Re^2: What is "Schwartzian Transform"

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 ) );
[download]

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.

Comment on Re^2: What is "Schwartzian Transform" Download Code

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Re^3: What is "Schwartzian Transform" by kelan (Deacon) on Jul 21, 2005 at 12:28 UTC
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;` [download] 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;` [download]	[reply] [d/l] [select]
Re^4: What is "Schwartzian Transform" by merlyn (Sage) on Jul 21, 2005 at 15:56 UTC
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.	[reply] [d/l]


"be consistent"
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