Beefy Boxes and Bandwidth Generously Provided by pair Networks
go ahead... be a heretic
 
PerlMonks  

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
Finding out if a number is prime has the typical algorithm of testing all integers less than the square root of the number for whether they are a factor (and of course eliminating a perfect square). The author is trying to trade off storage for execution time by saving previous calculations as a hash (key = number, value = boolean for whether prime). I have some thoughts:

(1) only prime factors need to be tested as factors and so test factors (all of them used per test) should also have their results kept in the lookup, so the lookup storage belongs to the function 'is_prime' (could be a package variable) rather than held at the scope shown in the snippet.

(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.

One world, one people


In reply to Re: Unable to Understand grep and hash use for prime no. by anonymized user 468275
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 musing on the Monastery: (4)
As of 2022-01-20 12:19 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?
    In 2022, my preferred method to securely store passwords is:












    Results (56 votes). Check out past polls.

    Notices?