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POLargest prime factor program takes aaaages - Java
 

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  1. POLargest prime factor program takes aaaages - Java
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    <p>So this is problem 3 from project Euler. For those who don't know, I have to find out the largest prime factor of 600851475143. I have the below code:</p> <pre><code>import java.lang.Math; // 600851475143 public class LargestPrimeFactor { public static void main(String[] stuff) { long num = getLong("What number do you want to analyse? "); long[] primes = primeGenerator(num); long result = 0; for(int i = 0; i &lt; primes.length; i++) { boolean modulo2 = num % primes[i] == 0; if(modulo2) { result = primes[i]; } } System.out.println(result); } public static long[] primeGenerator(long limit) { int aindex = 0; long[] ps = new long[primeCount(limit)]; for(long i = 2; i &lt; limit + 1; i++) { if(primeCheck(i)) { ps[aindex] = i; aindex++; } } return ps; } public static boolean primeCheck(long num) { boolean r = false; if(num == 2 || num == 3) { return true; } else if(num == 1) { return false; } for(long i = 2; i &lt; Math.sqrt(num); i++) { boolean modulo = num % i == 0; if(modulo) { r = false; break; } else if(Math.sqrt(num) &lt; i + 1 &amp;&amp; !modulo) { r = true; break; } } return r; } public static int primeCount(long limit) { int count = 0; if(limit == 1 || limit == 2) { return 0; } for(long i = 2; i &lt;= limit; i++) { if(primeCheck(i)) { count++; } } return count; } public static long getLong(String prompt) { System.out.print(prompt + " "); long mrlong = input.nextLong(); input.nextLine(); return mrlong; } } </code></pre> <p>But when I test the program with something (a lot) smaller than 600851475143, like 100000000, then the program takes its time - in fact, 100000000 has taken 20 minutes so far and is still going. I've obviously got the wrong approach here (and yes, the program <em>does</em> work, I tried it out with smaller numbers). Can anyone suggest a less exhaustive way?</p>
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      1. POLargest prime factor program takes aaaages - Java
      1. USMuhammad Maqsoodur Rehman
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    1. CODid you read some math books on prime numbers? They give more clever algorithms!
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      1. POLargest prime factor program takes aaaages - Java
      1. USBasile Starynkevitch
    2. COWhy don't you simply try the [Sieve of Eratosthenes](http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes) to implement your prime checking?
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      1. POLargest prime factor program takes aaaages - Java
      1. USKazekage Gaara
    3. COWhy are you generating primes up to num rather than the square root of num? If the number is not prime, it must have a factor less than or equal to its square root. Also, look into sieving, as Kazekage said.
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      1. POLargest prime factor program takes aaaages - Java
      1. USDavid Conrad
 

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