Logic Error in Java For-loop

Question

I am trying to make the loop continue to execute here until the user types the letter S at the keyboard. It seems to be giving three outputs instead of one for each iteration. What am I doing wrong here;

// Loop until an S is typed

public class ForTest {

    public static void main(String[] args) 
        throws java.io.IOException {

        int i;

        System.out.println("Type S to stop.");

        for(i = 0; (char) System.in.read() != 'S'; i++) 
//          System.out.println("print");
            System.out.println("Pass # " + i);
//          System.out.println("print");


    }

}

Output coming up is if I press 'a':

Type S to stop.
a
Pass # 0
Pass # 1
Pass # 2

Possible duplicate of https://stackoverflow.com/questions/12898455/while-loop-to-check-character-input — Scary Wombat, Jul 04 '18 at 06:55
it doesn't print that just tried but when I enter anything it prints twice means the iteration runs twice — Danyal Sandeelo, Jul 04 '18 at 06:55
it prints the value twice because of IO delay, till the value is processed, it's printing 2 values — Danyal Sandeelo, Jul 04 '18 at 06:58
Thanks guys for the comments and answers. I read about the inputstream flaws last week and it has just been highlighted with this code. I am fairly new to Java so just trying to familiarize myself with it. — Othello, Jul 04 '18 at 07:29

Laurent B · Answer 1 · 2018-07-04T08:04:30.493

2

The InputStream.read method block until the end of input which is on a Windows OS the control characters CR LF (\r\n).

This explains why you get 3 characters as a result.

See for yourself :

public static void main(String[] args) throws java.io.IOException {

    int i;

    System.out.println("Type S to stop.");

    char c = (char) 0;
    for (i = 0; c != 'S'; i++) {
        c = (char) System.in.read();
        System.out.println("Pass # " + i);
        System.out.println("char intValue : " + (int) c);
    }
}

Suggested read : Java: How to get input from System.console()

edited Jul 04 '18 at 08:04

answered Jul 04 '18 at 07:12

Laurent B

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This code works. I like the way you introduced the (int)c to make me follow the trail. Now I checked CR LF on the ascii table and I see they are printed each time hence the three outputs. Thanks – Othello Jul 04 '18 at 11:56

score 1 · Answer 2 · answered Jul 04 '18 at 07:50

1

System.in will use BufferedInputSteam to read the input from console bit by bit (including the line break, etc). In Mac system, I get 2 bits whenever i gave a single digit input.

Use Scanner and read all the bytes and convert as String.

answered Jul 04 '18 at 07:50

user4142018

540
1
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score 0 · Answer 3 · answered Jul 04 '18 at 07:21

0

I suggest you to avoid reading input in that way, use Scanner instead. You can reach your goal with this code:

    int i = 0;
    Scanner scan = new Scanner(System.in);
    System.out.println("Type S to stop.");
    while( !scan.next().equals("S")) {
        i++;
        System.out.println("Pass # "+i);
    }

answered Jul 04 '18 at 07:21

Leviand

2,745
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You still need to consume the linebreaks - this doesn't solve anything. – Boris the Spider Jul 04 '18 at 07:48
I tried it and it works. I had to import Scanner. Thanks – Othello Jul 04 '18 at 11:54

Logic Error in Java For-loop

3 Answers3