While loop input int validation

Question

I’m having a problem with my code. my while loop 3 times when its meant to loop once i've tried everything i know and it's not working

import java.util.Scanner;

public class validInput {
    public static void main (String[] args) {
        Scanner key = new Scanner(System.in);

        System.out.print("Enter a number: ");
        String num = key.next();
        boolean isNumeric = true;
         isNumeric = num.matches("-?\\d+(\\.\\d+)?");

        while ((!isNumeric)) {
            System.out.println("You must enter an integer");
            num = key.next();
        }
        System.out.print("Valid");
    }
}

# outputs Enter a number: my first mistake
# You must enter an integer
# You must enter an integer
# You must enter an integer
# my first mistake

"...when its meant to loop once" what makes you think so? The key.next(); consumes single token, so when you write my first mistake the first next() will consume only my. — Pshemo
– Pshemo, Commented Nov 21, 2022 at 17:34
BTW you never update isNumeric inside loop, so it wont end since condition of while ((!isNumeric)) is based on isNumeric. — Pshemo
– Pshemo, Commented Nov 21, 2022 at 17:34

aatwork · Accepted Answer · 2022-11-21 17:29:19Z

2

Check the while loop below. You forgot to update isNumeric value within loop.

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

    System.out.print("Enter a number: ");
    String num = key.next();
    boolean isNumeric = true;
     isNumeric = num.matches("-?\\d+(\\.\\d+)?");

    while ((!isNumeric)) {
        System.out.println("You must enter an integer");
        num = key.next();
        isNumeric = num.matches("-?\\d+(\\.\\d+)?");
    }
    System.out.print("Valid");
}

answered Nov 21, 2022 at 17:29

aatwork

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1 Comment

Pshemo Over a year ago

Or get rid of isNumeric variable and move test directly to condition like while ((!num.matches("-?\\d+(\\.\\d+)?"))){...}

ElishevaMiler · Accepted Answer · 2022-11-21 17:44:19Z

2

public class Eli {

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

    System.out.print("Enter a number: ");
    String num = key.nextLine();
    boolean isNumeric = true;
    isNumeric = num.matches("-?\\d+(\\.\\d+)?");

    while ((!isNumeric)) {
        System.out.println("You must enter an integer");
        num = key.nextLine();
        isNumeric = num.matches("-?\\d+(\\.\\d+)?");
    }
    System.out.print("Valid");

}

}

note: You have 2 mistakes *You wrote key.next() instead of key.nextLine() *You forgot to check inside the loop whether isNumeric and it didn't check correctness from the second input onwards

answered Nov 21, 2022 at 17:44

ElishevaMiler

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