Bug :- Scanner class asks for input infinite times

Question

import java.util.*;
public class Composite_Magic
{
    public static void main()
    {
        int i,j,m,n,fact=0,sum=0,temp=0;
        boolean k=false;
        Scanner sc=new Scanner(System.in);
        System.out.println("Enter 2 numbers as upper and lower bound and all composite numbers between them will be displayed");
        m=sc.nextInt();
        n=sc.nextInt();
        sc.close();
        if(m<n){
            for(i=m;i<=n;i++)
            {
                for(j=1;j<=i;j++)
                {
                    if(i%j==0)
                        fact++;
                }                    
                sum=i;
                while(k==false)
                {
                    temp=sum;
                    while(temp>0)
                    {
                        sum=sum+(temp%10);
                        temp=temp/10;
                    }
                    if(sum/10==0)
                        k=true;
                }
                if(sum==1 && fact>2) 
                    System.out.println(i);
            }
        }
        else
            System.out.println("Invalid Input");
    }
}

So I have asked for input only twice but it doesn't stop.

Is this a bug or some error that I'm making?

This is my complete program.

This is the screenshot of the terminal window

It will ask for input as long as you dont close the scanner. — SamHoque, Oct 26 '18 at 12:43
I am not calling my main() method anywhere else in the program. I'm running it by using void main() in the class options. — Dasyud, Oct 26 '18 at 12:43
Do you want me to share the complete program? It's lengthy and might not run as the logic might be wrong. — Dasyud, Oct 26 '18 at 12:45
So how do I close it? Normally the scanner asks for input and the program proceeds further. — Dasyud, Oct 26 '18 at 12:46
It prompts infinite times. The System.out.println() statement is shown only once. — Dasyud, Oct 26 '18 at 12:52

score 1 · Accepted Answer · answered Oct 26 '18 at 13:08

1

It's just that this loop:

        while(k==false)
        {
            temp=sum;
            while(temp>0)
            {
                sum=sum+(temp%10);
                temp=temp/10;
            }
            if(sum/10==0)
                k=true;
        }

seems to never end.
I don't know what you're trying to do with it, but k does not become true
or it will take a lot of time.
During this time you think that you're prompted to give new numbers but you're not.
You're just typing and pressing enter.
To prove this just type ppp. This should throw InputMismatchException but it does not.

answered Oct 26 '18 at 13:08

forpas

160,666
10
38
76

I just figured out that the problem is with this loop. That loop is used to check if a number is a magic number or not. Can you help me with the logic? – Dasyud Oct 26 '18 at 13:13
I have identified the mistake and now I need help fixing it. – Dasyud Oct 26 '18 at 13:21
@AdityaSoni this is another issue. The issue of this question is solved. Now you should post another question with another title and with all the relevant info about what you're trying to achieve and where the error is – forpas Oct 26 '18 at 13:23
Should I delete the question because it's useless? It already has -2 thanks to my stupidity. – Dasyud Oct 26 '18 at 13:36
@AdityaSoni I don't think it's useless but it's your question and you can do whatever you want with it. – forpas Oct 26 '18 at 13:37

SamHoque · Answer 2 · 2018-10-26T13:13:31.557

0

You have to close the scanner by using sc.close(); but still there is an issue with your loop I have re-implanted the code using my own code this should work now.

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.println("Enter 2 numbers as upper and lower bound and all composite numbers between them will be displayed");
    int m = sc.nextInt();
    int n = sc.nextInt();
    sc.close();
    if (m < n) {
        for (int i = m; i <= n; i++) {
            int f = 0;
            for (int i2 = 1; i2 <= n; i2++) {
                if (n % i2 == 0)
                    f++;
            }
            if (f > 2) {
                int num = i;
                do {
                    num = sumOfDigits(num);
                } while (num > 9);
                if (num == 1) {
                    System.out.println(i);
                }
            }
        }
    } else {
        System.out.println("Invalid Input");
    }
}

public static int sumOfDigits(int n) {
    int s = 0;
    while (n > 0) {
        s += n % 10;
        n /= 10;
    }
    return s;
}

which makes an output of

edited Oct 26 '18 at 13:13

answered Oct 26 '18 at 12:44

SamHoque

2,978
2
13
43

Can you try the input as 10 and 100 and tell me whats the output? Is it a problem with my JDK? – Dasyud Oct 26 '18 at 12:54
Your problem is with closing the Scanner not with output. – SamHoque Oct 26 '18 at 12:54
Okay looks like 10 and 100 creates an infinite loop, can you tell me what you are trying to calculate here? – SamHoque Oct 26 '18 at 12:55
So why isn't Scanner closing itself and how is it working for you? I never had to use sc.close() before in my life. – Dasyud Oct 26 '18 at 12:56
I tried with number 1 and 2. the loop is happening on your calculations. – SamHoque Oct 26 '18 at 12:57
I'm trying to find composite magic numbers between 10 and 100. A composite number has more than 2 factors and a magic number has eventual sum of digits=1. e.g:-55=5+5=10=1+0=1 – Dasyud Oct 26 '18 at 12:58
I tried with 3 and 4 too and it didn't prompt again. How is the calculation affecting the Scanner? – Dasyud Oct 26 '18 at 12:59
Okay give me a moment. – SamHoque Oct 26 '18 at 12:59
Edited my answer, please run the program and see if thats what you need to do – SamHoque Oct 26 '18 at 13:02
This is only half the program. You are only checking for composite numbers here. The number displayed should be both composite and a magic number. – Dasyud Oct 26 '18 at 13:08
Edited again.. try now – SamHoque Oct 26 '18 at 13:13
Your current output is correct for the magic number only part. I completed the program. Thanks for your help buddy! – Dasyud Oct 26 '18 at 13:37

Bug :- Scanner class asks for input infinite times

2 Answers2