While Loop Not Terminating With Boolean Value Changing

Question

A working example is to have a while loop that continues to get a number from the user until they guess the number correctly, then it will return true and exit out the while loop.

However, if I create a method that returns whether the variable is true or false, the while loop does not terminate. But, if the IF condition is inside the while loop, it terminates and work as intended.

Code that Works as Intended:

 private void print() { // Method name not decided yet
        boolean playerGuessedCorrectly = false;
        while(!playerGuessedCorrectly) {
             [...] // Other code that asks the user to enter a new number
             // userGuess is the next int entered by the user
            // computerNumber is a random number between 1 and 100
             if(userGuess == computerNumber) {
                      System.out.print("You have guessed my number.");
                      playerGuessedCorrectly = true; 
              } // while loop terminates upon true
        } 
}

This code works and the while loop stops working when it is changed to true. However, when I put the same code in a method, the while loop continues going around and does not terminate:

Not Working Code:

private void print() {
        boolean playerGuessedCorrectly = false;
        while(!playerGuessedCorrectly) {
              checkIfGuessIsMyNumber(playerGuessedCorrectly);
        } // value of playerGuessedCorrectly is sent to method checkIfGuessIsMyNumber();

private boolean checkIfGuessIsMyNumber(boolean playerGuessedCorrectly) {
    // userGuess is the next int entered by the user
    // computerNumber is a random number between 1 and 100
    if(userGuess == computerNumber) {
        System.out.print("You have guessed my number.");
        return playerGuessedCorrectly = true;
    } else {
        playerGuessedCorrectly = false;
    }
}

To clarify, the while loop terminates when IF userGuess == computerNumber is inside the while loop, however, it does not work when it is split up into a different method and returns the value.

Note that when I print out the value given by the method, it has printed true. So I am confused to why it doesn't terminate when the value in the method is true, but it terminates with the if condition inside the while loop.

Sam Estep · Accepted Answer · 2015-06-27 12:58:23Z

This line:

playerGuessedCorrectly = false;

and this line:

return playerGuessedCorrectly = true;

don't work because Java passes parameters by value. You're actually assigning false to the copy of playerGuessedCorrectly that is local to the checkIfGuessIsMyNumber function, not to the original version in the print function.

Following the not-working code line by line:

boolean playerGuessedCorrectly = false;

This works as intended.

while(!playerGuessedCorrectly) {
    // ...
}

This would work as intended if the value of playerGuessedCorrectly was updated properly.

checkIfGuessIsMyNumber(playerGuessedCorrectly);

This is where you go wrong. This copies the value of the currently existing playerGuessedCorrectly variable into the parameter of the checkIfGuessIsMyNumber function. In that function, the new copy of playerGuessedCorrectly is reassigned different values, and you have a return statement, but note that you never actually use the return value of that function. If you instead did something like this:

playerGuessedCorrectly = checkIfGuessIsMyNumber(playerGuessedCorrectly);

your code would work as intended. You would also probably want to clean up the body of your checkIfGuessIsMyNumber function:

if(userGuess == computerNumber) {
  System.out.print("You have guessed my number.");
  return true;
} else {
  return false;
}

Mureinik · Accepted Answer · 2015-06-27 12:56:24Z

Parameters in Java are passed by value. So when you call playerGuessedCorrectly = false, you just override the local copy of the playerGuessedCorrectly parameter.

A different approach to enclose your logic in a function is to have it return the appropriate value:

private void print() {
    boolean playerGuessedCorrectly = false;
    while(!playerGuessedCorrectly) {
        playerGuessedCorrectly = checkIfGuessIsMyNumber();
    }
}

private boolean checkIfGuessIsMyNumber() {
   // userGuess is the next int entered by the user
   // computerNumber is a random number between 1 and 100
   if(userGuess == computerNumber) {
         System.out.print("You have guessed my number.");
         return true;
   } else {
         return false;
   }
}

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