Print below series in single loop and using single variable only

Question

I'm trying to print the following sequence infinitely:

However, when I run this, the pattern only prints once.

Here's my code:

public class PrintSequence {
    public static void main(String args[]) {
        int i=0;
        while(true) 
        {
            if(i<=100) {
                System.out.println(i+ "   " + (100-i));    
                i++;
            }

        }

    }
}

The assignment only allows me to use a single loop and a single variable.

Please improve your question: show us what you've tried and use it to help ask a more specific and higher quality question. Please check out the [ask] and the [help] for more on best practices for this site. — Hovercraft Full Of Eels, Feb 01 '20 at 18:19
`void print_number(void) {for (int i = 0; i == 0; ++i) {puts("0 100 1 99 2 98 . . . . . . 97 3 98 2 99 1 100 0 99 1 98 2 97 3 . . . . . . 3 97 2 98 1 99 0 100 1 99 2 98 3 97 . . . . . .");}}` Not helpful, but you haven't shown an attempt, and it technically fulfills what you've asked. — Thomas Jager, Feb 01 '20 at 18:21
@HovercraftFullOfEels .I have added my code as well. pls check — ayaz husain, Feb 01 '20 at 19:05
Out of curiosity, why did you tag the C language? This doesn't appear to be related to that topic in the least. — EJoshuaS - Stand with Ukraine, Feb 01 '20 at 19:06
@EJoshuaS-ReinstateMonica Because it can be solved using any language. — ayaz husain, Feb 01 '20 at 19:08
@ayazhusain That doesn't mean that you should tag it in the language you're not using. Only tag a question with more than one language if it has something to do with interoperability between those two languages. — S.S. Anne, Feb 01 '20 at 19:14
BTW the problem is that once you reach `i == 100` there's no way to reset `i` to 0, so the loop will keep going but it'll never print anything. Add `else { i = 0; }` and this'll work. — EJoshuaS - Stand with Ukraine, Feb 01 '20 at 19:14
@EJoshuaS-ReinstateMonica thanks for editing and making it more readable. — ayaz husain, Feb 01 '20 at 19:15
@EJoshuaS-ReinstateMonica , it won't work, it should be printed in reverse order. — ayaz husain, Feb 01 '20 at 19:18
Sorry, I missed that - so you want it to reverse every time? Yeah, I see that now that I look at the expected output again. You could add a Boolean variable indicating what direction you were printing and switch the direction in the `else` statement. — EJoshuaS - Stand with Ukraine, Feb 01 '20 at 19:21
@EJoshuaS-ReinstateMonica I can use single loop and single variable only. — ayaz husain, Feb 01 '20 at 19:24
In that case, just as a hint in the right direction: there's a solution using modular arithmetic. — EJoshuaS - Stand with Ukraine, Feb 01 '20 at 19:33
@HovercraftFullOfEels pls mark this question as open, it has complete and precise information now. — ayaz husain, Feb 01 '20 at 19:34

Andreas · Accepted Answer · 2020-02-01T21:22:30.540

There are 2 tricks needed to solve this:

Realize that a full cycle from 0,100 back to 0,100 is 200 iterations, then use % remainder operator to write an infinite loop within range 0-199.
Use Math.abs()¹ to turn a negative number into a positive number.

Here is 2 lines of code that will produce the desired infinite output:

for (int i = 0; ; i = (i + 1) % 200)
    System.out.printf("%-6d%d%n", 100 - Math.abs(100 - i), Math.abs(100 - i));

Explanation

for (;;) is a loop that runs "forever" (infinitely). It is the same as for (; true; ), and the same as while (true), but using for allows keeping int i = 0, the loop, and i = (i + 1) % 200 together in one line of code.

i = (i + 1) % 200 means that when i = 199, the next value is i = 0, starting the sequence all over again, forever iterating through values 0..199.

100 - i makes the sequence 0..199 become 100..-99.

Math.abs(100 - i) makes the sequence become 100..0..99, i.e. the second column of the output.

100 - Math.abs(100 - i) makes the sequence become 0..100..1, i.e. the first column of the output.

printf("%-6d%d%n", ...) makes it print nice in 2 columns.

^{1) If Math.abs() is not allowed, use a ?: ternary conditional operator instead:

printf("%-6d%d%n", (i < 100 ? i : 200 - i), (i < 100 ? 100 - i : i - 100))}

Arvind Kumar Avinash · Answer 2 · 2020-02-02T11:19:53.100

You can do it as follows:

public class Main {
    public static void main(String[] args) {
        int i = 0;
        while (true) {
            if (i <= 100) {
                System.out.println(i + "\t" + (100 - i));
                i++;
            } else if (i <= 200) {
                System.out.println((200 - i) + "\t" + (i - 100));
                i++;
            }
            if (i == 200) {
                i = 0;
            }
        }
    }
}

Salient features:

It exactly meets your requirement.
It does not require any library.

Print below series in single loop and using single variable only

2 Answers2