in reply to What is "Schwarzian Transform" (aka Schwartzian)

I think it is called "Schwartzian Transform". That's why you couldn't find it ;-). Basically it's a sorting technique for arrays of multiple fields. With it you can sort them according to any fields you prefer. Suppose you have this data: 
-r--r--r--   1 yourname       8318 Jan 30  1996 file1.txt
-r--r--r--   1 yourname      11986 Jan 30  1996 file2.txt
-r--r--r--   1 yourname      46852 Feb 27  1996 file3.txt
-r--r--r--   1 yourname      72698 Feb 27  1996 file4.txt

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And you want to sort them according to size. You would do it with ST like this: 
@sorted_by_size = 
  map { $_->[0] } 
  sort { $a->[1] <=> $b->[1] } 
  map { [$_, -s] } 
  @files;

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So in principle you would do ST in the following step: 
1. Map the initial list into a list of ref to lists with the original 
+and modified values 
2. Sort the list of references 
3. Map the list of ref back into a plain list with the initial values

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Check this out by the very creator himself - Randal Schwartz (merlyn).
Regards,
Edward

Replies are listed 'Best First'.
Re^2: What is "Schwartzian Transform"
by monarch (Priest) on Jul 21, 2005 at 07:51 UTC
    How is the sort named after one of the many perlmonks here any more advantageous than a simple:
    use strict;

my @data = (
['-r--r--r--','1','yourname','8318','2000-01-01','ant.txt'],
['-r--r--r--','1','yourname','11986','1992-12-30','tiger.txt'],
['-r--r--r--','1','yourname','72698','2004-03-03','duck.txt'],
['-r--r--r--','1','yourname','46852','1788-01-26','goose.txt']
);

print( "Which column to sort on?" );
my $column = ( <STDIN> =~ m/^\d+$/ );
exit( 1 ) if ( ! defined($column) );

my @sorted = sort { $a->[$column] cmp
                    $b->[$column] } @data;

print( join( "\n", map { join( ",", @{$_} )
                         } @sorted ) );

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    IE, just select the column (or hashref) you want to sort on.

    update: I very much like kelan's answer to this post, it indeed shows one area where this other process comes in useful.

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[d/l]

      The usual use is when the sort key must be derived from the data using a process that takes significant time/resources. The idea of the ST is to precalculate all of the sort keys before the sort operation so you only spend the time once: 

      @sorted =
  map  { $_->[ 1 ] }
  sort { $a->[ 0 ] <=> $b->[ 0 ] }
  map  { [ expensivefunc( $_ ), $_ ] } @data;

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      Doing a naive sort, you would be calling that expensive function twice for every comparison, which would end up being a lot more than when precalculating: 
      @sorted = sort { expensivefunc( $a ) <=> expensivefunc( $b ) } @data;

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[d/l]
[select]
        In all recent examples, I've moved the "original key" to tuple element 0, so that the first step is consistently  map $_->[0] regardless of how many "expensive" keys are computed.

        Just a minor nit.

        -- Randal L. Schwartz, Perl hacker
        Be sure to read my standard disclaimer if this is a reply.

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[d/l]
Re^2: What is "Schwartzian Transform"
by GrandFather (Saint) on Jul 21, 2005 at 04:06 UTC

    In that case I won't mention whose node I copied the text from that I pasted into Wikipedia and into the title of my node. :)

    Perl is Huffman encoded by design.
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      No reason to perpetuate the error by leaving it uncorrected...

      Update: s/Schwarzian/Schwartzian/

      the lowliest monk

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        The good Zaxo has admited to his "finger" problem and corrected the error. See his node below

        Perl is Huffman encoded by design.
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