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Thread: Printing Sum of Primes of a number

  1. #1
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    Default Printing Sum of Primes of a number

    I'm trying to print the sum of the prime numbers of a given number from the console. I think my test in my while loop might be wrong, but I'm not 100% sure. Any help, I would appreciate. Thanks in advance.

    import java.util.*;
     
    public class PrimeSum
    {
        public static int isPrime(int num)
        {
            int sum = 0;
            int factor = 1;
            while(factor <= num)
            {
                if(num % factor != 0)
                {
                    sum += factor;
                    factor ++;
                }
                else
                {
                    factor ++;
                }
            }
            return sum;
        }
     
        public static void main(String[] args)
        {
            System.out.println("The program gets the sum of all prime numbers.");
     
            Scanner scan = new Scanner(System.in);
     
            System.out.print("Enter a number: ");
            int num = scan.nextInt();
     
            int sum = isPrime(num);
     
            System.out.println(sum);
        }
    }


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: Printing Sum of Primes of a number

    Did you mean the sum of all prime factors of a number, or the sum of all prime numbers less than (and/or equal) to the number? The two will provide very different results.

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    Default Re: Printing Sum of Primes of a number

    The later...

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    Super Moderator Norm's Avatar
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    Default Re: Printing Sum of Primes of a number

    Try debugging your code by printing out the results of the tests so you can see what is happening. Add some println()s to the loop in the isPrime() method.

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    Default Re: Printing Sum of Primes of a number

    Ok, I'm not that interested in coding this out, so I'll run you through what is going on with your code.

    Lets call isPrime(5). This should return 3 (unless you consider 1 a prime number, which would result in 4).
    Now what that does return is 9. Wrong answer . So, what is happening?

    We start the method.
    Sum = 0. Factor = 1. num = 5.
    Start the loop. 1 <= 5? Yes. num%factor != 0? No, that equals 0. Increase factor.
    2 <= 5? Yes. num%factor != 0? Yes, that equals 1. sum = 0+2. Increase factor.
    3 <= 5? Yes. num%factor != 0? Yes, that equals 2. sum = 2+3. Increase factor.
    4 <= 5? Yes. num%factor != 0? Yes, that equals 1. sum = 5+4. Increase factor.
    5 <= 5? Yes. num%factor != 0? No, that equals 0. Increase factor.
    6 <= 5? No. Return 9.

    So, if you arent aware of what mod ( % ) does, it returns the remainder from division. Which means. When we do 5%1, we are actually doing 5/1 and returning the extra. So, 5/1 = 5 with a remainder of 0. 5/2 = 2 with a remainder of 1. 5/3 = 1 with a remainder of 2. 5/4 = 1 with a remainder of 1. 5/5 = 1 with a remainder of 0. Mod is getting its answer from the remainder of the integer division. Hopefully that can help you.

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    Default Re: Printing Sum of Primes of a number

    So that shouldn't even be part of the test. I'm confused then about what I need to do, I have been scratching my head all day it seems... I thank you for the help.

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    Default Re: Printing Sum of Primes of a number

    I tried going back to the basic of is a number prime and working it that way, and I sort of see the problem... How do I kept the sum going even though that loop is changing it?

    import java.util.*;
     
    public class PrimeSum3
    {
        public static void main(String[] args)
        {
            Scanner scan = new Scanner(System.in);
            System.out.println("Enter a number: ");
            int input = scan.nextInt();
            boolean prime = true;
     
            int num = 0;
            int start = 2;
            while(start < input)
            {
                if(input % start == 0)
                {
                    prime = false;
                }
                if(prime == true)
                {
                    num = start;
                    System.out.print(num + " " + start);  //for test purposes
                    num += num;
                }
                System.out.print(" " + num + " .");  //for test purposes
                start++;
            }
     
            if(prime == false)
            {
                System.out.println(input + " is not prime");
            }
            else if(prime == true)
            {
                        System.out.println(input + " is prime");
            }
            System.out.println(num);
        }
    }

    //Output
    2 2 4 .3 3 6 .4 4 8 .5 is prime
    8

  8. #8
    Super Moderator Norm's Avatar
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    Default Re: Printing Sum of Primes of a number

    Your debug output should be a bit more verbose. It's hard to tell what the list of numbers you've posted means:
    println("num=" + num + ", start=" + start);

    I have no idea what this means:
    System.out.print(" " + num + " .");

  9. #9
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    Default Re: Printing Sum of Primes of a number

    You need to go step-by-step. First find whether a number is prime (so, write a method which tells whether a given number is prime) and then add it to the sum (write a method which adds only prime numbers).

    The following works for you:

    public class SumOfPrimes{

    public boolean isPrime(int x){
    for(int i=2;i<x;i++){
    if(x%i == 0)
    return false;
    }
    return true;
    }

    public int sumOfAllPrimes(int x){
    int sum = 0;
    for(int i=2;i<x;i++){
    if(isPrime(i)){
    sum = sum+i;
    System.out.println("sum: "+sum);
    }
    }
    return sum;
    }

    public static void main(String[] args){
    int number = 6;
    SumOfPrimes sop = new SumOfPrimes();
    System.out.println("sum of all primes less than the given number is: "+sop.sumOfAllPrimes(number));
    }
    }

  10. The Following User Says Thank You to kamalivy For This Useful Post:

    CodeNewb (July 12th, 2010)

  11. #10
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    Default Re: Printing Sum of Primes of a number

    I doubt I would of ever figured it out... Thanks for you help.

    I like this by the way, I'm just learning java and I think this is neat...
    SumOfPrimes sop = new SumOfPrimes();
    System.out.println("sum of all primes less than the given number is: "+sop.sumOfAllPrimes(number));

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