How to fetch key of a hash through values in Hash of Arrays

Question

Below is the small code snippet i have created using Hash Key and Hash Value which is an array. And my input is Hash value (Array in this case) , i have to search for all the arrays in $ENV hash and fetch the hash key

Example if i search for m3d1 , the output should be TEST3

  use strict;
  use warnings;
  use Data::Dumper;
  my %ENV;
  
  $ENV{"TEST3"}=["m3d1","m3d2"] ;
  $ENV{"TEST4"}=["m4d1","m4d2"] ;
  $ENV{"TEST5"}=["m5d1","m5d2"] ;
  $ENV{"TEST6"}=["m6d1","m6d2"] ;
  print Dumper \@keys;
  print Dumper \@values;

Is it possible , or is there a better design ?

Don't use %ENV as a variable name. There's an existing variable by that name. — ikegami
– ikegami, Commented Feb 5, 2021 at 7:52

ikegami · Accepted Answer · 2021-02-05 06:53:42Z

1

Of course it's possible. We can visit every value in the data structure until we find the one that matches.

You didn't say what the strings represent, to I'm going to call TEST3 a group, and I'm going to call m3d1 a host. The following snippets don't assume a host is preset in only one of group.

my @groups;
for my $group (keys(%ENV)) {
   for my $host (@{ $ENV{$group} }) {
      if ($host eq $target_host) {
         push @groups, $group;
      }
   }
}

die("Not found\n") if !@groups;

say for @groups;

But this isn't efficient. If you were planning on doing many lookups, this would be slow.

Let's start by turning the structure inside out.

my %groups_by_host;
for my $group (keys(%ENV)) {
   for my $host (@{ $ENV{$group} }) {
      push @{ $groups_by_host{$host} }, $group;
   }
}

The above produces

my %groups_by_host = (
   m3d1 => [ "TEST3" ],
   m3d2 => [ "TEST3" ],
   m4d1 => [ "TEST4" ],
   m4d2 => [ "TEST4" ],
   m5d1 => [ "TEST5" ],
   m5d2 => [ "TEST5" ],
   m6d1 => [ "TEST6" ],
   m6d2 => [ "TEST6" ],
);

Then, searching becomes instantaneous.

my $groups = $groups_by_host{$target_host}
   or die("Not found\n");

say for @$groups;

edited Feb 5, 2021 at 6:53

answered Feb 5, 2021 at 6:44

ikegami

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9 Comments

LearningCpp Over a year ago

i get Global symbol "$key" requires explicit package name at test.pl .guess its groups , Thanks

ikegami Over a year ago

Fixed. // I'm not sure what you're asking

LearningCpp Over a year ago

Thank you , this seems to be a better approach . my $target_host="m4d1"; my $groups = $groups_by_host{$target_host} or die("Not found\n"); say for @$groups; I get the below error , Does it print the Key ? Bareword "say" not allowed while "strict subs" in use at test.pl line 34.

ikegami Over a year ago

my ($name) = @$db_instance;

ikegami Over a year ago

Or use $name = $group; instead of push @groups, $group;

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