Why is my program entering a while loop when the condition is false?

Question

When I input a string operator whether it be addition(+), subtraction(-), multiplication(*), division(/) or module(%), it still enters the while loop even when I enter a valid input. I don't know what the problem could be because the while loop is working fine where I have to enter an int-value for variable num2.

import java.util.Scanner;

public class PolishNotationCalc {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        int num1;
        int num2;
        String operator;

        System.out.println("Polish notation calculator");

        System.out.print("Please enter an operation(+, -, *, /, %) ");
        operator = input.nextLine();

        while (!operator.equals("+") || !operator.equals("-") || !operator.equals("*") || !operator.equals("/") || !operator.equals("%")) {
            System.out.println("Please enter a valid operation ");
            operator = input.nextLine();
            if (operator.equals("+") || operator.equals("-") || operator.equals("*") || operator.equals("/") || operator.equals("%"))
                break;
        }

        System.out.print("");
        System.out.print("Please enter the first number ");
        num1 = input.nextInt();

        System.out.print("Please enter the second number ");
        num2 = input.nextInt();

        while (num2 == 0 && operator.equals("/")) {
            System.out.println("Please pick a non zero number: ");
            num2 = input.nextInt();
        }
        while (num2 == 0 && operator.equals("%")) {
            System.out.println("Please pick a non zero number: ");
            num2 = input.nextInt();

        }

        if (operator.equals("+"))
            System.out.println(num1 + " + " + num2 + " = " + (num1 + num2));

        else if (operator.equals("-"))
            System.out.println(num1 + " - " + num2 + " = " + (num1 - num2));

        else if (operator.equals("*"))
            System.out.println(num1 + " * " + +num2 + " = " + (num1 * num2));

        else if (operator.equals("/"))
            System.out.println(num1 + " / " + num2 + " = " + (num1 / num2));

        else if (operator.equals("%"))
            System.out.println(num1 + " % " + num2 + " = " + (num1 % num2));

    }

}

Answer 1

It's simple math that sometimes people just forgets.

Lets simplify the cases.

1 OR x = 1;

1 AND x = x;

Lets be more concrete. Let we have some variable, A, B and C.

In the first case, OR, we have:

Which means that if at least one is true, the all expression is true. In you case having one operation that does not appear means that the loop must be true. Therefore, you should put something like this:

Which means not A AND not B AND not C.

I hope I have helped.

Have a nice day. :)

Answer 2

The loop condition has a different meaning than you thought. It's always true for any input string. It's the kind of condition that you will never have to write (hopefully).

To understand this better, let's first define when an operator can be accepted, in English:

An operator is accepted if it's one of the five operators: +, -, *, /, %

"One of" basically means "or". Therefore the corresponding code is

operator.equals("+") ||
operator.equals("-") ||
operator.equals("*") ||
operator.equals("/") ||
operator.equals("%")

And we can wrap it in a method:

private static boolean acceptable(String operator) {
    return operator.equals("+") ||
           operator.equals("-") ||
           operator.equals("*") ||
           operator.equals("/") ||
           operator.equals("%");
}

Now the read-check-loop logic is very simple:

String operator;

do {
    System.out.println("Please enter a valid operation ");
    operator = input.nextLine();
} while (!acceptable(operator));

@SaclyrBarlonium, this is what I'm talking about. :P

Side Note

IMHO, every programmer should know De Morgan's laws so well that they can instinctively detect logical inconsistencies in the code. The inconsistency in this case is between the loop condition and the if statement in the loop body. If we put them side-by-side, we know that they are not equivalent, according to De Morgan's Law:

!operator.equals("+") || !operator.equals("-") || !operator.equals("*") || !operator.equals("/") || !operator.equals("%")
 operator.equals("+") ||  operator.equals("-") ||  operator.equals("*") ||  operator.equals("/") ||  operator.equals("%")

But they should be equivalent, because the intent are the same: to continue the loop when the user entered a invalid operator and to terminate otherwise.

Answer 3

You should substitute the || (or) with && (and)

SO

while (!operator.equals("+") || !operator.equals("-") || !operator.equals("*") || !operator.equals("/") || !operator.equals("%")) {

Becames

while (!operator.equals("+") && !operator.equals("-") && !operator.equals("*") && !operator.equals("/") && !operator.equals("%")) {

EDIT

You need an AND not an OR. Lets say you input "+",which is a valid operator. You'll enter the while because "+" NOT EQUALS "-", your second condition. This because the while conditions are in OR. If you put these conditions in AND (&&), the "+" does not enter the while because one of the conditions is not true ( !operator.equals("+") )

Answer 4

if you write your boolean selection in english it reads "While the operator does not equal "+" or it does not equal "-" or it does not equal "/" or it does not equal "*" or it does not equal "%" do the loop.

You need it to say "While the operator does not equal "+" AND it does not equal "-" AND it does not equal "/" AND it does not equal "*" AND it does not equal "%" do the loop.

Change || to && and it should work

For a while loop to be enacted all the parameters need to be true. So if one of the parameters is false, the while loop does not activate.

Answer 5

 while (!operator.equals("+") || !operator.equals("-") || !operator.equals("*") || !operator.equals("/") || !operator.equals("%")) {
        System.out.println("Please enter a valid operation ");
        operator = input.nextLine();
        if (operator.equals("+") || operator.equals("-") || operator.equals("*") || operator.equals("/") || operator.equals("%"))
            break;
    }

For example when you enter "+" it becomes while(false || true|| true || true). so it always goes into while loop

Answer 6

operator is always not equal to at least one of these strings: e.g. if it is equal to +, then it's not equal to -. If you combine one or more true conditions with ||, the overall result will be true.

You need to use && instead of ||, so that the loop breaks if any one of the conditions match:

while (!operator.equals("+") && !operator.equals("-") && ... ) {

You then don't need to check the value of operator again inside the loop; just let the new loop guard check it and break.

---

A more syntactically concise alternative would be to use a collection:

List<String> allowedOperators = Arrays.asList("+", "-", "*", "/", "%");

while (!allowedOperators.contains(operator)) {
  System.out.println("Please enter a valid operation ");
  operator = input.nextLine();
}

Similarly, when you are checking for num2 == 0:

List<String> zeroDenominatorOps = Arrays.asList("/", "%");

if (zeroDenominatorOps.contains(operator)) {
  while (num2 == 0) {
    // ...
  }
}

Answer 7

Try this by changing ! to the whole parameters inside the while loop,It will start working

  while (!(operator.equals("+") || operator.equals("-") || operator.equals("*") || operator.equals("/") || operator.equals("%"))) {
            System.out.println("Please enter a valid operation ");
            operator = input.nextLine();
            if ((operator.equals("+") || operator.equals("-") || operator.equals("*") || operator.equals("/") || operator.equals("%")))
                break;
        }