how to remove duplicate values and create a new perl hash? - perl

Question

my %hash1 = (
a=>192.168.0.1,
b=>192.168.0.1,
c=>192.168.2.2,
d=>192.168.2.3,
e=>192.168.3.4,
f=>192.168.3.4
);
i have a perl hash like given above. keys are device names and values are ip addresses.How do i create a hash with no duplicate ip addresses (like %hash2) using %hash1? (devices that have same ips are removed)
my %hash2 = ( c=>192.168.2.2, d=>192.168.2.3 );

Answer 1

First of all you need to quote your IP addresses, because 192.168.0.1 is V-String in perl, means chr(192).chr(168).chr(0).chr(1).
And my variant is:
my %t;
$t{$_}++ for values %hash1; #count values
my #keys = grep
{ $t{ $hash1{ $_ } } == 1 }
keys %hash1; #find keys for slice
my %hash2;
#hash2{ #keys } = #hash1{ #keys }; #hash slice

Answer 2

How about:
my %hash1 = (
a=>'192.168.0.1',
b=>'192.168.0.1',
c=>'192.168.2.2',
d=>'192.168.2.3',
e=>'192.168.3.4',
f=>'192.168.3.4',
);
my (%seen, %out);
while( my ($k,$v) = each %hash1) {
if ($seen{$v}) {
delete $out{$seen{$v}};
} else {
$seen{$v} = $k;
$out{$k} = $v;
}
}
say Dumper\%out;
output:
$VAR1 = {
'c' => '192.168.2.2',
'd' => '192.168.2.3'
};

Answer 3

A solution using the CPAN module List::Pairwise:
use strict;
use warnings;
use List::Pairwise qw( grep_pairwise );
use Data::Dumper;
my %hash1 = (
a => '192.168.0.1',
b => '192.168.0.1',
c => '192.168.2.2',
d => '192.168.2.3',
e => '192.168.3.4',
f => '192.168.3.4'
);
my %count;
for my $ip ( values %hash1 ) { $count{ $ip }++ }
my %hash2 = grep_pairwise { $count{ $b } == 1 ? ( $a => $b ) : () } %hash1;
print Dumper \%hash2;
It's pretty straightforward. First you count the IPs in an auxiliary hash. And then you select only those IPs with a count of one using grep_pairwise from List::Pairwise. The syntax of grep_pairwise is like grep:
my #result = grep_pairwise { ... } #list;
The idea of grep_pairwise is to select the elements of #list two by two, with $a representing the first element of the pair, and $b the second (in this case the IP). (Remember that a hash evaluates to a list of ($key1, $value1, $key2, $value2, ...) pairs in list context).