i had implemented Sieve of Eratosthenes long time back...
what i did was the following :-
say N is the range and we want to find all prime number within this range.
take size of temp array[] to half = N/2...as we only care of odd
numbers.Prime number 2 can be handled explicitly.
run outer loop for
for(i=3 ; i<(sqrt(N))/2;i=i+2) // consider only odd "i"
{
for(j=i^2; (j/2)< N/2 ; j+= i*2) // here I am excluding even multiple
of "j" by incrementing it by 2*i
set(j,false);
}
when i ran this algo for N=2000000 , it took 45.302 ms
On Sat, Dec 8, 2012 at 2:44 AM, Don <dondod...@gmail.com> wrote:
> I know that the Sieve of Eratosthenes is a fast way to find all prime
> numbers in a given range.
> I noticed that one implementation of a sieve spends a lot of time
> marking multiples of small primes as composite. For example, it takes
> 1000 times as long to mark off all of the multiples of five as it
> takes to mark off the multiples of 5003. In addition, when it is
> marking off the multiples of larger primes, most of them are multiples
> of small primes. In fact, if it could skip over multiples of 2,3,5,7,
> and 11, the sieve would be about 5 times faster.
>
> Can someone describe a way to make a sieve faster by not having to
> deal with multiples of the first few prime numbers?
>
> Don
>
> --
>
>
>
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