Scanner doesn't work properly after reading double

Question

Scanner doesn't wait for input after calling nextDouble() but it works fine after nextLine().

import java.util.*;

public class calc {
    public static final Scanner input = new Scanner(System.in);
    public static void main(String[] args) {
        while(true) {
            calculate();
            System.out.print("Would you like to run again? Y/N: ");
            String repeat = input.nextLine().toLowerCase();
            if (!(repeat.equals("y") || repeat.equals("yes"))) {
               System.out.println("\nBye!\n");
               break;
            }
        }
    }
    public static void calculate() {
        System.out.print("\nAdd, Subtract, Multiply, Divide or Modulus? : ");
        double fsnum, scnum, ans;
        String choice = input.nextLine().toLowerCase();
        if(choice.equals("add") || choice.equals("subtract") || choice.equals("multiply") || choice.equals("divide") || choice.equals("modulus")) {
            System.out.print("Enter the first number: ");
            fsnum = input.nextDouble();
            System.out.print("Enter the second number: ");
            scnum = input.nextDouble();
            switch(choice) {
                case "add":
                    ans = fsnum+scnum;
                    break;
                case "subtract":
                    ans = fsnum-scnum;
                    break;
                case "multiply":
                    ans = fsnum*scnum;
                    break;
                case "divide":
                    ans = fsnum/scnum;
                    break;
                case "modulus":
                    ans = fsnum%scnum;
                    break;
                default:
                    ans = 0;
                    break;
            }
            System.out.println("The answer is: " + ans);
        } else {
            System.out.println("ERROR: Please select a valid operation.");
        }
    }
}

Output

Add, Subtract, Multiply, Divide or Modulus? : add
Enter the first number: 3
Enter the second number: 4
The answer is: 7.0
Would you like to run again? Y/N:
Bye!

As you can see the last input to run again is skipped and the program seems to assume it as "" or null.
This doesn't seem to happen if an invalid operation is chosen. -

Add, Subtract, Multiply, Divide or Modulus? : yaya
ERROR: Please select a valid operation.
Would you like to run again? Y/N: n

Bye!

Any solutions?

score 1 · Accepted Answer · edited May 23 '17 at 11:58

1

Try putting input.nextLine() immediately after scnum = input.nextDouble(); I think you need to get rid of the newline character (which is not read by nextDouble()). See Using scanner.nextLine() for a better explanation :)

edited May 23 '17 at 11:58

Community

1
1

answered Jun 27 '15 at 13:21

atomSmasher

1,425
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Ahhh... That explains a lot of previous encounters as well. Thanks! – deleted Jun 27 '15 at 13:31

score 1 · Answer 2 · answered Jun 27 '15 at 13:29

1

Input.nextDouble() doesn't scans the new line charcater.It waits for you to give a numeric value as soon as it encounters new line character it returns leaving the new line character in the buffer.So add a dummy input.nextLine() after nextDouble()

answered Jun 27 '15 at 13:29

Sarfaraz Khan

2,048
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score 1 · Answer 3 · edited May 25 '20 at 22:46

1

Adding to what atomSmasher said you would easily solve this issue by replacing the calls to input.nextLine() by input.next().

edited May 25 '20 at 22:46

BSMP

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answered Jun 27 '15 at 14:04

alainlompo

4,130
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Scanner doesn't work properly after reading double

3 Answers3

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