Hash sorting (by value) woes

sschneid has asked for the wisdom of the Perl Monks concerning the following question:

Hi again, monks!

I'm having a heck of a time trying to sort a hash. Here are the details:

First off, for reasons uncontrollable by me, I'm being provided a hash containing the contents of /etc/passwd in the following format:

$etcpw = {
    'blah' => 'blah:x:3:1:blah:/home/blah:/sbin/nologin',
    'tra'  => 'tra:x:1:1:tra:/home/tra:/sbin/nologin',
    'la'   => 'la:x:2:1:la:/home/la:/sbin/nologin'
};
[download]

My objective is to sort it based on UID, but the following (my best attempt) doesn't quite seem to work:

map {
    print "$etcpw->{$_}\n";
} sort { ($etcpw->{$a} =~ /:(\d+)/) <=> ($etcpw->{$b} =~ /:(\d+)/) } k
+eys %{$etcpw};
[download]

I imagine it's probably pretty obvious, but I'm banging my head against the monitor trying to figure it out. Can someone tell me where I'm going wrong?

Thanks,
-s.

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Re: Hash sorting (by value) woes by dragonchild (Archbishop) on Feb 24, 2005 at 16:59 UTC
That's going to evaluate each side of the spaceship operator (`<=>`) to 1 and make no changes. You really want something like: `map { print "$etcpw->{$_}\n"; } sort { my ($first) = ($etcpw->{$a} =~ /:(\d+)/); my ($second) = ($etcpw->{$b} =~ /:(\d+)/); $first <=> $second } keys %{$etcpw};` [download] Being right, does not endow the right to be rude; politeness costs nothing. Being unknowing, is not the same as being stupid. Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence. Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.	[reply] [d/l] [select]
Re^2: Hash sorting (by value) woes by sschneid (Deacon) on Feb 24, 2005 at 17:05 UTC
That makes sense. Thanks. -s.	[reply]
Re^3: Hash sorting (by value) woes by ikegami (Patriarch) on Feb 24, 2005 at 17:14 UTC
Here's an optmization that only executes the regexp once per string: `@sorted_keys = map { $_->[0] } sort { $a->[1] <=> $b->[1] } map { $etcpw->{$_} =~ /:(\d+)/; [ $_, $1 ] } keys %{$etcpw}; print "$etcpw->{$_}\n" foreach @sorted_keys;` [download] We can even drop a step: `print "$etcpw->{$_->[0]}\n" foreach sort { $a->[1] <=> $b->[1] } map { $etcpw->{$_} =~ /:(\d+)/; [ $_, $1 ] } keys %{$etcpw};` [download]	[reply] [d/l] [select]
Re^4: Hash sorting (by value) woes by Roy Johnson (Monsignor) on Feb 24, 2005 at 17:33 UTC
Re^5: Hash sorting (by value) woes by ikegami (Patriarch) on Feb 24, 2005 at 17:42 UTC
Some notes below your chosen depth have not been shown here
Re: Hash sorting (by value) woes by jonadab (Parson) on Feb 24, 2005 at 17:14 UTC
This is a good situation for using a Schwartzian Transform. The code you have is close to right, but it's confusing and hard to follow a bit, partly because of all that logic in the sort block that isn't really part of the sorting. I'd move the regex matching out to a preliminary map, put just the sorting code in the sort block, and then map back to the part you want in the upper map... `my @sortedkeys = map { $$_[0] } sort { $$a[1] <=> $$b[1] } map { $$etcpw{$_} =~ /[:](\d+)/; [$_, $1] } keys %$etcpw;` [download] Much easier to follow, if you ask me. As far as why your original code doesn't work, I think it's because pattern matches don't do the same thing in scalar context as they do in array context. "In adjectives, with the addition of inflectional endings, a changeable long vowel (Qamets or Tsere) in an open, propretonic syllable will reduce to Vocal Shewa. This type of change occurs when the open, pretonic syllable of the masculine singular adjective becomes propretonic with the addition of inflectional endings." — Pratico & Van Pelt, BBHG, p68	[reply] [d/l]
Re: Hash sorting (by value) woes by RazorbladeBidet (Friar) on Feb 24, 2005 at 17:03 UTC
Another way (probably a little slower): `map { print "$etcpw->{$_}\n"; } sort { [ split /:/, $etcpw->{$a} ]->[2] <=> [ split /:/,$etcpw->{$b} + ]->[2] } keys %{$etcpw};` [download] -------------- It's sad that a family can be torn apart by such a such a simple thing as a pack of wild dogs	[reply] [d/l]
Re^2: Hash sorting (by value) woes by dragonchild (Archbishop) on Feb 24, 2005 at 17:07 UTC
`[ split /:/, $etcpw->{$a} ]->[2]` [download] can be rewritten as `(split /:/, $etcpw->{$a})[2]` [download] Being right, does not endow the right to be rude; politeness costs nothing. Being unknowing, is not the same as being stupid. Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence. Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.	[reply] [d/l] [select]