Console input in Java

Question

This is simple code but I am not able to figure out why it is accepting two lines of input:

import java.io.*;
import java.util.*;

public class Solution {

public static void main(String[] args) {
    Scanner scan = new Scanner (System.in);
    String inputString = scan.nextLine();
    int n = Integer.parseInt (inputString);
    for (int i = 1; i <= n; ++i) {
        inputString = scan.nextLine();
        int num = Integer.parseInt (inputString);
        System.out.println ("Checking prime of: " + num);

        for (int j = 2; j*j < num; ++j) {
            if (num % j == 0) {
                System.out.println ("Not prime");
                break;
            }
        }
        System.out.println ("Prime");
    }
}
}

Now when I run with the following input:

The program prints the following:

Checking prime of: 12
Not prime
Prime
Checking prime of: 5
Prime
Checking prime of: 7
Prime

Note the second Prime above when it has not consumed any input.

I must be making a simple mistake but not able to figure out what is wrong. If someone could point out what I am doing wrong that would be much appreciated.

You have a statement System.out.println ("Prime"); after the for loop so it always prints "Prime", no matter what you enter and whether the actual number is a prime or not. Note that the break; in the inner for loop only jumps out of the inner loop, not out of the outer loop. — Jesper
– Jesper, Commented May 12, 2017 at 18:30

Frakcool · Accepted Answer · 2017-05-12 18:32:05Z

6

This line is being printed always, every cycle.

System.out.println ("Prime");

Add a flag variable instead, inside your for 1st loop

boolean isPrime = true;

And inside your if:

if (num % j == 0) {
    isPrime = false;
    break;
}

Then have another condition

if (isPrime) {
    System.out.println ("Prime");
} else {
    System.out.println ("Not prime");
}

That's it

answered May 12, 2017 at 18:32

Frakcool

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2 Comments

ash Over a year ago

You are so right, I cannot believe I missed that. Thank you so much for pointing it out. I thought it was something relating to how I was reading console input.

Frakcool Over a year ago

@ash I'm glad it helped ^^ Don't forget to accept the answer

RoaaGharra · Accepted Answer · 2017-05-12 18:32:47Z

0

for (int j = 2; j*j < num; ++j) {
            if (num % j == 0) {
                System.out.println ("Not prime");
                break;
            }
        }
System.out.println ("Prime");

The last line is called either way. You need to either initialize a flag to help you figure out if the loop ended early, or simply return whenever you print "Not prime".

answered May 12, 2017 at 18:32

RoaaGharra

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Comments

arcy · Accepted Answer · 2017-05-12 18:33:34Z

0

You do a scan on the 2nd line of the main method, then again at the top of your loop. The first one consumes the first line of input, but all you do with it is parse it into an int, you don't check it and don't output anything for it.

answered May 12, 2017 at 18:33

arcy

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Comments

ZUKA · Accepted Answer · 2017-05-12 19:17:26Z

    import java.io.*;
        import java.util.*;

        class Solution {

         public static void main(String[] args) {
          Scanner scan = new Scanner (System.in);
            String inputString = scan.nextLine();
         int n = Integer.parseInt (inputString);
       a: for (int i = 1; i <= n; ++i) {
          inputString = scan.nextLine();
        int num = Integer.parseInt (inputString);
        System.out.println ("Checking prime of: " + num);

       for (int j = 2; j*j <= num; ++j) {
            if (num % j == 0) {
             System.out.println ("Not prime");
            continue a;
          }
     }
    System.out.println ("Prime");
   }
}
 }

here you have pay attention over two points. 1. about break statement that is it will break u from innermost loop only and that is the reason "prime" is printed whether the number is prime or not . so use labelled loop concept as i have used with "continue" .

your logic in "for loop" of checking the number whether it is prime or not is little bit wrong correct it as i have corrected that for(int j=2;j*j<=num;++j) that is "<=" should be used in place of "<" otherwise on input 9,25etc.it will show prime which is wrong

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