Perl: Sort values in multidimensional hash - perl

Question

I have a subroutine that looks likes this:
...
sub UserLogins {
my %loginData;
my #logins = qx(last) or die;
foreach my $row (#logins) {
if ( $row=~ /^(\w+)\s+/ and (("$1" ne "reboot") and ("$1" ne "wtmp")) ) {
$loginData{$1}{"logins"}++;
}
}
return \%loginData
}
...
Using this subroutine in main script, I get following output:
...
$VAR1 = {
'user1' => {
'oldpassword' = 0,
'filesize' => '14360',
'logins' => 1
},
'user2' => {
'oldpassword' = 0,
'filesize' => '1220',
'logins' => 15
},
'user3' => {
'oldpassword' = 1,
'filesize' => '1780',
'logins' => 7
}
}
...
I wonder how I should sort my %loginData hash so user with largest number of logins gets printed first (in this case user2, user3, user1).
I have also tried to sort values in this way:
foreach my $test_sort (sort {$a <=> $b} values %loginData) {
say $test_sort;
}
But this function doesn't work at all.
Another thing I tried and didn't work:
print "$_\n" foreach sort {$loginData{$b}->{logins} <=> $loginData{$a}->{logins}} keys %loginData;
Update
This function actually works, but shows errors massages:
print "$_\n" foreach sort {$userData{$b}->{'logins'} <=> $userData{$a}->{'logins'}} keys %userData;
Errors:
Use of uninitialized values in numeric comparison (<=>)

Answer 1

Here's a canonical solution for accessing elements of a hash in some order based on an attribute of the value (in this case logins):
for (sort { $loginData{$b}->{logins} <=> $loginData{$a}->{logins} } keys %loginData)
{
...
}
Note reversal of $b and $a to achieve a reverse sort (most logins first).

Answer 2

You can't sort a hash. From the Perl docs:
Hash entries are returned in an apparently random order.
You can, compute a list of keys that index the hash in the order you like.
Something like this:
use Data::Dumper;
our $hash = {
'user1' => {
'oldpassword' => 0,
'filesize' => '14360',
'logins' => 1
},
'user2' => {
'oldpassword' => 0,
'filesize' => '1220',
'logins' => 15
},
'user3' => {
'oldpassword' => 1,
'filesize' => '1780',
'logins' => 7
}
};
sub KeysByLogins {
my $hash = shift;
map { $_->[1] }
sort { $a->[0] <=> $b->[0] }
map { [ $hash->{$_}->{logins}, $_ ] } keys %$hash;
}
foreach my $key (KeysByLogins($hash)) {
print Data::Dumper->Dump([$hash->{$key}], [$key]) . "\n";
}
Then...
$ perl foo.pl
$user1 = {
'filesize' => '14360',
'oldpassword' => 0,
'logins' => 1
};
$user3 = {
'logins' => 7,
'oldpassword' => 1,
'filesize' => '1780'
};
$user2 = {
'oldpassword' => 0,
'filesize' => '1220',
'logins' => 15
};