Why/How does setting the integer to -1 make my while loop work?

Question

This is my code. It is fairly simple, I am just asking the user for a guess, and telling them if they are correct, high or low to the number. To continuously ask, I made a while loop.

Although, I am not sure why or how it works the way it does. I initialize the int as -1 for guess, and it seems to work. Why wont this work without initializing it at all, and why a negative number to begin with. Doing this just seems to be a bit out of place. I am guessing this isn't common practice?

Thanks again! :)

import java.util.Scanner;
public class numGuess
{
public static void main(String[] args)
{
    //generate random number between 1 and 100
    int number = (int)(Math.random() * 101);

    Scanner input = new Scanner(System.in);
    System.out.println("Guess a number between 1 and 100");

    int guess = -1;
    while (guess != number)
    {
       System.out.print("\nEnter your guess: ");

       guess = input.nextInt();


       if (guess == number)
           System.out.println("you are correct, the number is " + number);
       else if (guess > number)
           System.out.println("your guess is too high");
       else
           System.out.println("your guess is too low");
    }
}

}

Answer 1

Java is a strongly typed language and thus requires a variable to be defined before it can be used.

You have to define guess before you can use it. The loop will work as long as guess is initially defined to be anything that is not number.

You could define guess like so to make sure it's different. You would never want to do this. I just want to show that pretty much any initial value could be used to initialize guess.

int guess = number - 1;

You could also use a do..while loop to not initially assign it. This would be the preferred way of doing it since you're not making any assumptions.

int guess;
do {
   System.out.print("\nEnter your guess: ");

   guess = input.nextInt();


   if (guess == number)
       System.out.println("you are correct, the number is " + number);
   else if (guess > number)
       System.out.println("your guess is too high");
   else
       System.out.println("your guess is too low");
}while(guess != number);

This the loop runs once and assigns guess from the user input directly instead of having to assign it before yourself.

Answer 2

There is no hard and fast rule to have -1 as guess value in your case. You could even initialize it to value greater than 101 or any number other than between 0 - 101.

Alternatively, you could use do while loop like below if you dont want to initialize it:

int guess;
do {
    System.out.print("\nEnter your guess: ");

   guess = input.nextInt();


   if (guess == number)
       System.out.println("you are correct, the number is " + number);
   else if (guess > number)
       System.out.println("your guess is too high");
   else
       System.out.println("your guess is too low");
} while (guess != number);

Answer 3

-1 is a sentinel value. That is, it's an invalid value for your program's purpose, and in particular, it's not the value of anything returned by (int) (Math.random() * 101) (which, BTW, generates a number between 0 and 100, not 1 and 100).

---

One way to avoid the "double duty" of guess is to change the loop to unconditionally loop, and then use break to break out of the loop if the guess is correct. Like so:

while (true) {
   System.out.print("\nEnter your guess: ");

   int guess = input.nextInt();

   if (guess == number) {
       System.out.println("you are correct, the number is " + number);
       break;
   } else if (guess > number) {
       System.out.println("your guess is too high");
   } else {
       System.out.println("your guess is too low");
   }
}

(Notice that I've added braces to your if block so that there can be multiple statements in the branches.) This is probably the cleanest way to do it.

Answer 4

You are using -1 as a flag (sentinel) value. It's a value that the user is not allowed to guess so it indicates no guess has happened yet. If you don't initialize it at all then it would start at zero, the flag value would be zero, the user shouldn't be allowed to guess zero and the random number should never be zero.

In other words you should be able to work with zero as your flag value. However, you're Math.random() doesn't seem to exclude zero. Take a look at Math.random() explained.

//generate random number between 1 and 100
int number = (int)(Math.random() * 101);

Should be

int number = (int)(Math.random() * 100) + 1;

Math.random() has a range from [0, 1). That means 0 is included but 1 is not. Your existing code will pick numbers from 0 to 100. You told the user you were going to pick from 1 to 100. Unless you intend to cheat that needs fixing.

It's also a nightmarish bug to test for since it only happens 1% of the time. Read the documentation for Math.random() carefully.

If you write a test that run's until value has been every number is supposed to be able to be it will only stop when you don't have a bug. As nightmarish as that is it turns out if you have it programmed correctly and let it loop 459 times there will only be a 1% chance that it hasn't picked 0 simply by chance. So you shouldn't have to wait to long for the test. I can show you the math behind that if you care.

Also, you may wish to consider a do-while loop so you don't need a flag in the first place. But do while or not you need to fix your Math.random() line.