Why is Java skipping my second condition in the while loop?

Question

Does anyone know why the first condition is being skipped by Java?

while((withdraw % 100) != 0 && (withdraw > startBalance))

Although I state that the withdrawal must be less than the startBalance, you can still type a number that is higher than the startBalance and the the newBalance will be negative.

Here's my code:

public static void main(String[] args) {

  Scanner input = new Scanner(System.in);
  int startBalance = 1000;
  System.out.println("Please enter how much you want to withdraw: ");
  int withdraw = input.nextInt();

  while((withdraw % 100) != 0 && (withdraw > startBalance)){
    System.out.println("Sorry, you can only withdraw a value multiple of 100 (we only have 100 SEK bills): ");
    withdraw = input.nextInt();
  }

  int newBalance = startBalance-withdraw;
  System.out.println("Thanks! Your new balance is: SEK " + newBalance);
}

Use & instead of && if you want the second condition to be evaluated — borchero, Sep 17 '15 at 14:29

StackFlowed · Accepted Answer · 2015-09-17T14:27:28.400

4

If the first condition is false then it wont consider the second condition. If first condition is true then it will also evaluate the second condition. This is because you are using && operation. If you use || then if the first one is false it will evaluate the next condition.

edited Sep 17 '15 at 14:27

answered Sep 17 '15 at 14:22

StackFlowed

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@T.J.Crowder nextInt(); takes care of that you can do that what you can't do it nextInt(); nextLine(); and then nextInt(); again that time you need to clear the new line. – StackFlowed Sep 17 '15 at 14:28
The usual issue is `nextInt()` failing to parse, but being retried an a loop without clearing the invalid input. – Marko Topolnik Sep 17 '15 at 14:31
@StackFlowed: Thanks. :-) – T.J. Crowder Sep 17 '15 at 14:35
Thank you! Now I see how silly the error was. Forgive me, I'm new to programming. – Pedro Marinho Oct 20 '15 at 19:04

score 3 · Answer 2 · answered Sep 17 '15 at 14:35

while((withdraw % 100) != 0 && (withdraw > startBalance))

Let me read out your condition in plain words:

"Keep looping as long as both of these conditions hold:

the requested amount is not round;
it is larger than start balance."

So, let's say we request 1,000,000. Is it "not round"? No. Do both conditions hold? No. Therefore, the loop is over.

As a side point, this has nothing to do with the distinction between & and && or the order of evaluation. It is just plain, common-sense logic.

Thanks for the explanation! I'm new to programming that's why I'm making these silly logical mistakes. — Pedro Marinho, Oct 20 '15 at 19:05

score 0 · Answer 3 · answered Sep 17 '15 at 14:33

try this code:

import java.util.Scanner;

public class Balance {

    public static void main(String[] args) {

          Scanner input = new Scanner(System.in);
          int startBalance = 1000;
          System.out.println("Please enter how much you want to withdraw: ");
          int withdraw = input.nextInt();

          while((withdraw % 100) != 0 || (withdraw > startBalance)){
            System.out.println("Sorry, you can only withdraw a value multiple of 100 (we only have 100 SEK bills): ");
            withdraw = input.nextInt();
          }

          int newBalance = startBalance-withdraw;
          System.out.println("Thanks! Your new balance is: SEK " + newBalance);
        }
}

I used

|| (or)

instead of

&& (and)

because we ALWAYS need to check if the user has enough balance :)

Thank you! I fixed it exactly like you did it. I'm new to programming that's why I make these kinds of mistakes. — Pedro Marinho, Oct 20 '15 at 19:06

score 0 · Answer 4 · answered Sep 17 '15 at 14:38

My assumption is you don't want the loop to check if the modulo % of 100 does not equal AND the withdrawal is greater than the start balance.

Double ampersands && check if both the arguments are true and unless your starting balance was less than 0, the conditions for the while loop will never be met.

Therefore, you want to use the or operators || which will check if either one or the other conditions are met which is what you are looking for.

Change: while((withdraw % 100) != 0 && (withdraw > startBalance))

To: while((withdraw % 100) != 0 || (withdraw > startBalance))

This will fix your issue.

Why is Java skipping my second condition in the while loop?

4 Answers4