Printing prime numbers from 1-100 into a separate file?

Question

I have to print a list of numbers 1-100 that are all prime. Those prime numbers have to be printed onto a separate .txt file.

import java.io.*;
public static boolean isPrime(int number)
{
int division = 0;

if(number<1)
{
  return false;
}

for(division=1; division<=number; division++)
{
  if(number%division==0)
  {
    division+=1;
  }
  if((number%division>2) || (number==1) || (number==-1))
  {
    return false;
  }


}
return true;

}

public static void main(String[] args) throws IOException
{
  PrintWriter fileToWrite = new PrintWriter("primeNumberList.txt");

  for(int currentNumber=1; currentNumber<=100; currentNumber++)
  {
    if(isPrime(currentNumber))
    {
      fileToWrite.println(currentNumber);
    }
  }
  fileToWrite.close();
  System.out.println("The prime numbers have been listed in the file primeList.txt");
}

}

I have this code, whenever I run it will only print the number to. Anything I can do to change it?

Answer 1

This looks like a homework question, so I'll avoid complete code solutions.

Also, your last sentence seems cut-off - I'm guessing you're saying that the numbers being printed are not prime.

One thing you might want to look at is the Sieve of Eratosthenes . isPrime , as you've defined it, won't work correctly.

The first conditional, if (number % divisor) == 0 , will always be true for 1 (so the iterator shouldn't start there), and if it's true for any value > 1, then number is not prime, and the function should return false .

Also worth noting is that the range should not be division<=number , because number%number will always be 0.

The second conditional in isPrime doesn't do anything useful from the perspective of primefinding.

The Sieve of Eratosthenes is a more elegant prime-finder than just running through all integers lower than the number, but its implementation is left as an exercise to the student.

Good luck!

Answer 2

Your isPrime function does not work, try the below code:-

public static boolean isPrime(int number)
{       
    for(int division = 2; division < number; division++) {
        if(*MATHS FOR PRIME*)
            return false;
    }

    return true;    
}

Start at 2, the first prime number and cycle though all numbers up to it and see if it divides evenly. I have taken out the maths but you should be able to find this with a quick google.

Answer 3

this is your problem:

if((number%division>2) || (number==1) || (number==-1))

the first condition is if the modulo of the number and division is larger than 2 then return false. , this will happen for every number larger than 3 on the first iteration of the loop. the correct loop will be something like this:

for(division=2; division<number; division++)
{
  if(number%division == 0)
  {
    return false;
  }
}