NAME

Math::Prime::Util::PrimeArray - A tied array for primes

VERSION

Version 0.35

SYNOPSIS

  use Math::Prime::Util::PrimeArray;

  # Create:
  tie my @primes, 'Math::Prime::Util::PrimeArray';

  # Use in a loop by index:
  for my $n (1..10) {
    print "prime $n = $primes[$n]\n";
  }

  # Use in a loop over array:
  for my $p (@primes) {
    print "$p\n";
    last if $p > $limit;   # stop sometime
  }

  # Use via array slice:
  print join(",", @primes[0..50]), "\n";

  # Use via each:
  use 5.012;
  while( my($index,$value) = each @primes ) {
    print "The ${index}th prime is $value\n";
    last if $p > $limit;   # stop sometime
  }

  # Use with shift:
  while ((my $p = shift @primes) < $limit) {
    print "$p\n";
  }

DESCRIPTION

An array that acts like the infinite set of primes. This may be more convenient than using Math::Prime::Util directly, and in some cases it can be faster than calling next_prime and prev_prime.

If the access pattern is ascending or descending, then a window is sieved and results returned from the window as needed. If the access pattern is random, then nth_prime is used.

Shifting acts like the array is losing elements at the front, so after two shifts, $primes[0] == 5. Unshift will move the internal shift index back one, unless given an argument which is the number to move back (it silently truncates so it does not shift past the beginning). Example:

  say shift @primes;     # 2
  say shift @primes;     # 3
  say shift @primes;     # 5
  say $primes[0];        # 7
  unshift @primes;       #     back up one
  say $primes[0];        # 5
  unshift @primes, 2;    #     back up two
  say $primes[0];        # 2

If you want sequential primes with low memory, I recommend using "forprimes" in Math::Prime::Util. It is much faster, as the tied array functionality in Perl is not high performance. It isn't as flexible as the prime array, but it is a very common pattern.

If you prefer an iterator pattern, I would recommend using "prime_iterator" in Math::Prime::Util. It will be a bit faster than using this tied array, but of course you don't get random access. If you find yourself using the shift operation, consider the iterator.

LIMITATIONS

The size of the array will always be shown as 2147483647 (IV32 max), even in a 64-bit environment where primes through 2^64 are available.

There are some people that find the idea of shifting a prime array abhorrent, as after two shifts, "the second prime is 7?!". If this bothers you, do not use shift on the tied array.

PERFORMANCE

  MPU forprimes:  forprimes { $sum += $_ } nth_prime(100_000);
  MPU iterator:   my $it = prime_iterator; $sum += $it->() for 1..100000;
  MPU array:      $sum += $_ for @{primes(nth_prime(100_000))};
  MPUPA:          tie my @primes, ...; $sum += $primes[$_] for 0..99999;
  MNSP:           my $seq = Math::NumSeq::Primes->new;
                  $sum += ($seq->next)[1] for 1..100000;
  MPTA:           tie my @primes, ...; $sum += $primes[$_] for 0..99999;

Memory use is comparing the delta between just loading the module and running the test. Perl 5.19.2, Math::NumSeq v61, Math::Prime::TiedArray v0.04.

Summing the first 0.1M primes via walking the array:

       7ms     52k    Math::Prime::Util      forprimes
     140ms      0     Math::Prime::Util      prime_iterator
      12ms   4400k    Math::Prime::Util      sum big array
     220ms    840k    Math::Prime::Util::PrimeArray
     130ms    280k    Math::NumSeq::Primes   sequence iterator
    7560ms   65 MB    Math::Prime::TiedArray (extend 1k)

Summing the first 1M primes via walking the array:

      0.1s    300k    Math::Prime::Util      forprimes
      1.8s      0     Math::Prime::Util      prime_iterator
      0.2s   40 MB    Math::Prime::Util      sum big array
      1.9s   1.1MB    Math::Prime::Util::PrimeArray
      7.5s   1.2MB    Math::NumSeq::Primes   sequence iterator
    110.5s  785 MB    Math::Prime::TiedArray (extend 1k)

Summing the first 10M primes via walking the array:

      0.8s   5.9MB    Math::Prime::Util      forprimes
     22.4s      0     Math::Prime::Util      prime_iterator
      1.5s  368 MB    Math::Prime::Util      sum big array
     19.1s   1.2MB    Math::Prime::Util::PrimeArray
   3680  s  11.1MB    Math::NumSeq::Primes   sequence iterator
          >5000 MB    Math::Primes::TiedArray (extend 1k)

Math::Prime::Util offers three obvious solutions: a big array, an iterator, and the forprimes construct. The big array is fast but uses a lot of memory, forcing the user to start programming segments. Using the iterator avoids all the memory use, but isn't as fast (this may improve in a later release, as this is a new feature). The forprimes construct is by far the fastest, but it isn't quite as flexible as the iterator (most notably there is no way to exit early, and it doesn't lend itself to wrapping inside a filter).

Math::NumSeq::Primes offers an iterator alternative, and works quite well for reasonably small numbers. It does not support random access. It is very fast for small values, but is very slow with large counts.

Math::Primes::TiedArray is remarkably impractical for anything other than very small numbers.

SEE ALSO

This module uses Math::Prime::Util to do all the work. If you're doing anything but retrieving primes, you should examine that module to see if it has functionality you can use directly, as it may be a lot faster or easier.

Similar functionality can be had from Math::NumSeq and Math::Prime::TiedArray.

AUTHORS

Dana Jacobsen <dana@acm.org>

COPYRIGHT

Copyright 2012-2013 by Dana Jacobsen <dana@acm.org>

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

cpanm Math::Prime::Util

perl -MCPAN -e shell
install Math::Prime::Util