How to correct the scope of a variable that is created within a do-while loop?

Question

I am struggling to get the correct scope for my variable "input".

I am making a calculator for a university task, and I've got everything working apart from when I tried to make it loop by wrapping my main code in a do-while loop. Because the variable "input" is declared in the "do" part of the loop, it didn't know what it was when I was trying to use it in the "while" condition. To fix this I then declared "input" as a string before my do-while loop to make it a global. However, now the scanner that takes the value of input will not work. AM I doing something stupid or am I missing something?

import java.util.Scanner;

public class Calculator {

    public static void main(String [] args) {

        String input;

        do {

            System.out.println("Welcome to the calculator. Please enter an operator (+, -, /, *) below:");

            Scanner myScanner = new Scanner(System.in);
            String oper = myScanner.nextLine();

            System.out.println("Now please enter two numbers:");
            double a = myScanner.nextDouble();
            double b = myScanner.nextDouble();

            switch (oper) {
            case "+" :
                System.out.println(CalculatorUtils.add(a, b));
                break;
            case "-" :
                System.out.println(CalculatorUtils.subtract(a, b));
                break;
            case "/" :
                System.out.println(CalculatorUtils.divide(a, b));
                break;
            case "*" :
                System.out.println(CalculatorUtils.multiply(a, b));
                break;
            }

        System.out.println("Do you want to complete another calculation? (y/n)");
        input = myScanner.nextLine();


        }
        while (input.contentEquals("y"));

    }


}

I expect this to be the output: Welcome to the calculator. Please enter an operator (+, -, /, *) below: + Now please enter two numbers: 32.5 12.5 45.0 Do you want to complete another calculation? (y/n) y (This is where the code would start again)

However I'm not being able to enter my input when being asked if I would like to do another calculation.

What do you mean by "not being able to enter my input"? Are your keypresses not recorded on the terminal? — Kayaman, Oct 23 '19 at 10:35
What do you mean by 'correct the scope'? There is no scope problem here that needs correcting. — user207421, Oct 23 '19 at 10:36
@AndrewMcDowell that wouldn't help. The `input` is read before the `while`, so it should work correctly. It's unclear what "not able to enter input" means actually here. — Kayaman, Oct 23 '19 at 10:40
@Kayaman I think he wants to say the program is getting exited before being asked for "do another calculation". — Kaustubh Khare, Oct 23 '19 at 10:44
Sorry if I wasn't clear enough. Currently when I ask the user if they want to complete another calculation, it will not let the user enter anything. It is as if I haven't put the line: input = myScanner.nextLine(); — Olivia Kelly, Oct 23 '19 at 11:02
Possible duplicate of [Scanner is skipping nextLine() after using next() or nextFoo()?](https://stackoverflow.com/questions/13102045/scanner-is-skipping-nextline-after-using-next-or-nextfoo) — Kevin Anderson, Oct 23 '19 at 12:29
@KevinAnderson I think you are correct after looking into it. — Olivia Kelly, Oct 23 '19 at 13:19

Kshitij Dhakal · Answer 1 · 2019-10-23T10:56:02.207

Here is the fix.

import java.util.Scanner;

public class Calculator {

    public static void main(String[] args) {

        String input;

        do {

            System.out.println("Welcome to the calculator. Please enter an operator (+, -, /, *) below:");

            Scanner myScanner = new Scanner(System.in);
            String oper = myScanner.nextLine();

            System.out.println("Now please enter two numbers:");
            double a = myScanner.nextDouble();
            double b = myScanner.nextDouble();

            switch (oper) {
                case "+":
                    System.out.println(CalculatorUtils.add(a, b));
                    break;
                case "-":
                    System.out.println(CalculatorUtils.subtract(a, b));
                    break;
                case "/":
                    System.out.println(CalculatorUtils.divide(a, b));
                    break;
                case "*":
                    System.out.println(CalculatorUtils.multiply(a, b));
                    break;
            }
            myScanner.nextLine();
            System.out.println("Do you want to complete another calculation? (y/n)");
            input = myScanner.nextLine();
myScanner.nextLine();


        }
        while (input.contentEquals("y"));

    }


}

It happens because second time you call myScanner.nextLine() it just scans enter from before. It will happen after myScanner.nextDouble() but not after myScanner.nextLine() because myScanner.nextLine() reads/scans until including next newLine character (\n) whereas myScanner.nextDouble() will just scan a double and leave.

Here is similar thread

score 0 · Answer 2 · answered Oct 23 '19 at 10:54

0

What you do not want to do is create a Scanner on every trip around the loop. Move the definition and initialization of your Scanner variable outside the loop:

    String input;
    Scanner myScanner = new Scanner(System.in);
    do {
        System.out.println("Welcome to the calculator. Please enter an operator (+, -, /, *) below:");

        String oper = myScanner.nextLine();

        // rest of loop...

    } while (input.contentEquals("y"));

This may or may not solve you're immediate problem, but it's still the right thing to do in general.

answered Oct 23 '19 at 10:54

Kevin Anderson

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Thank you, I have moved it outside of the do-while loop. It hasn't solved my problem, but it had improved my best practise – Olivia Kelly Oct 23 '19 at 12:16
I think you need to do `myScanner.nextLine()` after reading in the values of `a` and `b`. The explanation for why is somewhat technical and tedious, but it should help. – Kevin Anderson Oct 23 '19 at 12:30

How to correct the scope of a variable that is created within a do-while loop?

2 Answers2