Question

This is a program I made to simulate the Collatz Conjecture on Java:

import java.util.*;
public class Collatz {
public static void main(String args[]){
    Scanner raj= new Scanner(System.in);
    int n;
    int k=0;
    System.out.print("n? ");
    n = raj.nextInt();
    while(n > 1){
        if(n%2 ==1){
            n=3*n+1;
            System.out.println(n);
            k++;
        }
        if(n%2==0){
            n=n/2;
            System.out.println(n);
            k++;
        }

    }
    System.out.print("It took " + k + " iterations!");
}

}

When I put in n=6, I get

3
10
5
16
8
4
2
1
It took 8 iterations!

But when I put, say, n= 63728127, I get

191184382
95592191
286776574
143388287
430164862
215082431
645247294
322623647
967870942
483935471
1451806414
725903207
-2117257674
-1058628837
It took 14 iterations!

What went wrong? Why? How can I fix it? Thanks!

Answer 1

This is a classic case of integer overflow. Primitive ints have a limited range in Java. The solution is to always use something like BigInteger if you have to deal with large integers.

Incidentally, things would be so much easier if Java supported operator overloading like almost all other modern languages.

import java.util.*;
import java.math.BigInteger;


public class Collatz {
    public static void main(String args[]){
        Scanner raj= new Scanner(System.in);
        int k=0;
        System.out.print("n? ");

        BigInteger n = BigInteger.valueOf(raj.nextLong());

        while(n.compareTo(BigInteger.ONE) > 0){
            if(n.testBit(0)){
                n = n.multiply(BigInteger.valueOf(3));
                n = n.add(BigInteger.ONE);
                System.out.println(n);
                k++;
            }
            else {
                n = n.divide(BigInteger.valueOf(2));
                System.out.println(n);
                k++;
            }
        }
        System.out.print("It took " + k + " iterations!");
    }
}