CODEWITHSUNDEEP
javainput
TonyW
TonyW

Reputation: 18875

why my scanner can only take two numbers? Java

I am trying to use a Scanner object to read keyboard input (double type numbers). The program compiles, but it can only take two numbers. Below is my code, please help me find the reason. Thank you!

    public static void main(String[] args)
    {
        Scanner keyboard = new Scanner(System.in);
        ArrayList<String> container = new ArrayList<String>();
        double number = 0;

        System.out.println("Type in the polynomials in increasing powers:");

        while (!keyboard.nextLine().isEmpty())
        {           
//          number = keyboard.nextDouble();
            try {
                     number = keyboard.nextDouble();
            } catch(Exception e)              // throw exception if the user input is not of double type
            {
                System.out.println("Invalid input");
            }


            container.add("-" + number);

        }

Upvotes: 0

Views: 340

Answers (1)

Sotirios Delimanolis
Sotirios Delimanolis

Reputation: 279970

The method call nextLine() in 

while (!keyboard.nextLine().isEmpty())

will consume the first double value you enter. This

number = keyboard.nextDouble();

will then consume the second one.

When the loop iterates again, keyboard.nextLine() will consume the trailing end of line characters which it will trim(). isEmpty() will therefore return true.

The solution if you want to enter a number, press enter, and keep entering numbers is to read the line as a String and use Double.parseDouble(String) to get a double value.

Otherwise, you can also use

while (keyboard.hasNextDouble()) {
    number = keyboard.nextDouble();
    System.out.println(number);
    ...
}

And enter your numbers on a single line separated by spaces

22.0 45.6 123.123123 -61.31 -

Use a random non-numerical character at the end to tell it input is done. The above prints

22.0
45.6
123.123123
-61.31

and stops.

Upvotes: 2

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