Method #1: If there is no correlation between one record and the next, then reading from a random position and taking the next record after that should be fine.

Method #2: If there are important correlations between one record and the next, then one way of dealing with that would be to reorder the entire file in random order. For instance, read the file once in order to count the number of records, N, and while you're at it, generate an array that has the offset to each record. Generate a random permutation of the integers from 1 to N. Read back through the file and pull out the records in that order, writing them to a new copy of the file. Now just use method #1 on the randomized version of the file.

Is the file static, or is it changing a lot? If it's static, then method #2 should be fine. If it's changing all the time, and there are also correlations between successive records, then this becomes a more difficult problem. I think there are probably various ways to do it, but I suspect they all involve reinventing the wheel. Either you're going to reinvent filesystem-level support for random access to a file with varying record lengths, or you're going to reinvent a relational database. My suggestion would be to switch to a relational database. If that's not an option, and you really need to roll your own solution, then the optimal solution may depend on other details, e.g., do the changes to the file just involve steadily appending to it?

In reply to Re^3: Random sampling a variable length file. by bcrowell2
in thread Random sampling a variable record-length file. by BrowserUk

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.