prime numbers

[Randall Twede]

i am getting a strange output from my class

//import java.math.BigInteger;

class Prime implements Algorithm
{
	public String calculate(int howMany)
	{
		//BigInteger[] numbers = new BigInteger[howMany];
		StringBuffer output = new StringBuffer("Prime Numbers\n");
		long number = 3;
		int count = 2;
		
		output.append(1 + "\n");
		output.append(2 + "\n");
		while(count < howMany)
		{
			for(long i = 2; i < number; i++)
			{
				if((number % i) == 0)
				{
					break;
				}
				output.append(number + "\n");
				count++;
			}
			number++;
		}
		return output.toString();
	}
}

when howMany = 20 i get the following output

Prime Numbers
1
2
3
5
5
5
7
7
7
7
7
9
11
11
11
11
11
11
11
11
11

[Randall Twede]

i think i figured it out maybe. i am using longs so the % operator rounds results?

[Jeff Verdegan]

Randall Twede wrote:i think i figured it out maybe. i am using longs so the % operator rounds results?

Not sure what you mean by that, but whatever you mean, no that's not it.

Why does that output surprise you? What would you expect instead? You should add print statements or use a debugger so you can see what's actually happening?

Also note that when checking for divisors of X, you only need to go up to the square root of X.

[Randall Twede]

thats right that is not the problem. i will try some System.out.println
you are also correct that i dont have to loop so many times

i see the problem now. it is doing exactly what i told it too
thanks for the tip about only needing to go up to the square root

[Jesper de Jong]

Randall Twede wrote:it is doing exactly what i told it too

Yes, that's the biggest problem that people have with computers. They always do exactly what you tell them to do, not what you intended them to do.

[fred rosenberger]

Randall Twede wrote:it is doing exactly what i told it to

it always does.

[Randall Twede]

ok, i solved that one. now i am trying to solve problem3 a projectEuler.net
here is my solution of original problem

//import java.math.BigInteger;

class Prime implements Algorithm
{
	public String calculate(int howMany)
	{
		//BigInteger[] numbers = new BigInteger[howMany];
		StringBuffer output = new StringBuffer("Prime Numbers\n");
		long number = 3;
		int count = 2;
		boolean isPrime = true;
		
		output.append(1 + "\n");
		output.append(2 + "\n");
		while(count < howMany)
		{
			isPrime = true;
			//for(long i = 2; i < number; i++)
			for(long i = 2; i <= Math.sqrt((double)number); i++)
			{
				if((number % i) == 0)
				{
					isPrime = false;
					break;
				}
			}
			if(isPrime)
			{
				output.append(number + "\n");
				count++;
			}
			number++;
		}
		return output.toString();
	}
}

the new problem is this: What is the largest prime factor of the number 600851475143 ?
even using longs the number is too big. and Math.sqrt() requires a double and the number is too big

[Jesper de Jong]

600851475143 does fit in a long, but if you write it in source code you have to put an "L" at the end of it: long value = 600851475143L;

If you don't, Java is going to interpret 600851475143 as an int literal instead of a long, and it's going to tell you the number is too big to fit in an int.

To solve this problem, you should do something smarter than checking if all numbers smaller than 600851475143 are primes. That's the trick of this Project Euler exercise!

[Randall Twede]

oh....i forgot about the L thanks