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POPrime number calculation fun
 

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    <p>We're having a bit of fun here at work. It all started with one of the guys setting up a Hackintosh and we were wondering whether it was faster than a Windows Box of (nearly) same specs that we have. So we decided to write a little test for it. Just a simple Prime number calculator. It's written in Java and tells us the time it takes to calculate the first n Prime numbers.</p> <p>Optimised version below - now takes ~6.6secs</p> <pre><code>public class Primes { public static void main(String[] args) { int topPrime = 150000; int current = 2; int count = 0; int lastPrime = 2; long start = System.currentTimeMillis(); while (count &lt; topPrime) { boolean prime = true; int top = (int)Math.sqrt(current) + 1; for (int i = 2; i &lt; top; i++) { if (current % i == 0) { prime = false; break; } } if (prime) { count++; lastPrime = current; } if (current == 2) { current++; } else { current = current + 2; } } System.out.println("Last prime = " + lastPrime); System.out.println("Total time = " + (double)(System.currentTimeMillis() - start) / 1000); } } </code></pre> <p>We've pretty much lost the plot of the whole Hackintosh vs PC thing and are just having some fun with optimising it. First attempt with no optimisations (the above code has a couple) ran around 52.6min to find the first 150000 prime numbers. This optimisation is running around 47.2mins.</p> <p>If you want to have a go and post your results, then stick em up.</p> <p>Specs for the PC I'm running it on are Pentium D 2.8GHz, 2GB RAM, running Ubuntu 8.04.</p> <p><strong>Best Optimisation so far has been the square root of current, first mentioned by Jason Z.</strong></p>
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    1. USAndrew Eisenberg
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    1. COThere is an error in your code. Top != current / 2, it should be the square root of current. Might speed it up a bit.
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      1. USJason Z
    2. COThat's an optimisation rather than an error? Thanks, I'll try it out.
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    3. COSometimes else blocks slow down a lot so try to get rid of that too and see if it helps. Obviously current is increased by 1 in each loop so instead of that if..else I'd put if(current!=2) { current++; } current++; in its place.
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