Hailstone Sequence in Java with ArrayList

Question

Hello I am attempting to perform the hailstone sequence.
A hailstone sequence is basically: take a given integer n - if even, the next integer in the sequence is n/2, if odd, the next integer in sequence is n * 3 + 1.
The API I must follow for my assignment requires it to be performed as it is with a method returning an arraylist.
My problem is the code just hangs forever, when I added output in the method itself to see what was happening I see it always hangs when it is given the number 10 for some reason.
I am hoping that there is something small I am missing here perhaps in my conditions.

Here is some sample output when given n value of 15 it outputs this over and over again.

15 is odd so I make it 3n+1: 46
46 is even so I divide by 2: 23
23 is odd so I make it 3n+1: 70
70 is even so I divide by 2: 35
35 is odd so I make it 3n+1: 106
106 is even so I divide by 2: 53
53 is odd so I make it 3n+1: 160
160 is even so I divide by 2: 80
80 is even so I divide by 2: 40
40 is even so I divide by 2: 20
20 is even so I divide by 2: 10
15 is odd so I make it 3n+1: 46

My code

import java.util.ArrayList;
import java.util.Scanner;

public class HailstoneSequence {
    public static ArrayList<Integer> getHailstoneSequence(int n){
        ArrayList<Integer> results;
        results = new ArrayList<Integer>();
        results.add(n);

        //while the last number is not 1 perform these actions
        while((results.size() - 1) != 1){
            //for each number in the array
        for(int i=0; i< results.get(i); i++){
            //test if odd or even
            if((results.get(i)%2)==0){
                System.out.println(results.get(i)+" is even so I divide by 2: "+ (results.get(i)/2));

                   results.add((results.get(i)/2));

                   }
                else{
                    //odd
                    System.out.println(results.get(i)+" is odd so I make it 3n+1: "+ (3*(results.get(i))+1));
                    results.add((3*(results.get(i))+1));
                }

        }
        }
        return results;
    }

    public static void main(String[] args) {
        int n=0;
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the value of n ");
        n=sc.nextInt();
        sc.close();

        //create an initialize new array list to hold results of the hailstonesequence
        ArrayList<Integer> list;
        list = new ArrayList<Integer>();

       list = getHailstoneSequence(n);

        //for each number in the array
       for(int i=0; i< list.get(i); i++){

           if ((list.get(i)!= 1)){
            if((list.get(i)%2)==0){
                    System.out.println(list.get(i)+" is even so I divide by 2: "+ (list.get(i+1)));

                   }
                else{
                    //odd
                    System.out.println(list.get(i)+" is odd so I make it 3n+1: "+ (list.get(i+1)));

                }
           }
           else{break;}
        }

    }

    }

Maybe it hangs forever because that's what the algorithm does on certain input? — OneCricketeer, Dec 04 '16 at 16:04
Have you stepped through the code in your IDE debugger? That is the place to start. Please visit the [help] and read [ask] — Jim Garrison, Dec 04 '16 at 16:04
What's the purpose of the for loop after you call `getHailstoneSequence`? — OneCricketeer, Dec 04 '16 at 16:09

OneCricketeer · Accepted Answer · 2016-12-04T19:41:13.450

In your method for(int i=0; i< results.get(i); i++){ and in main for(int i=0; i< list.get(i); i++){

These do not loops over each element of the list, or at least not only once, and it'll eventually result in out of bounds if you never added to the list.

Say results.get(i) is 10, and that's the only number in the list... You then add 5 ten times because 10 is even and the loop is running ten times. You then are probably adding 16 5*10 times, etc, etc

Adding elements to lists while you iterate over them is generally a bad idea anyway. You only need to keep track of two numbers at a time, and can add to the list separate from the iteration process.

Here's a working sample

ArrayList<Integer> results = new ArrayList<Integer>();
results.add(n);
if (n == 1) return results;

int next;
if (n % 2 == 0) next = n / 2;
else next = 3*n + 1; 
results.add(next);

while (next != 1) {
    if (next % 2 == 0) next = next / 2;
    else next = 3*next + 1; 
    results.add(next);
} 
return results;

I am almost there, this did work so I am no longer stuck in an infinity loop but no matter what number is entered it always stops on 10 instead of 1 still :S — Ben Schmidt, Dec 04 '16 at 18:35
If you enter 10, should add 10, 5, 16, 8, 4, 2,1. It's the for loop in the main method that's probably the issue -- https://ideone.com/iWzlM9 — OneCricketeer, Dec 04 '16 at 19:37

Hailstone Sequence in Java with ArrayList

1 Answers1