-' || key == '+'))
  {
    if (presentValue == false)
    {
      presentValue = true;
      op = key;
      lcd.setCursor(15,0);
      lcd.print(op);
    }
  }

  else if (final == true && key != NO_KEY && key == '='){
    if (op == '+'){
      answer = num1.toInt() + num2.toInt();
    }
    else if (op == '-'){
      answer = num1.toInt() - num2.toInt();
    }
    else if (op == '*'){
      answer = num1.toInt() * num2.toInt();
    }
    else if (op == '/'){
      answer = num1.toInt() / num2.toInt();
    }    
      lcd.clear();
      lcd.setCursor(15,0);
      lcd.autoscroll();
      lcd.print(answer);
      lcd.noAutoscroll();
  }
  else if (key != NO_KEY && key == 'C'){
    lcd.clear();
    presentValue = false;
    final = false;
    num1 = "";
    num2 = "";
    answer = 0;
    op = ' ';
  }
}

Arduino: 1.8.7 (Windows 10), Board: "Arduino/Genuino Uno"

calculator_XX:2:20: error: Keypad.h: No such file or directory

compilation terminated.

The code was not uploaded cos of this "Keypad.h: No such file or directory"

exit status 1

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.

Use a library!

Any commenting would be appreciated, good job on making coding for new-comers an absolute nightmare.

Hello, you just need to switch the code from :
{'7','8','9','D'},
{'4','5','6','C'},
{'1','2','3','B'},
{'*','0','#','A'}
to :

{'1','2','3','A'},
{'4','5','6','B'},
{'7','8','9','C'},
{'*','0','#','D'}