Java Prime Number help

1649 Views 9 Replies 3 Participants Last post by wolf2009, Mar 10, 2009

wolf2009

Discussion Starter · Mar 8, 2009

I need help with making this prime number generator in java for univ HW. I'm using BlueJ for writing and testing the code.

I have made the code, but some logic is wrong. The user has to enter the starting number, 2 for HW, and the ending number. Then the code should display all the prime numbers between the two numbers. And it has to be done with loops and if-else statements.

Code:

Code:

import java.util.Scanner;
public class PrimeNumber
{
    public static void main(String [] args)
    {
        int num1, num2, count, i = 2, prChk = 0 , m = 0;
        Scanner keyIn = new Scanner (System.in);

        System.out.print("Enter First number : ");
        num1 = keyIn.nextInt();

        System.out.print("Enter Second number : ");
        num2 = keyIn.nextInt();

        count = num1;

        while(count <= num2)
        {
            while(i < count)
            {
                prChk = count % i;
                if(prChk == 0)
                    m = 0;
                else if(prChk == 1)
                    m = 1;
                i++;
            }
            if (m == 0)
                System.out.println(count + ": prime");
            else if(m ==1)
                System.out.println(count + ": not prime");
            count++;
        }
    }
}

What am I doing wrong ?

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Kuntz

· #2 · Mar 8, 2009

Only ODD numbers can be prime, so we should only be checking those numbers. You also need to break from your inner loop once you realize the number is not a prime. No need to keep checking it, since once a number is not prime, there is no further test that could overrule that decision.

Code:

Code:

        count = num1 | 1; // Make sure count is ODD

        while(count <= num2)
        {
            bool prime = true;
            i = 2;
            while(i < count)
            {
                //check if count is prime
                prChk = count % i;
                if(!prChk)
                {
                    prime = false;
                    break;
                }
                i++;
            }
            if (prime) System.out.println(count + ": prime");
            else System.out.println(count + ": not prime");
            count += 2; // Add to next odd number
        }

Also, if you are allowed to, a FOR loop is better than a WHILE loop in that instance, as it makes for cleaner code.

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wolf2009

Discussion Starter · #3 · Mar 8, 2009

I can use for loop. but I'm really a beginner in this class, although I have done some C++ but that is also about basic like classes .

What does this do ?
count = num1 | 1;

also i didn't get this if statement
if(!prChk)

with this if (prime)
don't you mean if(prime = true) ?

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Kuntz

· #4 · Mar 8, 2009

Quote:

What does this do ?
count = num1 | 1;

Oring a number by 1 turns any number into an Odd number.

16 | 1 = 17
2,156 | 1 = 2,157
2 | 1 = 3
1 | 1 = 1
7 | 1 = 7

If the person wants to see if the number 20 is Prime, there is no point to check anything, because any Even number can't be a prime number. All Even numbers can be divided by 2. So we should start the check at 21, then add 2 to count to get 23, then 25, skipping all the Even numbers...

Quote:

also i didn't get this if statement
if(!prChk)

if (prChk) is the same as if (prChk == true)
if (!prChk) is the same as if (prChk == false)

They are short cuts and save you from typing. It's also makes for cleaner code.

You could also have just done...

if (!(count % i))
{
}

...and saved yourself needing to type out the variable.

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Kuntz

· #5 · Mar 8, 2009

This is a bit cleaner code, also uses a FOR loop and increments i by 2 instead of one.

Code:

Code:

        for (int count=(num1|1);count<=num2;count+=2)
        {
            bool prime = true;
            for (int i=2;i<count;i+=2)
            {
                //check if count is prime
                if(!(count % i))
                {
                    prime = false;
                    break;
                }
            }
            if (prime) System.out.println(count + ": prime");
            else System.out.println(count + ": not prime");
        }

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The Bartender Paradox

· #6 · Mar 9, 2009

Quote:

Originally Posted by wolf2009

Code:

import java.util.Scanner;
public class PrimeNumber
{
    public static void main(String [] args)
    {
        int num1, num2, count, i = 2, prChk = 0 , m = 0;
        Scanner keyIn = new Scanner (System.in);

        System.out.print("Enter First number : ");
        num1 = keyIn.nextInt();

        System.out.print("Enter Second number : ");
        num2 = keyIn.nextInt();

        count = num1;

        while(count <= num2)
        {
            while(i < count)
            {
                prChk = count % i;
                if(prChk == 0)
                    m = 0;
                else if(prChk == 1)
                    m = 1;
                i++;
            }
            if (m == 0)
                System.out.println(count + ": prime");
            else if(m ==1)
                System.out.println(count + ": not prime");
            count++;
        }
    }
}

What am I doing wrong ?

You also forget to reset i for every time the count loop repeats. Every time the count loop repeats i is not reset so it stays at the previous value of count. So for every repetition in the count loop, the i loop is exicuted only once with i equal to count - 1.

Quote:

Originally Posted by Kuntz

Only ODD numbers can be prime

Well, except for two. Two is the only really odd prime number because its the only one that's even.

(I'll cease with the bad puns now.)

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wolf2009

Discussion Starter · #7 · Mar 10, 2009

@Kuntz, the compiler says it can't find the symbol -class bool

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The Bartender Paradox

· #8 · Mar 10, 2009

Quote:

Originally Posted by wolf2009

@Kuntz, the compiler says it can't find the symbol -class bool

Change the line that says "bool prime = true" to "boolean prime = true". "bool" is the C++ way of declaring a Boolean variable.

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Kuntz

· #9 · Mar 10, 2009

A BOOL is just an integer, so you can replace it with one. Some people use BOOL's in their programming, I personally don't, it's up to you. There is no performance difference.

Code:

Code:

for (int count=(num1|1);count<=num2;count+=2)
{
    int prime = 1;
    for (int i=3;i<count;i+=2)
    {
        if(!(count % i))
        {
            prime = 0;
            break;
        }
    }
    if (prime) System.out.println(count + ": prime");
    else System.out.println(count + ": not prime");
}

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wolf2009

Discussion Starter · #10 · Mar 10, 2009

Quote:

Originally Posted by The Bartender Paradox

Change the line that says "bool prime = true" to "boolean prime = true". "bool" is the C++ way of declaring a Boolean variable.

that did it

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