Re: [Algorithm] Sum of Primes < 1000000
Simon wrote:
Luc The Perverse schrieb:
I was messing around on a site where you answer questions for points. (No
prizes or anything)
Anyway, they say our programs *should* execute in less than 1 minute if well
formed.
Mine didn't - it was less than 10 minutes though and I got the right answer.
I was supposed to sum all primes less than 1 million.
Is there an algorithm out there, which I will be able to understand, which
works significantly faster than this one?
You may want to have a look at the Sieve of Eratosthenes (and its successors)
(see http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes). It finds all primes up
to n in time n log n. On my computer my implementation takes 150 milliseconds
for n=1000000.
As a matter of curiosity, did you use a boolean[] or BitSet?
I started with a boolean[], and mine was slower than yours, about 200
milliseconds. It dropped to under 100 milliseconds when I switched to
BitSet. I believe the improvement is due to more efficient caching.
Patricia
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