How can I get this while loop to run when using a sentinel value for a variable with scanner input inside of the loop, while calling methods?

2 months into my first java programming. I'm trying to create a while loop in a class with methods and scanner inputs. Attempting to use a sentinel value to break out of the while loop. I've attempted while loops, do-while loops, with and without if-else blocks.

It looks like it's requiring me to initialize the salePrice for it to work, but the salePrice is entered by the user inside the loop. I have tried to assigning a fake value to it before the while loop, it then lets me start the loop, but 0 does not break of out of it, it just goes to the second question in the loop.

Is there something fundamentally wrong with this logic structure? Do the methods create a different need? I'm only 2 months into coding, so it has to be a very simple answer... Do I need to put if-else logic within the while loop just to break out of the loop with a sentinel value in this case?

public class PurchaseCalculator { //create main method public static void main(String[] args) { // declare variables // double salePrice; double discount = .15; double maxPurchaseAmount; double salePrice; // create scanner object Scanner userInput = new Scanner(System.in); while (salePrice > 0){ // prompt user to enter price System.out.println("Enter the price of the item. Enter 0 to end program"); // read in price from user and store in sale price salePrice = userInput.nextDouble(); //create a scanner object for salesPrice input System.out.println("Price entered: $" + salePrice); //prompt user to enter max purchase amount System.out.println("Enter the maximum purchase amount"); // read in max amount from user and store in maxPurchaseAmount maxPurchaseAmount = userInput.nextDouble(); // call displaypricewithtax method displayPriceWithTax(salePrice); // call computePriceAfterDiscountMethod computePriceAfterDiscount(salePrice, discount); // call displaypurchasablelimit method displayPurchasableNumber(salePrice, maxPurchaseAmount); } // close while loop } //close main 

1 Answer

In Java any local variable¹ must be initialized before its value can be read. Not to be confused with class fields², which are initialized with default values.

1 - variables declared inside statement blocks ({...})
2 - instance variables or class variables

Option 1: change the while loop to a do-while loop. Using this, the check is done after the iteration, so a value is assigned to the variable:

double salePrice; do { ... salePrice = userInput.nextDouble(); ... while (salePrice > 10); 

To avoid execution of the rest of the iteration, an if can be used; but mostly the next option (Option 2) is used instead:

double salePrice; do { ... salePrice = userInput.nextDouble(); if (salePrice >0) { ... } while (salePrice > 10); 

Option 2: use an if inside the loop:

while (true) { // for ever ... salePrice = userInput.nextDouble(); if (salePrice <= 0) break; } 

Option 3: use a flag (boolean) to control the loop

boolean stop = false; while (!stop) { ... salePrice = userInput.nextDouble(); stop = salePrice <=0; ... } 

Option 4: assign a fake initial value:

double salePrice = 1.0; while (salePrice > 0) { ... salePrice = userInput.nextDouble(); ... } 

(I would suggest going by first or second option, eventually 3rd for some cases)