The Schwartzian Transform is a technique to optimize the sorting of complex data structures in Perl.

The simplest case would be to do a single line sort: 

my @newest_first = sort { -M $a <=> -M $b } </var/script/proc/*>;

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However, to sort, one must compare pairs of items, often comparing the same $a to many different $bs, and vice versa. All those -M checks take a lot of time.

Read a Schwartzian Transform like this starting with the last line.

  • <.../*> is a shorthand call to glob() that returns a list of files.
  • The map above that takes each filename and pairs it to its own -M check results. So you get a list of "foo.blah", timestamp pairs. The square brackets keep each pair in their own little referenced array.
  • The sort line sorts these pairs numerically descending by their second (index 1) element, the timestamps.
  • The map at the top will turn the sorted list of pairs into a list of filenames again, keeping the sorted order.

    --
    [ e d @ h a l l e y . c c ]

    In reply to Re: Re: &bull;Re: Keeping only the $n newest files/directories in a diretory? by halley
    in thread Keeping only the $n newest files/directories in a diretory? by Anonymous Monk

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