Java Prime Number Finder...

[4th gen]

I started writing (badly ) a prime number finder in java code today...below is where I'm at just now, however, it returns "4" as a prime number (which it obviously isn't), however, I can't sort it for the correct "4" output without either breaking the program or 'cheating' by starting > 4. Anyone got any ideas?

public class primefinder
{
	public static void main(String[] args)
	{
  int count1 = 0;
  int count2 = 0;
  
  for(int number=2; number<50; number++)
  {
    count1 = 0;
    for(int i=2; i<number/2; i++)
    {
       count2 = 0;
     if(number%2 == 0.0)
     {
          count2++;
          break;
        }
     else  
     {
       if(number%i == 0.0)
        count1++;
        }
    }
    if(count1 == 0 && count2 == 0)
      System.out.println(number+" is a prime number");
  }
    }
}

[Entity101]

public boolean isPrime(long number)
 {
   if ((number % 2 == 0) && (number != 2))
     return false;
   long endNum = (long)Math.sqrt((double)number);
   long curNum = 3;
   long remainder;
   while (curNum <= endNum)
   {
     remainder = number % curNum;
     if (remainder == 0)
       return false;
     curNum += 2;
   }
   return true;
 }

[4th gen]

Nice program! Did you write it?
Sadly mine works at finding prime numbers from a range of inputs (i.e. find all prime numbers from 2 - 1000), rather than checking whether an inputted number is prime...

edit: just saw your edit

[ilw]

I tried briefly

for (number = (a + (a % 2)); number <= b&#59; number+2)
{
	max = (number - (number % 2)) / 2   // round down &  halve 
         max = max - 1 + (max % 2)  // To make it odd 
	for (denominator = max; denominator >= 3; denominator - 2)
	{
 if number % denominator = 0 
 {	
 	break
 }
 
 if denominator ==3 
 	prime found ...
	}
}

doesn't find primes 1,2,3 but i think it should work after that, obviously its not written in proper code. I didn't really understand what you meant about '4' thing...

Even if you ignore the rest of my code, adding 2s judiciously instead of ++ will save you code and runtime.

[Lamsey]

Here's mine:

/* Returns the prime numbers from 2 to 100000
 *
 * By Lamsey
 */
public class PrimeFinder
{
  public static void main(String[] args)
  {
    boolean prime;
    for (int number = 2; number < 1000001; number++)
    {
      prime = true;
      if ((number % 2) == 0)
        prime = false;
      else
      {
        for (int x = 3; x <= (number / 2); x += 2)
        {
          if ((number % x) == 0)
          {
            prime = false;
            break;
          }
        }
      }
      if (prime)
      	System.out.println(number+" is prime.");
      else
      	System.out.println(number+" is not prime.");
    }
  }
}

[4th gen]

My finished one:

public class primefinder
{
	public static void main(String[] args)
	{
  int count1 = 0;
  int count2 = 0;
  long start_time = 0, end_time = 0;
  
  start_time = System.currentTimeMillis();
  
  for(int number=2; number<1000; number++)
  {
    count1 = 0;
    
    for(int i=2; i<=number/2; i++)
    {
       count2 = 0;
     if(number%2 == 0.0)
     {
          count2++;
          break;
        }
     else  
     {
       if(number%i == 0.0)
        count1++;
        }
    }
    if(count1 == 0 && count2 == 0)
      System.out.println(number+" is a prime number");
  }
  System.out.println("Finished");
  end_time = System.currentTimeMillis();
  System.out.println("Time taken = "+(end_time - start_time));	
    }
}

Sorry, ilw, but I never understood what you said

[4th gen]

Mine takes 31ms to find all primes <1000, Lamesy&#39;s takes 141
Mine takes 594ms to find all primes <10000, Lamesy&#39;s takes 703
Mine takes 41125 to find all primes <100000, Lamsey&#39;s takes 8641.

So, it looks like mine is faster for relatively small tasks, but Lamesy&#39;s is quicker for larger tasks...

Interestingly, removing the printline statement from Lamesy&#39;s (for when the number is not prime), brings his execution time for primes <100000 down to 3532ms, less than half...

I&#39;ve got to say his is better than mine

[4th gen]

After tweaking Lamesy&#39;s code further, I&#39;ve just seen 16ms execution for primes <1000 and the average is around about 20-23ms, which is pretty quick

edit:

public class lamseycode
{
  public static void main(String[] args)
  {
    boolean prime;
    double start_time, end_time;
    start_time = System.currentTimeMillis();
    for (int number = 1; number < 1000; number += 2)
    {
      prime = true;
      for (int x = 3; x <= (number / 2); x += 2)
      {
        if(number % x == 0)
        {
          prime = false;
          break;
        }
      }
      if (prime)
      System.out.println(number+" is prime.");
     }
     end_time = System.currentTimeMillis();
     System.out.println("Time taken = "+(end_time - start_time));
   }
}

[4th gen]

After taking out all printline statements (just running a count on the number of primes) and using Lamsey&#39;s code, the execution time for primes <10000 (ten thousand, as compared to 1000 in the last post) is now 47ms

public class lamsey
{
  public static void main(String[] args)
  {
    boolean prime;
    int primes = 0;
    double start_time = System.currentTimeMillis();
    for (int number = 1; number < 10000; number += 2)
    {
      prime = true;
      for (int x = 3; x <= (number / 2); x += 2)
      {
        if(number % x == 0)
        {
          prime = false;
          break;
        }
      }
      if (prime)
        primes++;
     }
     double end_time = System.currentTimeMillis();
     System.out.println("Time taken = "+(end_time - start_time));
     System.out.println("Primes found = "+primes);
   }
}

[Lamsey]

Great, now you have a program with no output. Very useful.

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