Using while loops to validate input

Question

I want to make the user enter only "y" "Y" "n" or "N". The resulting code asks the user to enter Y or N when they have entered the correct input, the opposite of what I expected by placing the ! in front of input.equalscaseignore(input).

import java.util.Scanner;


class inputVal {

    public static void main(String[] args) {
        String Input;
        Scanner keyboard = new Scanner(System.in);

        System.out.println("Please enter Y or N. ");
        Input = keyboard.nextLine();

        while (!Input.equalsIgnoreCase("Y") || !Input.equalsIgnoreCase("N"))
        {
            System.out.println("Please enter Y or N");
            Input = keyboard.nextLine();

        }


        keyboard.close();
    }

}

the problem is if user input N then !Input.equalsIgnoreCase("Y") become true ..and if user input Y then !Input.equalsIgnoreCase("N") become true so it always go to inside of the while — Madhawa Priyashantha
– Madhawa Priyashantha, Commented Oct 17, 2014 at 17:24

gtgaxiola · Accepted Answer · 2014-10-17 17:21:52Z

6

It is your or condition, you must change it to and

while (!Input.equalsIgnoreCase("Y") && !Input.equalsIgnoreCase("N"))

Just reading it out loud would help understand the logic:

WHILE Input doesn't equal (ignore case) "Y" AND Input doesn't equal (ignore case) "N"

answered Oct 17, 2014 at 17:21

gtgaxiola

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Comments

ajb · Accepted Answer · 2014-10-17 17:27:37Z

To elaborate on @gtgaxiola's answer:

!Input.equalsIgnoreCase("Y") || !Input.equalsIgnoreCase("N")

Suppose Input is "Y". The left operand of || is false, since Input equals "Y". But the right operand is true, since Input is not equal to "N". And false || true is true, since the result of || is true whenever either operand is true. In fact, this expression is always going to be true no matter what Input is.

That's why you need to use &&:

!Input.equalsIgnoreCase("Y") && !Input.equalsIgnoreCase("N")

because you want the result to be true only if both equalsIgnoreCase calls return false.

Alternatively, you could say

! (Input.equalsIgnoreCase("Y") || Input.equalsIgnoreCase("N"))

See also this article on DeMorgan's Laws.

(By the way, the convention in Java is to use names starting with lower-case letters for variables, i.e. input.)

sfedak · Accepted Answer · 2014-10-17 17:25:28Z

1

Your boolean logic is what's killing you here

while (!Input.equalsIgnoreCase("Y") || !Input.equalsIgnoreCase("N"))

You want to be in the loop while the input is not Y AND not N, therefore you need to change it to

while (!Input.equalsIgnoreCase("Y") && !Input.equalsIgnoreCase("N"))

answered Oct 17, 2014 at 17:25

sfedak

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Comments

Larry · Accepted Answer · 2014-10-17 17:24:38Z

0

My code logic is rusty but I believe that the issue is in the || operand.

One of the answers will always be FALSE therefor it won't enter the loop correctly.

answered Oct 17, 2014 at 17:24

Larry

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Comments

RunFranks525 · Accepted Answer · 2014-10-17 17:30:09Z

0

Here is great tool to trace your code with: http://cscircles.cemc.uwaterloo.ca/java_visualize/#

answered Oct 17, 2014 at 17:30

RunFranks525

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