Java Prime Number Finder...

[4th gen]

Originally posted by Lamsey@31 March 2004 - 14:28
Great, now you have a program with no output. Very useful.

I really wanted to know if 95,247 was prime or not

Well, the point of removing the output was that it now shows you just how long it takes to calculate them, not to display them...

Besides, if you wanted to show them, you could always output to text file...

[4th gen]

This code outputs the results to a text file, only slowing the program down by about 0.15 seconds for all primes < 100000

import java.util.*;
import java.io.*;

public class lamsey
{
  public static void main(String[] args) throws Exception
  {
    File outFile = new File("primes.txt");
	FileOutputStream outFileStream = new FileOutputStream(outFile);
	PrintWriter outStream = new PrintWriter(outFileStream);
    
    boolean prime;
    int primes=0;
    double start_time=System.currentTimeMillis();
    System.out.println("Working...please wait");
    for (int number=1;number<100000;number+=2)
    {
      prime=true;
      for (int x=3;x<=number/2;x+=2)
      {
        if(number%x==0)
        {
          prime=false;
          break;
        }
      }
      if(prime)
      {
        primes++;
        outStream.println(number+" is a prime number");
      }
    }
    double end_time = System.currentTimeMillis();
    System.out.println("Time taken = "+(end_time - start_time));
    System.out.println("Primes found = "+primes);
  }
}

[4th gen]

This code is a little bit quicker than mine and Lamesy&#39;s attempts so far:

import java.lang.*;

class sieve
{
	public static void main (String [] args)
    {
  final int max = 100000;
        final int startat = 1;
        int n = max;
        double limit;
        double start_time=0, end_time=0; 
        int primesfound=0;
        start_time = System.currentTimeMillis();
      
        boolean numberpool [] = new boolean [n];
        for(int i=2; i<n; i++)
        {
        	numberpool[i]=true;
        }

        limit = Math.sqrt(n);
        int j=2;
        for (int i=j+j; i<n; i=i+j)
        {
        	numberpool[i]=false;
        }

        for (j=3; j<=limit; j=j+2)
        {
        	if (numberpool[j]==true)
        	{
          for (int i=j+j; i<n; i=i+j)
          {
          	numberpool[i]=false;
             }
            }
        }

        System.out.println("Prime Numbers from " + startat + " to " + max + ":");
        for (int i=startat; i<max; i++)
        {
        	if (numberpool[i]==true)
        	{
          primesfound++;
            }
        }
        end_time = System.currentTimeMillis();
        System.out.println("Time taken = "+(end_time - start_time));
        System.out.println("Number of primes found "+primesfound);
    }
}

I should say it&#39;s not my code (well, I never wrote it anyway), but here are a few results:

Prime Numbers from 1 to 100000:
Time taken = 16.0ms
Number of primes found 9592

Prime Numbers from 1 to 1000000:
Time taken = 94.0ms
Number of primes found 78498

Prime Numbers from 1 to 10000000:
Time taken = 860.0ms
Number of primes found 664579