How can I stop my Collatz Sequence code at 1?

Question

I tried implementing the Collatz Sequence using a while loop, but I can't stop the sequence at 1. The code continues. I tried using all my possibilities, but I can't come up with a solution.

import java.util.Scanner;

public class cs{
    public static void main(String[] args)throws Exception{
        System.out.println("Starting number: ");
        Scanner s = new Scanner(System.in);
        int a = s.nextInt();
        System.out.print(" " + a);
        while(a>1)
        {
            if(a == 1) break; // this is not working though
            if((a % 2 ) == 0) {
            a = a / 2;
            System.out.print("  " + a);
            }

            Thread.sleep(1000);
            if((a % 2) != 0){
                a = (a * 3) + 1;
                System.out.print(" " + a);
            }

            Thread.sleep(1000);
        } 
    }
}

Answer 1

The second if condition here should be an else of the first one:

if((a%2)==0){
    // ...
}

// ...
if((a%2)!=0){

Like this:

    while (a > 1) {
        if ((a % 2) == 0) {
            a /= 2;
            System.out.print("  " + a);
        } else {
            a = (a * 3) + 1;
            System.out.print("  " + a);
        }
    }

I also removed the if (a == 1) line which was pointless, as due to the while (a > 1) condition, that if would never be true.

Lastly, I recommend to pay more attention to indenting your code correctly, and to add spaces around operators like I did, otherwise the code is too difficult to read, understand and debug.

Answer 2

The problem must be that a never equals 1. Try printing the value of a in each loop and see what it actually equals:

while(a > 1)
{
    // This line is not required because if a == 1 then the while loop would terminate anyway:
    //  if(a==1) break; //this is not working though

    if((a%2)==0){
        a = a/2;
        System.out.print("  "+a);
    }

    Thread.sleep(1000);

    if((a%2)!=0){
        a = (a*3)+1;;
        System.out.print("  "+a);
    }

    Thread.sleep(1000);

    System.out.println("a = " + a); // This checks what the value of a actually is
} 

Answer 3

Your loop is performing two steps in the Collatz sequence, instead of just one. Even if a is 1, it will then continue into the second if, and turn it to 4, so the loop will never terminate. Instead, you should use an else clause, just just perform one step per iteration of the loop:

while (a > 1)  {
    if ((a%2) == 0) {
        a = a / 2;
    } else {
        a = (a * 3) + 1;
    }

    Thread.sleep(1000);
    System.out.print("  " + a);
}

Answer 4

This is enough for your case:

while (a > 1 && (a % 2) != 0){
    a /= 2;
    System.out.print(" " + a);
}

because if you have a look at your while condition, you will see that a is bigger than 1 ==> can't be 1 at all, so, there is no need for the first if statement.

So your class will look like:

import java.util.Scanner;

public class cs{
    public static void main(String[] args)throws Exception{
        System.out.println("Starting number: ");
        Scanner s = new Scanner(System.in);
        int a = s.nextInt();
        System.out.print(" " + a);

        while (a > 1 && (a % 2) != 0){
            a /= 2;
            System.out.print(" " + a);
        }
}