(2) hash access slows disproportionately whereas this lookup could just be an array. That array could be packed down to one bit per element for the most efficient storage, given that it has only boolean values. Lookup (and registration) can be achieved without unpacking from that using bit operators such as '<<' and '&', thereby winning on processing as well as storage.

What you describe is a primes sieve. One common prime generation technique is the Sieve of Eratosthenes. Another (sometimes slightly faster, but always more complicated) is the Sieve of Atkin.

I believe danaj has a blazing fast segmented sieve in Math::Prime::Util, and I have a pretty basic non-segmented Sieve of Eratosthenes implementation in Math::Prime::FastSieve.


Dave

In reply to Re^2: Unable to Understand grep and hash use for prime no. by davido
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":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.