Scanner loop doesn't cycle when hitting enter twice

Question

boolean loop = false;
double numberOfStudents;


System.out.print("Enter a number: ");

if ((scnr.nextLine().trim().isEmpty()) ) {
    loop = true;
}

while (loop) {
    System.out.println("Enter a number");

    if (scnr.hasNextDouble() ){
        System.out.println("Loop has stopped");
        numberOfStudents = scnr.nextDouble();
        loop = false;
    }

}           

System.out.println("You're outside the loop!");

I'm trying to get the program to say "Enter a number" until the user has entered an actual number (no white spaces or letters or signs). When the user has entered a number, it sets numberOfStudents equal to that number and breaks out of the loop.

But if you hit enter twice, it doesn't iterate. It only displays "Enter a number" once.

What is wrong with the loop logic? Why isn't it looping until valid input is taken?

Answer 1

For the actual answer to your question of "Why doesn't 'Enter a number' display more than once?" see Tom's comment (update: Tom's answer).

I've rewritten your loop in a way which preserves your code, but also makes it a little easier to handle format exceptions (though at the risk of silently swallowing an exception -- should be acceptable for this use case).

Can be up to you to use this design, here is an SO post on why empty catch blocks can be a bad practice.

public static void main(String args[])
{
    boolean loop = true;
    double numberOfStudents;
    Scanner scnr = new Scanner(System.in);

    while(loop){
        System.out.print("Enter a number: ");

        String input = scnr.nextLine();
        try{
            numberOfStudents = Double.parseDouble(input);
            loop = false;
        }catch(NumberFormatException e){
        }
    }
    System.out.println("You're outside the loop!");
}

Output:

Enter a number:
Enter a number:
Enter a number:
Enter a number: 50
You're outside the loop!

Answer 2

While trobbins code basically solves your problem, it's bad practice to use exceptions for flow control.

I used a small regexp to check if the value is a number. But this example is not complete, it will still crash it the user enters for example two decimal points. So you would need to create a proper number check or just use integers where the check is much easier.

Someone in the comments pointed out that people may want to enter scientific notation like 5e10, so this would also be another case to check for. If this is just some code you need as a proof of concept or something quick and dirty, you can go with the exception handling method but in production code you should avoid using exceptions this way.

double numberOfStudents;
Scanner scnr = new Scanner(System.in);

while(true) {
    System.out.print("Enter a number: ");
    String input = scnr.nextLine().trim();

    if(input.matches("^[0-9\\.]{1,}$")) {
        System.out.println("Loop has stopped");
        numberOfStudents = Double.parseDouble(input);
        break;
    }
}

System.out.println("You're outside the loop!");

Answer 3

First of all: Since you're reading from System.in a call to the input stream will block until the user entered a valid token.

So let's check first scan using your scnr variable:

scnr.nextLine()

nextLine() reads everything til the next line delimiter. So if you just press return, then it will successfully read it and will perform the next stuff.

The next call is:

scnr.hasNextDouble()

This call expects a "real" token and ignores white spaces, except as a delimiter between tokens. So if you just press return again it doesn't actually read that input. So it still waits for more (for the first token). That is why it stucks in your loop and you won't get another "Enter a number" output.

You can fix that by either enter a real token, like a number, or by changing the loop like trobbins said.

I hope you now understand your program flow a bit more :).

Answer 4

The following code should help you:

    double numberOfStudents = 0;
    Scanner scnr = new Scanner(System.in);

    boolean readValue = false;            //Check if the valid input is received
    boolean shouldAskForNumber = true;    //Need to ask for number again? Case for Enter
    do {
        if (shouldAskForNumber) {
            System.out.print("Enter a number:");
            shouldAskForNumber = false;
        }

        if (scnr.hasNextDouble()) {
            numberOfStudents = scnr.nextDouble();
            readValue = true;
        } else {
            String token = scnr.next();
            if (!"".equals(token.trim())) {   //Check for Enter or space
                shouldAskForNumber = true;
            }
        }
    } while (!readValue);

    System.out.printf("Value read is %.0f\n", numberOfStudents);
    System.out.println("You're outside the loop!");

Update

Understood the following statement in question different way:

But if you hit enter twice, it doesn't loop back. It only displays "Enter a number" once.

The code is set to print "Enter a number" only once if the user hits RETURN/ENTER or enters space character. You may remove the special check and use the code if needed.

Answer 5

import java.util.Scanner;

public class Testing {
    public static boolean checkInt(String s)
    {
        try
        {
            Integer.parseInt(s);
            return true;
        } catch (NumberFormatException ex)
        {
            return false;
        }
    }
    public static void main(String[] args) {
        boolean loop = false;
        double numberOfStudents;
        Scanner scnr = new Scanner(System.in);

            String input = "";

            while (!(checkInt(input))) {
                System.out.println("Enter a number");
                input = scnr.nextLine();
            }
            numberOfStudents = Integer.parseInt(input);

        System.out.println("Number of students: " + numberOfStudents );
    }
}

//this code is working fine, if you want you check it out. //In your code your taking another input if the first is an int/double; if the first input is not a number then you have mentioned to take input again..

Answer 6

Use a debugger to see what the code is actually doing. Here's a guide on debugging in Eclipse. After you have finished debugging your code, you will probably know what the problem is.

Answer 7

The following code is working,

boolean loop = true;
    double numberOfStudents;
        Scanner scnr=new Scanner(System.in);
       while(loop) {
            System.out.println("Enter a number");
               if ((scnr.nextLine().trim().isEmpty()) ) {
            loop = true;
        }
            if (scnr.hasNextDouble() ){
                System.out.println("Loop has stopped");
                numberOfStudents = scnr.nextDouble();
                loop = false;
            }
        }
    System.out.println("You're outside the loop!");

The output is,

run:
Enter a number
hj
po
Enter a number
lhf
Enter a number
o
Enter a number
p
Enter a number
a
Enter a number
34
Loop has stopped
You're outside the loop!

Answer 8

Below code will help you

    boolean loop = true;
    double numberOfStudents;
    Scanner scnr = new Scanner(System.in);
    System.out.print("Enter a number: ");
    String input = scnr.nextLine();
    while(!scnr.hasNextDouble()){
        System.out.print("Enter a number: ");
        try{
            numberOfStudents = Double.parseDouble(input);
            break;
        }catch(NumberFormatException e){
        }
        input = scnr.nextLine();
    }
    System.out.println("You're outside the loop!");

Answer 9

You have to scan the next line if you want to get more values form the scanner again. The code should be like:

while (loop) {
        System.out.println("Enter a number");
        if(!(scnr.nextLine().trim().isEmpty())){
            if (scnr.hasNextDouble() ){
                System.out.println("Loop has stopped");
                numberOfStudents = scnr.nextDouble();
                loop = false;
            }
        }
    }