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javaif-statementnumbersstatements
Matt
Matt

Reputation: 1

Sieve of Eratosthenes java prime number if statement

I am trying to do the Sieve of Eratosthenes using a for loop then an if statement by checking for prime numbers up to 30 however I have a problem. Because of my code 2,3 and 5 are all shown as not prime numbers because they are divisible by 2,3 and 5 of course. How do I edit my code to make these come up as prime numbers?

Here is my code

public class prime {

public static void main(String[] args) {
    for(int a=2; a<=30; a++){
        if(a%2 == 0 || a%3 == 0 || a%5 == 0){
            System.out.println(a +" = Not Prime");
        }
        else
        {
            System.out.println(a + " = Prime");
        }
    }
}

}

Thanks

Upvotes: 0

Views: 187

Answers (3)

Matt
Matt

Reputation: 1

Thanks for the help, I added some code in in the if statement to check if its 2, 3 or 5. 

public class prime {

public static void main(String[] args) {
    for(int a=2; a<=30; a++){
        if(**a != 2** && a%2 == 0 ||**a !=3** && a%3 == 0 ||**a != 5** && a%5 == 0){
            System.out.println(a +" = Not Prime");
        }
        else
        {
            System.out.println(a + " = Prime");
        }
    }
}

}

Upvotes: 0

Atafar
Atafar

Reputation: 717

The definition of a prime is that it is not divisible by another natural number other than 1 or itself. So, you need to devise a test for this, e.g., a != 2, etc., or, a / 2 != 1. In your code:

if (a != 2 && a % 2 == 0 || ...

Upvotes: 0

Scott Hunter
Scott Hunter

Reputation: 49803

Add an if statement to explicitly test for a being 2,3 or 5 before you test for divisibility.

Upvotes: 1

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