sorting a hash by its values, date

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

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Re: sorting a hash by its values, date by ikegami (Patriarch) on Aug 16, 2004 at 23:31 UTC
sort's second argument is a list. %H, as a list, would look like `('key2', 'value2', 'key1', 'value1', 'key3', 'value3')`. That means that by_date would be provided not just with the values, but with the keys as well. Here's a solution: `sub by_date { my ($monA, $monB, $dayA, $dayB, $yearA, $yearB); ($monA, $dayA, $yearA) = split(/\//, $a->[1]); # <-- slight change ($monB, $dayB, $yearB) = split(/\//, $b->[1]); # <-- slight change $yearA <=> $yearB or $monA <=> $monB or $dayA <=> $dayB } @H = map { [ $_, $H{$_} ] } keys(%H); foreach $k (sort by_date @H)) { # $k->[0] is the key. # $k->[1] is the value. ... }` [download]	[reply] [d/l] [select]
Re: sorting a hash by its values, date by Joost (Canon) on Aug 16, 2004 at 23:34 UTC
Well, first of all you can't really sort a hash. A hash is always unsorted. Also, you're sorting the hash keys and values together, which is probably not what you want. If you want to process the hash values in a sorted order, do this: `foreach $value (sort by_date values %H) { # ... }` [download] If you need the keys, sorted by their corresponding values: `foreach $key (sort { by_date($H{$a},$H{$b}) } keys %H) { }` [download] If it still doesn't work, try showing some of the input and expected output of your code. By the way, code can be much easier formatted using `<code>` tags. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: sorting a hash by its values, date by aquarium (Curate) on Aug 17, 2004 at 03:05 UTC
i gather you should have the problem worked out by now, to sort the values of the hash. To make it much simpler to work with the dates in the hash, you could store YYYYMMDD format instead and treat as a number (not a string). Then, sorting and comparisons will be much simpler. Convert to MM/DD/YYYY only when needed, for display etc.	[reply]
Re: sorting a hash by its values, date by Eimi Metamorphoumai (Deacon) on Aug 16, 2004 at 23:25 UTC
The problem is that you can't sort a hash, you sort an array of, for instance, the keys of the hash. So replacing `sort by_date %H` with `sort by_date keys %H` should work.	[reply]
Re^2: sorting a hash by its values, date by levbao (Initiate) on Aug 16, 2004 at 23:48 UTC
Thank you so much for your answer. But the date is the values of the hash, not the keys of the hash.	[reply]
Re: sorting a hash by its values, date by Random_Walk (Prior) on Aug 17, 2004 at 08:56 UTC
I have stolen ikegama's idea of using map along with aquarium's idea of reordering the date so it sorts easily and come up with the following little snippet. The date is re-ordered in the map block and munged together with the key to make it easy to sort the entire thing as an array. once sorted it is easy to reconstruct the data as you like. This does depend on the day, date and year being 2,2,4 digits, if they are not then a little more needs adding in the map block to fix this cheers. `#!/usr/local/bin/perl -w use strict; my %h = ( key1 => "10/14/2003", key2 => "11/23/2001", key3 => "12/23/2001", key4 => "12/22/2001", key5 => "02/21/1984", key6 => "08/13/1969", key7 => "09/11/1973", key8 => "09/30/2000" ); my @bydate = map { my @tmp = split /\//, $h{$_}; $tmp[2].":".$tmp[0].":".$tmp[1].":".$_ } keys(%h); foreach (sort @bydate) { my ($yr, $mn, $dy, $key) = split /:/; print "$mn/$dy/$yr = $key\n"; }` [download] update I have been playing with pack/unpack and thought this was a nice place to use it. So here is a more efficient version `#!/usr/local/bin/perl -w use strict; my %h = ( key1 => "10/14/2003", key2 => "11/23/2001", key3 => "12/23/2001", key4 => "12/22/2001", key5 => "02/21/1984", key6 => "08/13/1969", key7 => "09/11/1973", key8 => "09/30/2000" ); my @bydate = map { my @tmp = split /\//, $h{$_}; pack 'A4A2A2A', $tmp[2], $tmp[0], $tmp[1], $_; } keys(%h); foreach (sort @bydate) { print "@{[unpack ('A4A2A2A',$_)]}\n"; }` [download]	[reply] [d/l] [select]

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