Labelled "continue" doesn't seem to work

Question

Just started learning java and I can't understand what's wrong with my code. PrimeIterator is supposed to generate infinite amount of prime numbers (starting from a number 3) but when I print the output I get: 3, 5, 7, 9, 11, 13, 15 etc.

public class Prime {

    PrimeIterator iter = new PrimeIterator();

    private class PrimeIterator implements java.util.Iterator<Integer>
    {
        int numb = 1;

        public boolean hasNext() 
        {
            return true;
        }

        public Integer next() 
        {
            nextCandidate:
            do{
                numb += 2;
                int numbSqrt = (int)java.lang.Math.sqrt(numb);

                for (int i = 3; i <= numbSqrt; i = i+2)
                {
                    if (numb % i == 0)
                    {
                        continue nextCandidate;
                    }
                }
            }while(false);
            return numb;
        }

        public void remove() {}
    }

    void printPrimes()
    {
        System.out.print(2);
        while(iter.hasNext())
        {
            try 
            {
                Thread.sleep(500);
            } catch (InterruptedException e) 
            {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            System.out.print(", " + iter.next());   
        }
    }
}

I wanted to use labelled "continue" statement for my do-while loop. However my intuition tells me that I use it incorrectly.

maybe I'm missing it, but it doesn't look like you're printing output at all — Sam I am says Reinstate Monica, Feb 21 '14 at 16:33
Instead of continue I would recommend breaking. or return false — Moonhead, Feb 21 '14 at 16:33
Please everyone, he has labeled his `do` loop with the `nextCandidate` label. The continue will affect the outer loop. — everton, Feb 21 '14 at 16:33
Your code won't compile, because it's missing `void remove()` as specified by `Iterator`. It would also be nice to have the `main` function so we don't have to create it in order to help you. — aliteralmind, Feb 21 '14 at 16:37
@KRUKUSA Neither of these works. Also, you can't return false in that method. — matt-pielat, Feb 21 '14 at 16:37
@aliteralmind Oracle documentation told me that remove is optional. Rest of the code coming right up. — matt-pielat, Feb 21 '14 at 16:38
Start your for loop at 2 instead of 3 because 2 is a prime number. — Moonhead, Feb 21 '14 at 16:39
*Removing an element* is optional, but *implementing the `remove` function* is still required. Implement it to throw an `UnsupportedOperationException` (and document it as such). — aliteralmind, Feb 21 '14 at 16:49

score 1 · Accepted Answer · answered Feb 21 '14 at 16:38

The trouble is with the while(false) continuation condition. Being a do while(false) statement, this means that it can never loop more than once. That is, when you try to jump the execution to the labeled statement, it won't cycle again through the do while because the continuation condition (false) is not validated, even if you think that continue will make the execution loop again.

So it will never increment numb more than once per next() method execution.

I'd do something like the following:

nextCandidate:
do{
    numb += 2;
    int numbSqrt = (int)java.lang.Math.sqrt(numb);

    for (int i = 3; i <= numbSqrt; i = i+2)
    {
        if (numb % i == 0)
        {
            continue nextCandidate;
        }
    }
    break;

}while(true);

Ok. Please also note the observation @Sam I am posted regarding truncation of `sqrt` (point 3. in his answer). — Andrei Nicusan, Feb 21 '14 at 16:44

Sam I am says Reinstate Monica · Answer 2 · 2014-02-21T16:45:10.760

1

Here are the problems that I see

You're not even printing output.
(int)java.lang.Math.sqrt(5) will end up truncating to 2. You should add 1 to your square root, since it's a problem if you don't iterate enough, but it's not a problem if you iterate more than you need to.
when you do find a prime number, your for loop will end, and while(false) will terminate the do-while loop

edited Feb 21 '14 at 16:45

answered Feb 21 '14 at 16:38

Sam I am says Reinstate Monica

30,851
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100

@AndreiNicusan you're right. I was afraid that if `i` was 4, than it would say that 4 was prime, but then again, `i` will never be `4` – Sam I am says Reinstate Monica Feb 21 '14 at 16:46
I wanted to increase my numbers by 2 to kinda optimize the whole thing. I couldn't nicely implement generating number 2 in my PrimeIterator subclass. – matt-pielat Feb 21 '14 at 16:46
actually, not what I think about it more, the square root thing won't actually end up being a problem either – Sam I am says Reinstate Monica Feb 21 '14 at 16:49
@Sam, I think you may have produced a full trifecta of wrongness. He produces output through an iterator that can be printed or otherwise used by the caller (a more flexible approach than directly printing). The sqrt truncation is a feature, not a bug. Your third point describes the desired behavior rather than a problem. – phatfingers Feb 21 '14 at 17:05

Labelled "continue" doesn't seem to work

2 Answers2