リスト（配列等）の内容を元に別の内容に変換したリストが欲しい時map関数（メソッド）が使える。
まずは簡単なサンプルを
Perlでは
my @a = (1,2,3,4,5); my @b = map ++$_, @a;
これで変数@bの内容は(2,3,4,5,6)になってる。
Rubyでは
a = [1,2,3,4,5]
b = a.map {|i| i + 1}
Pythonでは
a = [1,2,3,4,5] b = map(lambda i: i + 1, a)
PHPでは
$a = array(1,2,3,4,5);
$b = array_map(create_function('$i', 'return ++$i;'), $a);
いずれも変数bは2から6までの整数のリストになっている。
次はもうちょっと複雑な例
あるディレクトリの中のすべてのファイルのファイル名とファイルサイズを格納したリスト（隠しファイルは除く）を出力したいとする。
構造的にはこんな感じ
[['ファイル名', 'ファイルサイズ'].....]
Perl:
#!/usr/bin/env perl
BEGIN {
use File::Basename;
chdir dirname $0;
}
use strict;
use warnings;
use IO::Dir;
my $dir = IO::Dir->new(".");
my @ fileAndSize = map [$_, -s $_], grep /^[^.]/, $dir->read();
print "$_->[0] => $_->[1]byte\n" for (@ fileAndSize);
$dir->close();
Ruby:
#!/usr/bin/env ruby
Dir.chdir File.dirname(ENV["_"])
Dir.open(".") do |dir|
fileAndSize = dir.select {|i| i =~ /^[^.]/}.map {|i| [i, FileTest.size(i)]}
fileAndSize.each {|l| puts "#{l[0]} => #{l[1]}byte"}
end
Python:
#!/usr/bin/env python
import os, re
os.chdir(os.path.dirname(os.environ['_']))
fileAndSize = map(lambda i: [i, int(os.path.getsize(i))], filter(lambda i: re.match('^[^.]', i), os.listdir(".")))
for i in fileAndSize: print "%s => %sbyte" % (i[0], i[1])
PHP:
#!/usr/bin/env php
<?php
chdir(dirname($_ENV[_]));
$files = glob("*");
$fileAndSize = array_map(create_function('$file', 'return array($file, filesize($file));'), $files);
foreach ($fileAndSize as $i) echo "$i[0] => $i[1]byte\n";
?>
言語によってはディレクトリをオープンしてたりglobを使ってたりするがすべて同じ出力になるはず。
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