in reply to Hash in Perl

Create a second hash, with countries as the keys; the hash value for each country will be an anonymous array containing the cities belonging to that country:

#! perl
use strict;
use warnings;

my %h1 = 
(
    'Chicago, USA'       => undef,
    'Frankfurt, Germany' => undef,
    'Berlin, Germany'    => undef,
    'Washington, USA'    => undef,
    'Helsinki, Finland'  => undef,
    'New York, USA'      => undef,
);

my %h2;

for (keys %h1)
{
    my  ($city, $country) = split ', ';
    push @{ $h2{$country} }, $city;
}

for (sort keys %h2)
{
    print "$_: ", join(', ',  sort { $a cmp $b } @{ $h2{$_} }), "\n";
}

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Output:

16:29 >perl 819_SoPW.pl
Finland: Helsinki
Germany: Berlin, Frankfurt
USA: Chicago, New York, Washington

16:31 >

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Note: Perl autovivifies the new $h2{$country} elements as needed.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

Replies are listed 'Best First'.
Re^2: Hash in Perl
by rammohan (Acolyte) on Jan 02, 2014 at 06:41 UTC
    why you again declare undef function.Actually we declare all values in hash then we need again that undef.?
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      ... we declare all values in hash ...

      Declare them as what? As Anonymous Monk pointed out above, the data you show is not a hash, it’s a list (of strings). Since you said this data is in a hash, I made a guess as to how the hash was contstructed. A hash is a collection of key/value pairs. Every key must have a value (and every value must belong to a key). I assumed that the data was stored as hash keys, so I supplied an undef value for each — but that could have been any value, as it isn’t used.

      If your hash is really key/value pairs of the form: Chicago => 'USA' (another guess), adapting the answer I gave above should be quite straightforward.

      Hope that helps,

      Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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        Thank you well explanation. 
        my%hash = (Script => "Perl",
              script_1=>"Ruby",
              script_2=> "PHP");

 for my$k (keys %{my$href}) {
          print "$k => ${$href}{$k}\n";
        }

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        it showing Global symbol $href requires explicit package name.why this error.?
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