Java While循环以负数中断

Question

I am having a problem with this java project I am working on for tomorrow. I am supposed to make a while loop that prompts the user for the prices of items and end the loop with any negative number, that negative number cannot be counted. Then I have to prompt the user for their membership discount level plat 15% off/gold 7%/silver 3%/non member 0% off using variables 3,2,1,0 respectively. For invalid input of status of membership, the user needs to enter again with a valid one using a do-while loop. Then I use the appropriate discount % off the sum of the items then tax it. The last few steps are simple enough, but I am stuck at the loops.

I did some searching and found this block of code on this site that does the job for the first while loop but does not end in the way I need it to. Using While Loop instead of a For Loop in Java to ask User Input

I modified it to fit my needs but I still cannot find a way to make the throwable = to any negative number ie <0. Is that even possible? I am pretty sure there is a way to do this much simpler without try/catch which I have not learned yet. Here is my very slightly modified version

Scanner askPrice = new Scanner(System.in);
BigDecimal totalPrice = new BigDecimal("0");
Scanner keyboard = new Scanner(System.in);
while (true) {
    System.out.println("Enter item price or negative number to quit:") ;
    try {
        double price = askPrice.nextDouble();
        totalPrice = totalPrice.add(new BigDecimal(String.valueOf(price)));
    }
    catch (Throwable t) {
        break;
    }
    System.out.println(totalPrice.toString());
}

I tried turning the try and catch statements into if / else and replacing the catch throwable with else(askPrice <0), but I was getting tons of errors.

I greatly appreciate any help that can be offered. Thanks.

Answer 1

This sounds like homework. I know a lot of teachers are against students using 'break' to exit a loop. Here is how I would do it. A try-catch statement is unnecessary.

public static void main(String[]args) {

Scanner keyboard = new Scanner(System.in);
    double input = 0.0;
    double totalPrice = 0.0;

    do {
        totalPrice += input;
        input = keyboard.nextDouble();
    }

    while(input >= 0);
}

Using a while loop:

public static void main(String[]args) {

Scanner keyboard = new Scanner(System.in);
    double totalPrice = 0.0;
    double input = keyboard.nextDouble();

    while(input >= 0) {
    totalPrice += input;
    input = keyboard.nextDouble();
    }
}

Answer 2

Here is a simple do while loop that you are looking for

double price = 0;
do {
    try {
        price = askPrice.nextDouble();
        // if the price was  > 0 we need to add it to the total
        if(price > 0)
            totalPrice = totalPrice.add(new BigDecimal(String.valueOf(price)));  
    } catch(InputMismatchException e) {
        // exit our loop, if the user entered an invalid double. Maybe you want to display a message instead?
        price = -1; 
    }
} while(price >= 0);

The key is a do-while loop vs a while loop. The do-while loop will always run one time prior to analysing the loop condition. This allows you to accept the users input inside the loop and keep your code clean. I belive it is good practice to avoid using the break statement when possible. It will generally keep you from writing bad code (with the exception of switch statements of course).

Edit: For the record I think the try catch statement is not only necessary but mandatory for any user input. When a user enters an invalid number do you want a nasty error with a stack trace being thrown or your program to gracefully handle the bad input and notify the user. Most important it gets new programmers thinking about error handling, a very important part of software development.

Answer 3

Try/catch is for handling exceptions and totally unnecessary here.

    Scanner askPrice = new Scanner(System.in);
    BigDecimal totalPrice = new BigDecimal("0");
    Scanner keyboard = new Scanner(System.in);

    while (true) {

        System.out.println("Enter item price or negative number to quit:") ;
        double price = askPrice.nextDouble(); 
        if (price<0)
            break;
        else {
            totalPrice = totalPrice.add(new BigDecimal(String.valueOf(price)));       
        }
        System.out.println(totalPrice.toString());
   } 

Answer 4

You don't you check for negative number.

And you need to move your last System.out outside the loop

    Scanner askPrice = new Scanner(System.in);
    BigDecimal totalPrice = new BigDecimal("0");

    while (true) {

        System.out.println("Enter item price or negative number to quit:");
        try {
            double price = askPrice.nextDouble();
            if (price < 0)
                break;

            totalPrice = totalPrice.add(new BigDecimal(String.valueOf(price)));
        } catch (Throwable t) {
            break;
        }
    }
    System.out.println(totalPrice.toString());

Answer 5

I'm not sure of understand, but if you want exit with a negative number as input, use this:

    Scanner askPrice = new Scanner(System.in);
    BigDecimal totalPrice = new BigDecimal("0");
    Scanner keyboard = new Scanner(System.in);
    double price = 0.0d;
    try {
        while (0.0d <= price) {
         System.out.println("Enter item price or negative number to quit:") ;
         price = askPrice.nextDouble();        
         totalPrice = totalPrice.add(new BigDecimal(String.valueOf(price)));       
        }
     }catch (Throwable t) {           
        System.err.println(t.getMessage());         
     }
     System.out.println(totalPrice.toString());

Some points:

• try/catch always out of a cycle, beacuase the code is more readable and faster
• inside catch, you have to put a syserr, not sysout

• in the orginal code, the eternal lopp finish when the user put a dollar char, what throw an Exception in converting as number with instruction

  askPrice.nextDouble()

In your case, the instructions

 double price = askPrice.nextDouble();        
 totalPrice = totalPrice.add(new BigDecimal(String.valueOf(price))); 

don't throw any exception with negative number.

Answer 6

Normally it is better to write your own code instead of copying code for simple examples. That way you will learn a lot. In the code you have a missing closing brace for while loop, I hope there is a brace closing later.

You can temporarily fix the code by throwing an exception after getting the price:

double price = askPrice.nextDouble();
if(price < 0) throw new ArithmeticException("Negative Price!");

I am not sure whether that is what you want.