Collatz Conjecture Recursion Error

Question

I've lurked around for years, but today I have a question concerning my code. I am currently trying to create a collatz program that puts the number of steps for a certain number in an array, but at the same time puts the number of steps for every single number it passes through. Here's my code:

public class GenerousRecursion {

  public static short steps;
  public static int length;
  public static short[] array = new short[101];

  public static void main(String[] args) {
    length = 100;
    for (int count = 2; count < length + 1; count++){
      steps = 0;
      System.out.println(count + ": " + findCollatz(count));
    }
  }

  public static short findCollatz(int number) {
    if (number < length){
      if (array[number] > 0) {
        steps = array[number]++; return steps;
      }
      else if(number % 2 == 0) {
        array[number] = findCollatz(number / 2);
        steps ++;
        return steps;
      }
      else {
        array[number] = findCollatz(3 * number + 1);
        steps ++;
        return steps;
      }
    }

    else {
      if(number % 2 == 0) {
        findCollatz(number / 2);
        steps ++;
        return steps;
      }
      else {
        findCollatz(3 * number + 1);
        steps ++;
        return steps;
      }
    }
  }
}

Here's a great video on the Collatz Conjecture: Numberphile

So here is the error being thrown (reduced), but I don't understand, because I am not anywhere near the bounds of any int or short:

Exception in thread "main" java.lang.StackOverflowError
at GenerousRecursion.findCollatz(GenerousRecursion.java:22)
at GenerousRecursion.findCollatz(GenerousRecursion.java:33)
at GenerousRecursion.findCollatz(GenerousRecursion.java:27)

I just listed these first three lines because these same three lines draw errors for hundreds of lines.

Whats the problem and how do I fix it? Thanks abunch!

Edit: When I ran the debugger, my program continously throws exceptions whenever the array is refrenced.

`StackOverflowError` is unrelated to the bounds of `int` or `short`. Are you familiar with the concept of the stack? — Marko Topolnik, Sep 10 '16 at 14:09
@Marko Topolnik I thought I did, but apparently not! I would appreciate some guidance if you wouldn;t mind. — Erik, Sep 10 '16 at 14:19
@MarkoTopolnik Thank you that was really helpful. So does that mean that my program is just a complete memory hog, and as a result will just crash no matter what I do? — Erik, Sep 10 '16 at 14:28
write out what happens when you call `findCollatz(1)`, and you should be able to spot the problem. Just start at the method call, and then run through each line to see if it fires, and if so, what it does. — Mike 'Pomax' Kamermans, Sep 10 '16 at 18:00

Joop Eggen · Accepted Answer · 2016-09-10T18:10:48.860

As stated in the video-clip continuing with 1 will end in an endless loop. Try the following.

static int[] collatzCounts = new int[100];
static final int NO_COUNT = -1;
static {
    Arrays.fill(collatzCounts, NO_COUNT);
    collatzCounts{1] = 0; // Define collatz(1) = 0 (1, 4, 2, 1, ...) 
}

public static void main(String[] args) {
    for (int n = 2; n < 120; n++) {
        int steps = countCollatz(n);
        System.out.println(n + ": " + steps);
    }
}

public static int countCollatz(int n) {
    IntFunction f = k ->
            k % 2 == 0
                ? 1 + countCollatz(k / 2)
                : 1 + countCollatz(3 * k + 1);
                //: 2  + countCollatz((3 * k + 1) /  2);

    //if (n == 1) {
    //    return 0;
    //}
    if (n < collatzCounts.length) {
        if (collatzCounts[n] == NO_COUNT) {
            collatzCounts[n] = f.apply(n);
        }
        return collatzCounts[n];
    } else {
        return f.apply(n);
    }
}

countCollatz simply counts the steps needed - to reach 1 actually. Though till further proof there might be a cycle of higher numbers.

I have used a Java 8 lambda expression, the IntFunction f, as it is more natural to repeat the calculation, once to fill the array, once for too large numberss.

Collatz Conjecture Recursion Error

1 Answers1