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Practice. Developing "number guessing game" step by step

In the lesson we will practise using the basic Java tools learned in previous articles. To do it let's develop the "Guess game". Its rules are as follows:

  • Computer proposes a number from 1 to 1000.
  • Human player tries to guess it. One enters a guess and computer tells if the number matches or it is smaller/greater than the proposed one.
  • Game continues, until player guesses the number.

Step 1. Class & main function

Let's call the class "NumberGuessingGame" and add empty main function. It's completely valid program you can compile and run, but it doesn't print anything to the console yet.

public class NumberGuessingGame {

      public static void main(String[] args) {

 

      }

}

 

Step 2. Secret number

To propose a secret number, we declare a variable secretNumber of type int and use Math.random() function to give it random value in range 1..1000.

public class NumberGuessingGame {

      public static void main(String[] args) {

            int secretNumber;

            secretNumber = (int) (Math.random() * 999 + 1);

            System.out.println("Secret number is " + secretNumber); // to be removed later

      }

}

Secret number is 230

Don't worry, if you don't understand how things with random work. It's not a subject of the lesson, so just believe it. Note, that program exposes secret number to player at the moment, but we will remove the line printing the proposal in the final version.

Step 3. Asking user for a guess

In order to get input from user, we declare another variable guess of type int. Code, reading input from user is not to be discussed in detail here, so take it on trust.

import java.util.Scanner;

 

public class NumberGuessingGame {

      public static void main(String[] args) {

            int secretNumber;

            secretNumber = (int) (Math.random() * 999 + 1);

            System.out.println("Secret number is " + secretNumber); // to be removed

            // later

            Scanner keyboard = new Scanner(System.in);

            int guess;

            System.out.print("Enter a guess: ");

            guess = keyboard.nextInt();

            System.out.println("Your guess is " + guess);

      }

}

Secret number is 78
Enter a guess: 555
Your guess is 555

Step 4. Checking if guess is right

Now let's check if human player's guess is right. If so program should print corresponding message. Otherwise, tell user that a guess is smaller/greater than the proposed number. To test the program let's try all three cases (remember that we peeked the secret number).

import java.util.Scanner;

 

public class NumberGuessingGame {

      public static void main(String[] args) {

            int secretNumber;

            secretNumber = (int) (Math.random() * 999 + 1);

            System.out.println("Secret number is " + secretNumber); // to be removed

            // later

            Scanner keyboard = new Scanner(System.in);

            int guess;

            System.out.print("Enter a guess: ");

            guess = keyboard.nextInt();

            System.out.println("Your guess is " + guess);

            if (guess == secretNumber)

                  System.out.println("Your guess is correct. Congratulations!");

            else if (guess < secretNumber)

                  System.out

                             .println("Your guess is smaller than the secret number.");

            else if (guess > secretNumber)

                  System.out

                             .println("Your guess is greater than the secret number.");

      }

}

Secret number is 938
Enter a guess: 938
Your guess is 938
Your guess is correct. Congratulations!

Secret number is 478
Enter a guess: 222
Your guess is 222
Your guess is smaller than the secret number.

Secret number is 559
Enter a guess: 777
Your guess is 777
Your guess is greater than the secret number.

Things seem to be working ok.

Step 5. Add tries

At the moment user has only one attempt to guess a number, which is, obvious, not sufficient. Our next step is about giving user as many attempts as one needs. For this purpose let's use do-while loop, because user must enter a guess at least once.

import java.util.Scanner;

 

public class NumberGuessingGame {

      public static void main(String[] args) {

            int secretNumber;

            secretNumber = (int) (Math.random() * 999 + 1);

            System.out.println("Secret number is " + secretNumber); // to be removed

            // later

            Scanner keyboard = new Scanner(System.in);

            int guess;

            do {

                  System.out.print("Enter a guess: ");

                  guess = keyboard.nextInt();

                  System.out.println("Your guess is " + guess);

                  if (guess == secretNumber)

                        System.out.println("Your guess is correct. Congratulations!");

                  else if (guess < secretNumber)

                        System.out

                                   .println("Your guess is smaller than the secret number.");

                  else if (guess > secretNumber)

                        System.out

                                   .println("Your guess is greater than the secret number.");

            } while (guess != secretNumber);

      }

}

Secret number is 504

Enter a guess: 777

Your guess is 777

Your guess is greater than the secret number.

Enter a guess: 333

Your guess is 333

Your guess is smaller than the secret number.

Enter a guess: 504

Your guess is 504

Your guess is correct. Congratulations!

Final step. Make it glow

Just before we consider the program to be complete, let's remove the code, used for debug purposes.

import java.util.Scanner;

 

public class NumberGuessingGame {

      public static void main(String[] args) {

            int secretNumber;

            secretNumber = (int) (Math.random() * 999 + 1);           

            Scanner keyboard = new Scanner(System.in);

            int guess;

            do {

                  System.out.print("Enter a guess (1-1000): ");

                  guess = keyboard.nextInt();

                  if (guess == secretNumber)

                        System.out.println("Your guess is correct. Congratulations!");

                  else if (guess < secretNumber)

                        System.out

                                   .println("Your guess is smaller than the secret number.");

                  else if (guess > secretNumber)

                        System.out

                                   .println("Your guess is greater than the secret number.");

            } while (guess != secretNumber);

      }

}

Enter a guess (1-1000): 500

Your guess is greater than the secret number.

Enter a guess (1-1000): 250

Your guess is smaller than the secret number.

Enter a guess (1-1000): 375

Your guess is smaller than the secret number.

Enter a guess (1-1000): 437

Your guess is smaller than the secret number.

Enter a guess (1-1000): 468

Your guess is smaller than the secret number.

Enter a guess (1-1000): 484

Your guess is greater than the secret number.

Enter a guess (1-1000): 476

Your guess is greater than the secret number.

Enter a guess (1-1000): 472

Your guess is correct. Congratulations!

Extra tasks

If you would like to practise on your own, there are suggestions of possible improvements:

  1. Check, if user enters the number in range 1.1000 and force one to re-enter a guess in case it is out of bounds.
  2. Limit the number of attempts.
  3. Add more than one round and wins/loses score.

Algorithmic part. Binary search algorithm

How would you answer the question: "How much attempts one needs the most to guess the number?" The right answer is 10. Check binary search algorithm explanation to see why.

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