So would preallocating buckets be helpful or hurtful here

If you know the final size, preallocating to that size will certainly do no harm and speed things a little.

Hashes get 8 buckets to start, then double in size each time as they fill. When the doubling occurs, the key/value pairs have to be copied from the old to the new hash which is a relatively expensive operation. Moving stright to the final number of buckets avoids that.

Of course, even if you pre-sized the hash to 2^18(262144) buckets, there's no guarantee that will be the final size. It could be that hash collisions at that size will prompt another doubling dependant upon the keys.

You could go straight to the next size up directly to be safe, but then you are potentially wasting memory. Which if the ultimate criteria is speed may be acceptable.

But without knowing the OPs actual criteria there's no way to know for sure.

If runtime performance is his only goal, using an array rather than a hash would be twice as fast. And despite being sparse, would actually use less memory.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

The start of some sanity?

In reply to Re^4: Populating a hash by BrowserUk
in thread Populating a hash by Anonymous Monk

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.