Re: What is "Schwartzian Transform"

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
[download]

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;
[download]

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
[download]

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

Regards,
Edward

Comment on Re: What is "Schwartzian Transform" Select or Download Code

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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 ) ); [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.	[reply] [d/l]
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]
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.	[reply]
Re^3: What is "Schwartzian Transform" by tlm (Prior) on Jul 21, 2005 at 04:29 UTC
No reason to perpetuate the error by leaving it uncorrected... Update: `s/Schwarzian/Schwartzian/` the lowliest monk	[reply]
Re^4: What is "Schwartzian Transform" by GrandFather (Saint) on Jul 21, 2005 at 04:35 UTC
The good Zaxo has admited to his "finger" problem and corrected the error. See his node below Perl is Huffman encoded by design.	[reply]


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