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I'm trying to figure out why this mystery method in Java prints out "43211234". I understand how the program reaches "43211" but I'm not sure how the last System.out.print(x % 10) after the "if" is ran 3 times and for each time, it brings the value of x back to the value it was previous to its current one until it reaches "1234" as the value of x. Could this have something to do with recursion since the method is being called in the "if" 3 times? I'm guessing it's something along those lines because the last executes exactly 3 times as well. I would greatly appreciate your help. Thanks.

class Main 
 public static void main(String args) 
 mystery(1234);

public static void mystery(int x) 
 System.out.print(x % 10);
 if((x / 10) != 0) 
 mystery(x / 10);
 
 System.out.print(x % 10);
 

I understand how the program reaches "43211"

so you know what recursion is.

Everytime mystery() is called, the 1st print is called and then it calls (recursively) itself, before the 2nd print.

When the recursion stops because (x / 10) != 0 is false, the 2nd print is called for the 1st time and then goes back to the previous unfinished recursive calls and executes the remaining print for each one.

Not sure if my answer will be any more useful than the previous ones, but I'll try. So, basically, your program uses 2 types of recursion: backward recursion and forward recursion. My answer here is not to describe them to you, but to give you a starting point for more information on them.

Let's trace your program's execution:
mystery(1234) -> print(4) -> mystery(123); At this point, the System.out.print(x % 10); at the end of the method has not been called yet since your program has gone further in the recursion. It will be executed once the program returns from deep inside your recursion, and will be executed with whatever's left.

mystery(1234):
 print(4);
 mystery(123):
 print(3);
 mystery(12):
 print(2);
 mystery(1);
 print(1);
 print(1); //This is the first System.out.print(x % 10); from the end of the method, that is executed
 print(2);
 print(3);
 print(4);

Your mystery() method does the following:

• print the final digit of the input number (num % 10 gives the last digit)


• make a recursive call mystery(x / 10), assuming x / 10 is not zero


• then on the way out from the recursion, print the final digit of the input again

Putting this together, with an input of 1234, it means we would print those digits in reverse, then print them again in order.

If this answer still leaves you with doubts, I suggest running your code, beginning with a two digit input like 12, until it is clear what is happening.

This is your recursion stack. This is perfectly fine.

 mystery(x / 10); input 1234 prints 4
 -> mystery(x / 10); input 123 prints 3
 -> mystery(x / 10); input 12 prints 2
 -> mystery(x / 10); input 1 prints 1

Just make sure your remove the second sysout that you that you in your code. That is the reason it is printing the same numbers in again.