Thread: 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);
    }
}

The later...

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.

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.

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.

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

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 + " .");

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));
}
}

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));