Perl: the Markov chain saw PerlMonks

### Re: is it prime?

by johngg (Canon)
 on May 06, 2012 at 12:15 UTC ( #969135=note: print w/replies, xml ) Need Help??

in reply to is it prime?

One speedy way is to use the Sieve of Eratosthenes.

```use strict;
use warnings;

use 5.010;

say
qq{\$_ is },
isPrime( \$_ ) ? q{} : q{not },
q{prime}
for 75, 169, 229, 367, 369, 8794, 9227;

sub isPrime
{
my \$toTest    = shift;
my \$sqrtLimit = sqrt \$toTest;

my \$sieve = q{};
vec( \$sieve, 0      , 1 ) = 1;
vec( \$sieve, 1      , 1 ) = 1;
vec( \$sieve, \$toTest, 1 ) = 0;

my \$marker = 1;
while ( \$marker < \$sqrtLimit )
{
my \$possPrime = \$marker + 1;
\$possPrime ++ while vec( \$sieve, \$possPrime, 1 );

my \$fill = 2 * \$possPrime;
while ( \$fill <= \$toTest )
{
vec( \$sieve, \$fill, 1 ) = 1;
\$fill += \$possPrime;
}
last if vec( \$sieve, \$toTest, 1 );

\$marker = \$possPrime;
}

return not vec( \$sieve, \$toTest, 1 );
}

The output.

```75 is not prime
169 is not prime
229 is prime
367 is prime
369 is not prime
8794 is not prime
9227 is prime

I quickly adapted this from a much older program I had lying around that lists all the primes up to a limit.

```# Script to find prime numbers up to a given limit using
# the algorithm called "The Sieve of Eratosthenes." The
# "sieve" is filled by marking multiples of each prime found
# as "not a prime" so the first prime we find is 2 and so
# all even numbers are marked. The next number along the
# "sieve" that is not marked will be 3 so all multiples of
# 3 are then marked, und so weiter.
#
use strict;
use warnings;

# Read number to find primes up to from command-line and
# make sure it is a positive integer. Algorithm has found
# and marked all non-primes up to \$limit once the value
# being examined exceeds the square root of \$limit so
# set up that value to avoid recalulation each time.
#
my \$limit = shift or die qq{No limit supplied\n};
die qq{Limit is not a positive integer\n} unless
\$limit =~ /^\+?\d+\$/;
my \$sqrtLimit = sqrt \$limit;

# Initialise the "sieve," for which we use a vector. The
# numbers 0 and 1 are not prime so set their positions
# in the vector to true. Assume that our \$limit is a
# prime for now by setting to false. We now have a vector
# of the correct length.
#
my \$sieve = q{};
vec( \$sieve, 0, 1 ) = 1;
vec( \$sieve, 1, 1 ) = 1;
vec( \$sieve, \$limit, 1 ) = 0;

# In initialising the "sieve" we have reached number 1
# so iterate in a while loop until we pass \$sqrtLimit.
#
my \$reached = 1;
while ( \$reached < \$sqrtLimit )
{
# Examine the next number after \$reached and keep
# moving along until we find a number that hasn't
# been marked as "not a prime."
#
my \$prime = \$reached + 1;
++ \$prime while vec( \$sieve, \$prime, 1 );

# This is a prime so print it out, mark multiples
# of the prime we have found as "not a prime" up
# to our \$limit. Update where we have reached.
#
print qq{\$prime is a prime\n};
my \$fill = 2 * \$prime;
while ( \$fill <= \$limit )
{
vec( \$sieve, \$fill, 1 ) = 1;
\$fill += \$prime;
}
\$reached = \$prime;
}

# We have passed \$sqrtLimit so all primes up to \$limit have
# been found. It just remains to print them out.
#
foreach my \$value ( \$reached + 1 .. \$limit )
{
print qq{\$value is a prime\n} unless vec(\$sieve, \$value, 1);
}

I hope this is of interest.

Update: Not so speedy, apparently! If only I had a computer science background and understood all the O(log n) gubbins :-D

Cheers,

JohnGG

Replies are listed 'Best First'.
Re^2: is it prime?
by JavaFan (Canon) on May 06, 2012 at 15:12 UTC
One speedy way is to use the Sieve of Eratosthenes.
It's speedy if you want all primes below a certain number, or if you need to determine primality of "a lot of" numbers, but it isn't for checking a single number.

Checking all the possible divisors can be done in O(√N) (throw in a factor of O(log n) to do the division if you have really big numbers). However, just initializing the sieve will take you Ω(N) time. And then you still have to do the work: Ω(N/pi) for the ith prime.

The fastest way is probably just to use bigprimes.net, which has the first 1.4 billion primes on file.

Re^2: is it prime?
by tobyink (Canon) on May 06, 2012 at 19:41 UTC

One speedy way is to use the Sieve of Eratosthenes.

Speedy? Benchmarking mine against yours...

```  NUMBER : TOBYINK       JOHNGG
NUMBER : RESULT  TIME  RESULT  TIME
75 : NO  0.000065  NO  0.000215
169 : NO  0.000057  NO  0.000649
229 : YES 0.000070  YES 0.000888
367 : YES 0.000073  YES 0.001437
369 : NO  0.000052  NO  0.000805
8794 : NO  0.000051  NO  0.008682
9227 : YES 0.000113  YES 0.031170
10807 : NO  0.000131  NO  0.047135
11939 : YES 0.000121  YES 0.045032
14803 : NO  0.000122  NO  0.054764
19937 : YES 0.000143  YES 0.070692
19938 : NO  0.000040  NO  0.020510
39783 : NO  0.000057  NO  0.065555
47083 : NO  0.000230  NO  0.170125
199933 : YES 0.000346  YES 0.778786

For some of those higher numbers, mine is about 2000 times faster than yours. The sieve is efficient if you need to generate a list of all prime numbers below x, but very slow as a mechanism for determining whether a given number is prime.

Benchmark script:

perl -E'sub Monkey::do{say\$_,for@_,do{(\$monkey=[caller(0)]->[3])=~s{::}{ }and\$monkey}}"Monkey say"->Monkey::do'

I'd only ever used Eratosthenes' Sieve to find a list of all primes up to a limit and, stupidly, assumed that it would be equally good at testing whether a single number was prime. Silly me!

You have shown your method to be much faster but there's still room for improvement. You reject all even numbers the first time through your loop so there's no point in using even numbers again as divisor. Instead, take the evens test out of the loop then employ a C-style loop to test only odd divisors.

The code. I've tweaked things to return 0 if not prime in order to make testing easier.

The benchmark results.

```Testing with value 2 which is a prime
ok 1 - eratosthenes
ok 2 - tobyink
ok 3 - tobyinkPlus
Testing with value 8357 which is not a prime
ok 4 - eratosthenes
ok 5 - tobyink
ok 6 - tobyinkPlus
Testing with value 9293 which is a prime
ok 7 - eratosthenes
ok 8 - tobyink
ok 9 - tobyinkPlus
Rate eratosthenes      tobyink  tobyinkPlus
eratosthenes 0.154/s           --        -100%        -100%
tobyink       1291/s      838266%           --         -35%
tobyinkPlus   1984/s     1288047%          54%           --
1..9

I hope this is of interest.

Cheers,

JohnGG

Necroposting, but maybe of interest still.

First, no surprise, there are modules that will be much faster than doing it yourself, and easier overall. But let's take a look at pure Perl methods. We can improve the SoE method about 2-3x by using the simple vector sieve from RosettaCode. The sieve using a string is faster yet, but let's keep it simple since clearly this is not the right way to go about testing primality. I also included my version of the all, mod-2 wheel, and mod-6 wheel trial division methods. Although going to a mod-30 wheel can improve things a little more and still be reasonable code, I didn't include it. The mod-6 wheel runs about 2x faster than Toby's original code.

For modules, some choices include:

• Math::Prime::Util        scads faster than anything else, especially for large inputs. Needs Math::Prime::Util::GMP to be very fast for bigints, though it works fine without. I'm biased, being the author.
• Math::Prime::XS           trial division with a mod-30 wheel in XS. Very fast for small numbers, slows down rapidly, no bigint support.
• Math::Pari                  reasonably fast, supports bigints. Since it is based on the ancient Pari 2.1, be careful as it will sometimes give you incorrect results.
• Math::Primality           works, but quite slow as it is designed for bigints
• Math::Prime::FastSieve  if your range is small (e.g. under 1000M) and you have lots of numbers to test, this can work well by doing an efficient sieve in XS then do fast bit tests to determine primality.
For generation, both Math::Prime::Util and Math::Prime::FastSieve should be much faster than trying to download a list from the net, as a previous suggestion mentioned, and probably faster than loading from disk as well.

Code:

Timing results:

```Validating tests on small primes
Validating tests on small composites
TOBYINK     DIV2        DIV6        MPU
NUMBER   :  RES TIME    RES TIME    RES TIME    RES  TIME
75 :  N 0.000004  N 0.000002  N 0.000001  N 0.000002
169 :  N 0.000002  N 0.000002  N 0.000001  N 0.000000
229 :  Y 0.000004  Y 0.000001  Y 0.000001  Y 0.000000
367 :  Y 0.000004  Y 0.000002  Y 0.000002  Y 0.000000
369 :  N 0.000002  N 0.000001  N 0.000001  N 0.000000
8794 :  N 0.000002  N 0.000001  N 0.000001  N 0.000000
9227 :  Y 0.000008  Y 0.000005  Y 0.000004  Y 0.000000
10807 :  N 0.000008  N 0.000005  N 0.000004  N 0.000000
11939 :  Y 0.000009  Y 0.000005  Y 0.000005  Y 0.000000
14803 :  N 0.000009  N 0.000005  N 0.000004  N 0.000000
19937 :  Y 0.000011  Y 0.000006  Y 0.000005  Y 0.000001
19938 :  N 0.000002  N 0.000001  N 0.000001  N 0.000001
39783 :  N 0.000002  N 0.000001  N 0.000001  N 0.000000
47083 :  N 0.000014  N 0.000008  N 0.000006  N 0.000000
199933 :  Y 0.000030  Y 0.000017  Y 0.000014  Y 0.000008
75640351 :  Y 0.000554  Y 0.000311  Y 0.000246  Y 0.000001
760149189769 :  Y 0.056347  Y 0.031018  Y 0.024714  Y 0.000011
635921898906263 :  Y 1.605829  Y 0.904163  Y 0.721130  Y 0.000004

Create A New User
Node Status?
node history
Node Type: note [id://969135]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others musing on the Monastery: (9)
As of 2020-10-26 13:40 GMT
Sections?
Information?
Find Nodes?
Leftovers?
Voting Booth?
My favourite web site is:

Results (251 votes). Check out past polls.

Notices?