Beefy Boxes and Bandwidth Generously Provided by pair Networks
laziness, impatience, and hubris
 
PerlMonks  

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
That's why Davido's module uses XS and C++. An optimised Perl implementation would have a challenge to re-invent the processor in some ways, whereas in C++, structs can be used to map one datatype over another without any conversion, bit-shifting or reassignment making it easier to simulate a 2^N bit machine, where N is arbitrary.

I think you've missed the point. You said it would be more space efficient to use a bit array vs. a hash. That's right if your inputs are small, as a bit array (in Perl or C) takes only only N bits (N being the max entry), vs. a hash which in Perl is quite large per entry. Storing the result of every prime below 10M takes about 67MB as a hash, while only 1.5MB as a bit array (could be even less with some optimizations). But if I store the result of 10k random 32-bit numbers, then the hash wins by a huge amount.

My example was 1e18. This one entry now takes 1e18 bits to store just the single result. You can't do that, whether it is C or C++ or Perl. You need a different way to think about it. Your first solution is also a non-starter, as you certainly don't want to generate or store the 2.47e16 primes under that value. Unfortunately Math::Prime::FastSieve doesn't work at these large sizes, while Math::Prime::XS and Math::Prime::Util (ntheory) do.

Primality is not the same problem as prime generation.


In reply to Re^4: Unable to Understand grep and hash use for prime no. by danaj
in thread Unable to Understand grep and hash use for prime no. by shankonit

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Are you posting in the right place? Check out Where do I post X? to know for sure.
  • Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
    <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>
  • Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
  • Want more info? How to link or or How to display code and escape characters are good places to start.
Log In?
Username:
Password:

What's my password?
Create A New User
Domain Nodelet?
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others lurking in the Monastery: (1)
As of 2022-01-17 02:29 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?
    In 2022, my preferred method to securely store passwords is:












    Results (50 votes). Check out past polls.

    Notices?