How to program calculator array to not include sentinel?

[ekennedybills]

Task is to create a basic calculator using double value and sentinel. I cannot figure out how to get the array to avoid counting the sentinel. To me, it looks like I've put in safeguards such as only looping if the value is >=0, but it still counts, lists, and even adds the sentinel number to the final result. I've tried changing the while statement to !=-1 with no change in the results. I'm a complete Java newbie (obviously), and have tried many options, but can't find the right one.

package arraypractice;

import java.util.ArrayList;
import java.util.Scanner;

public class CashRegister

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    {
    final int SENTINEL=-1;
    double priceFromUser=0;
    double total=0;
    double average=0;
    double count=0;
    boolean done=false;
    String trash="";
    String lists="";
    ArrayList<Double> list = new ArrayList<>();
    Scanner in = new Scanner(System.in);

    //Prompt user to enter a price, or -1 when finished entering prices.

    while (!done)
    {


    while (priceFromUser >= 0)
    {
        System.out.println("Please enter item price, or enter -1 to quit.");

        if(in.hasNextDouble())//if the input is a double
        {
            priceFromUser=in.nextDouble(); //read value from user
            list.add(priceFromUser); //add valid price to the list
        }
        else                

        {
            trash=in.nextLine(); //throw away the input 
            System.out.println("You did not enter a price.");

        }



    //add valid input to list


    //total the prices added by user, and keep track of how many were added

        {
             total += priceFromUser;
             count++;
        }

    }

    //print the array list of entries, the total cost, and the average cost.

    list.forEach(System.out::println);

    System.out.println("The total price of the items is $" + total +". The average cost per item is: $" + (total/count));
    done=true;

}

Output is:

run-single:
Please enter item price, or enter -1 to quit.
25
Please enter item price, or enter -1 to quit.
25
Please enter item price, or enter -1 to quit.
25
Please enter item price, or enter -1 to quit.
25
Please enter item price, or enter -1 to quit.
-1
25.0
25.0
25.0
25.0
-1.0
The total price of the items is $99.0. The average cost per item is: $19.8

[Dani]

I will preface this by saying that I don't know Java at all. I touched upon it in school 20+ years ago, but I've been doing PHP ever since.

However, on line 27, you say that if the input passed in is a double, execute this code block blah.

The user enters -1.

Is -1 not a double? It is a valid double, and therefore the code block executes, setting the priceFromUser to be -1 and adding it to the list.

Then, when the loop starts back up again on line 23, priceFromUser is now -1, so the loop breaks and prints the tally.

Also, note you're definining this constant SENTINEL variable, but you're not actually using it anywhere in your code.

Perhaps do something like this instead, changing the while loop into a do-while loop:

System.out.println("Please enter item price to start, since there needs to be at least 1 item:");

do
{

        // Keep asking for a price until the user enters a valid price
        while (!in.hasNextDouble())               
        {
                trash=in.nextLine(); //throw away the input 
                System.out.println("You did not enter a valid price. Please try again:");
        }

        priceFromUser=in.nextDouble(); //read value from user
        list.add(priceFromUser); //add valid price to the list            

          System.out.println("Please enter another item price, or enter -1 to quit.");

} while (priceFromUser != SENTINEL)

Note this isn't valid code because I'm not a Java programmer, but hopefully it helps you out a bit.

[JamesCherrill]

i think you'll find that still adds the -1 to the total before exiting the loop.

Given the need to exit the loop after validating the input but before processing it I would suggest this pattern:

while true
   get and validate input
   if (input = SENTINEL) break;
   process input