Perl: Sort List, Matrix, Object

By Xah Lee. Date: . Last updated: .

This page shows you how to sort in Perl

here's a example of sort (Perl 5.14):

#-*- coding: utf-8 -*-
# perl

# sort a list

@li = (1,9,2,3);

@li2 = sort {$a <=> $b} @li; # original list is not changed

print join(' ', @li2); # 1 2 3 9

In Perl, sort is a function. It returns the sorted result as another list.

“sort” takes the form sort {…} @myList. Inside the enclosing braces is the body of the ordering function, where variables $a and $b inside are predefined by the language to represent two elements in the list. The operator <=> returns -1 if left operand is less than the right operand. If equal, it returns 0, else 1. It is equivalent to Python's “cmp” function.

Another form of sort is sort orderFunctionName @list, which uses a function name in place of the comparison block. The function should have 2 parameters, and return one of {-1, 0, 1}.

Compare as Number or as String

Perl has 2 comparison operators.

Example:

# -*- coding: utf-8 -*-
# perl

print "3" <=> "20"; # prints -1

print "\n";

print "3" cmp "20"; # prints 1

In Perl, numbers and strings are mutually automatically converted if needed.

Sort Matrix

# -*- coding: utf-8 -*-
# perl

# sort a matrix

use Data::Dumper;
use strict;

my @li1 = ([2,6],[1,3],[5,4]);

my @li2 = sort { $a->[1] <=> $b->[1] } @li1;

print Dumper(\@li2);            #  [[1, 3], [5, 4], [2, 6]]

The ([2,6],[1,3],[5,4]) is the syntax for nested list. The square brackets inside creates array references.

The $a->[1] is the syntax to get the element of a array reference.

The \@li2 in Dumper(\@li2) gets the reference to the array @li2.

Reverse Sort Order

To reverse sort order, all you have to do is to reverse the placement of $a and $b in your comparison. Example: sort {$b <=> $a} @li

Or, you can use the reverse function afterward, if you don't mind doing extra computation.

# -*- coding: utf-8 -*-
# perl

use Data::Dumper;

@aa = (3, 4, 5);

@bb = reverse(@aa);

print Dumper(\@bb);

Sort Complex Objects

Here's a more complex example of sort.

Suppose you have a list of strings.

'my283.jpg'
'my23i.jpg'
'web7-s.jpg'
'fris88large.jpg'
…

You want to sort them by the number embedded in them.

You need to define a ordering function, and pass it to sort. The function should takes two strings, and compare the integer inside the string. Here's the solution:

#-*- coding: utf-8 -*-
#  perl

@li = (
'my283.jpg',
'my23i.jpg',
'web7-s.jpg',
'fris88large.jpg',
);

# sorts a list of strings by their embedded number

@li2 = sort { ($a =~ m/(\d+)/)[0] <=> ($b =~ m/(\d+)/)[0]} @li;

print join(' ', @li2);  # prints web7-s.jpg my23i.jpg fris88large.jpg my283.jpg

decorate-sort-dedecorate, Schwartzian transform

Normally, the key for comparison is computed 2 or more times for each element.

Here's a more efficient way, called decorate-sort-dedecorate (aka Schwartzian transform).

# -*- coding: utf-8 -*-
# perl

# sort a array of string, by comparing the number part inside the string

@li = ('my283.jpg','my23i.jpg','web7-s.jpg','fris88large.jpg');

# this is “decorate-sort-dedecorate”, aka Schwartzian transform
@li2 = map { $_->[0] } sort { $a->[1] <=> $b->[1] } map { [ $_, ($_=~m/(\d+)/)[0] ] } @li;
#          ↑ take item               ↑ sort            ↑ create list of pairs [item,key]

use Data::Dumper;
print Dumper(\@li2); # ('web7-s.jpg', 'my23i.jpg', 'fris88large.jpg', 'my283.jpg')

In the above Perl code:

In this way, the cost to compute the same key multiple times is avoided. This method is good when computing the key is expensive.

perldoc -f sort

General Function to Sort Matrix

Perl: Sort Matrix