Sorting an Array Reference to Hashes - perl

Question

After executing these lines in Perl:
my $data = `curl '$url'`;
my $pets = XMLin($data)->(pets);
I have an array reference that contains references to hashes:
$VAR1 = [
{
'title' => 'cat',
'count' => '210'
},
{
'title' => 'dog',
'count' => '210'
}
]
In Perl, how do I sort the hashes first by count and secondarily by title. Then print to STDOUT the count followed by the title on each newline.

Answer 1

Assuming you want counts in descending order and titles ascending:
print map join(" ", #$_{qw/ count title /}) . "\n",
sort { $b->{count} <=> $a->{count}
||
$a->{title} cmp $b->{title} }
#$pets;
That's compact code written in a functional style. To help understand it, let's look at equivalent code in a more familiar, imperative style.
Perl's sort operator takes an optional SUBNAME parameter that allows you to factor out your comparison and give it a name that describes what it does. When I do this, I like to begin the sub's name with by_ to make sort by_... ready more naturally.
To start, you might have written
sub by_count_then_title {
$b->{count} <=> $a->{count}
||
$a->{title} cmp $b->{title}
}
my #sorted = sort by_count_then_title #$pets;
Note that no comma follows the SUBNAME in this form!
To address another commenter's question, you could use or rather than || in by_count_then_title if you find it more readable. Both <=> and cmp have higher precedence (which you might think of as binding more tightly) than || and or, so it's strictly a matter of style.
To print the sorted array, a more familiar choice might be
foreach my $p (#sorted) {
print "$p->{count} $p->{title}\n";
}
Perl uses $_ if you don't specify the variable that gets each value, so the following has the same meaning:
for (#sorted) {
print "$_->{count} $_->{title}\n";
}
The for and foreach keywords are synonyms, but I find that the uses above, i.e., foreach if I'm going to name a variable or for otherwise, read most naturally.
Using map, a close cousin of foreach, instead isn't much different:
map print("$_->{count} $_->{title}\n"), #sorted;
You could also promote print through the map:
print map "$_->{count} $_->{title}\n",
#sorted;
Finally, to avoid repetition of $_->{...}, the hash slice #$_{"count", "title"} gives us the values associated with count and title in the loop's current record. Having the values, we need to join them with a single space and append a newline to the result, so
print map join(" ", #$_{qw/ count title /}) . "\n",
#sorted;
Remember that qw// is shorthand for writing a list of strings. As this example shows, read a map expression back-to-front (or bottom-to-top the way I indented it): first sort the records, then format them, then print them.
You could eliminate the temporary #sorted but call the named comparison:
print map join(" ", #$_{qw/ count title /}) . "\n",
sort by_count_then_title
#$pets;
If the application of join is just too verbose for your taste, then
print map "#$_{qw/ count title /}\n",
sort by_count_then_title
#$pets;