This is apparently a very fast prime list generator ( Fastest way to list all primes below N ), but I can't understand some parts of it, mostly because of the syntax.
def rwh_primes1(n):
""" Returns a list of primes < n """
sieve = [True] * (n/2)
for i in xrange(3,int(n**0.5)+1,2):
if sieve[i/2]:
sieve[i*i/2::i] = [False] * ((n-i*i-1)/(2*i)+1)
return [2] + [2*i+1 for i in xrange(1,n/2) if sieve[i]]
Why sieve
is defined as [True]
(a boolean) multiplied by an integer?
What means if sieve[i/2]
?
What means sieve[i*i/2::i]
, especially the ::i
part?
looks like it's just array notation you're looking for.
[a] * 5
just becomes [a,a,a,a,a]
if sieve[i/2]
is checking if the value of sieve
at i/2
is True
or False
and the ::
defines the stride.
see this answer.
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.