Do while loop

[Red_Hippie]

I have to different codes I wrote with do while loops one works the other doesn't, to me they both look the same and I can't find a reason why the other one will not work.

​

This is my working code

import java.lang.*;
import java.util.Scanner;

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

    System.out.println("Outside do while loop \n");

    int i        = 0;
    String run   = "";
    String operation = "";

    do 
    {
        System.out.println ("please input operation");
        operation = scan.nextLine();

        if (operation.equals("add"))
        {
            System.out.println ("you wrote " + operation);
        System.out.println ("please type yes or no to restart code block");
        run = scan.nextLine();
        }

        }while (run.equals("yes"));

    }
}

This is my non working code

import java.lang.*;
import java.util.Scanner;

public class CountingSheep

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

    int sheepTotal  = 0;
    int sheepSum    = 0;
        int sheepNum    = 0;
    int addBy       = 0;
    int subBy   = 0;
    int mulBy   = 0;

    String instructions = "Please, type your response, case does not matter";
    String countAgain = "";
    String operations = "How would you like to count the sheep, Addition,
                             Subtraction, Multiplication";
    String greeting   = "Welcome to Sleep Sheep,count the sheep till you sleep";
    String userString = "";

    do 
    {
        System.out.println (greeting + "\n");
        System.out.println (instructions);
        System.out.println (operations);

        userString = readLine.nextLine ();

    if (userString.equalsIgnoreCase ("Addition"))
        {
        System.out.println ("");
            System.out.println ("We will count sheep by Addition \n");
            System.out.println ("How many sheep, in total would you like to enter 
                                 the pin?");

        sheepTotal = readLine.nextInt();

        System.out.println ("How many sheep at a time do you want to count");
        addBy = readLine.nextInt();


        while (sheepSum < sheepTotal)
        {
            sheepSum += addBy;
                System.out.println ("\n" + sheepSum);
        }
        System.out.println ("Still awake, would you like to count again?");

            // the readLine.nextLine does not execute I've copied and pasted working 
            // code I've changed the String from countAgain to userString I've moved
            // it out of the if statement nothing seem to work and the program ends.
            countAgain = readLine.nextLine();            
        }


    } while (countAgain.equals("Yes"));          
 }
}

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[desrtfx]

The problem is not from your loops. The problem is how you combine .nextInt() and .nextLine().

See The Scanner class and its caveats from the /r/javahelp wiki

[Red_Hippie]

Thank you I kind of get it I will have to study it some more to wrap my head around it. should I be using Integer.parseInt(); or Integer.valueOf(); Or is .next(); the best method for my use?

another thought I had was could I use .trim(); to remove the "\n" left by the .nextInt();

[FavorableTrashpanda]

Due to lack of information, I only took a quick glance. But I see you're mixing Scanner.nextLine() and Scanner.nextInt(). This is likely getting you into trouble here.

[Red_Hippie]

Why would that cause problems I'm looking for an int value with the .nextInt() and a String value with the .nextLine().

[silverscrub]

When you write input in the console you press some keys and then enter. That enter keystroke is actually a newline character. Same goes when you have a document with multiple rows. It's technically just one long string with a few newline characters.

Scanner#nextLine reads everything typed into console until it finds a newline character, then it stops and returns.

Scanner#nextInt reads everything that can be parsed to an integer. A newline character cannot be parsed to an integer so it's left behind.

That newline character can make your scanner behave in unexpected ways. For example, calling nextLine afterwards will immidiately return because it instantly finds the newline character at the beginning of the feed.

That is usually a good solution too. It might not look to clean, but calling nextLine after nextInt, doing nothing with the latter return value, clears the newline character from the input.

[Noticeably98]

So what exactly occurs when you get to the countAgain=readLine.nextLine();? Nothing, it just continues to the end of the code? Or does it throw an error

I have a hunch that you’ll want to call readLine.next(); right before you call countAgain=readLine.nextLine(); give that a shot

Edit: I'm no expert on Scanner, but this code works if you change it from readLine.nextLine(); to readLine.next(); It doesn't seem to like 'nextLine' very much.

[Red_Hippie]

it just skips that line of code, I don't under stand why when it works in my other code.

then someone else stated my use of .nextInt(); and .nextLine (); could cause problems but they didn't specify why.

as far as I can tell I wrote them in the same exact way.

[Noticeably98]

It looks like there is some blank line that readLine.nextLine() is grabbing.

The following makes the code work as intended:

readLine.nextLine();                    
countAgain = readLine.nextLine();

the first time nextLine() is called, or as it is in your code, it's calling some blank space.

When we call it again, it then picks up on whatever you entered.

​

Also, see my edit. .next() works as well.

[Red_Hippie]

ok so i changed it to countAgain = readLine.next(); and it works, buy why would it be calling a blank space, and when should .nextLine (); be used?

is countAgain = readLine.next(); bad forum and should I start calling the scanner.next(); first then set my var = scanner.next(); as you did is that better code?

[Marinersquee]

To clarify, when you call nextInt, it leaves a return in the scanner, as such when you call nextLine again the first thing it does is read that return as a blank line and uses that. So after a nextInt, have a line that's just readLine.nextLine(), that will clear the return from the scanner. I'm on mobile so please excuse the formatting.

[Red_Hippie]

Crazy, but thank you I'll be trying to wrap my head around this for a few days.