Re: [Algorithm] Sum of Primes < 1000000

Patricia Shanahan <>
Fri, 05 Jan 2007 20:23:34 GMT
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

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 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.


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