Java printf complication

Question

For this code to run as intended use your cmd/terminal not an IDE. (or you will not see the correct effect of \r)

When the 9 in the terminal reaches the right most position after 4 spaces it will turn around and move back 4 spaces and on and on. However when it reaches the start point after a full cycle there is a remaining 9 on the second space that is not cleared by \r. What could be causing this?

public class Core {
    public static void main(String[] args) throws InterruptedException {

        int Array[][] = new int[4][6];
        int score;
        boolean enemy = true;
        boolean dir = true;

        // EnemyLine
        while (enemy) {
            for (int i = 1;;) {
                Thread.sleep(1000);

                if (i == 1)
                    dir = true;
                else if (i == 4)
                    dir = false;
                if (dir == true) {
                    int a = Array[0][i] = 9;
                    System.out.printf("%" + i + "d", a);
                    i++;
                    System.out.print("\r");
                    continue;
                } else if (dir == false) {
                    int a = Array[0][i] = 9;
                    System.out.printf("%" + i + "d", a);
                    i--;
                    System.out.print("               \r");
                    continue;
                }
            }
        }

    }
}

I don't see a question here or a problem (besides a homework problem). Run it in a debugger/IDE and watch all the variables and output window to ascertain what is happening. Or, just step through it on paper (or in your head, it's not too complex). — KevinDTimm
– KevinDTimm, Commented Oct 14, 2015 at 20:15
Well the problem is that on the return leg \r does not clear every 9. I am unable to find what is causing this also I write in eclipse and can not find the error. (the code doesn't function as intended in eclipse anyway as for some reason it jumps a line but in terminal all output happens on one line) — jim layhey
– jim layhey, Commented Oct 14, 2015 at 20:31
Couple of unrelated comments on your code: You have if (dir == true) { ... } else if (dir == false) { ... } Well since dir is a boolean it can only be true or false so your if..else could be if (dir == true) { ... } else /*dir must be false*/ { ... } Also, don't test if some_boolean == true just test some_boolean: if (dir) { ... } else { ... } That's confusing, since "dir" (direction?) doesn't tell you much: what does dir == true` mean? If it means "moving right" and false means "moving left" rename dir to movingRight ... if (movingRight) { ... } else { /* moving left */ } — Stephen P
– Stephen P, Commented Oct 14, 2015 at 22:32
I too will go by Stephen suggestion. @heathbm. Clearly you need to improve your programming logic. And when programming a better naming always helps. Though I likes the usage of \r or carriage return — Acewin
– Acewin, Commented Oct 14, 2015 at 23:07

Acewin · Accepted Answer · 2015-10-14 21:55:01Z

3

Explaining why you are getting such result

Basically in your loop you are changing where to place carriage return \r.

After loop completion when you come back to i = 1. your are changing after how many characters you are going to do a carriage return.

1 -> _9\r
2 -> __9\r
3 -> ___9\r
4 -> ____9      \r
x3 -> ___9       \r
x2 -> __9        \r
x1 -> _9\r

thus the last statement does not clear the 9 at x2

Change needed in if (dir == true)

System.out.print(" \r" ); // enter a space before \r

or it would be best to use

System.out.print("               \r" ); // preferable

you can also remove your sysout \r from the condition loop and use it before your Thread sleep

System.out.print("       \r" );
Thread.sleep(1000);

answered Oct 14, 2015 at 21:55

Acewin

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1 Comment

jim layhey Over a year ago

Thanks for the feedback, I now use your second method and now I understand where I initially went wrong. much appreciated.

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