Hash tests

Determine whether a hash value exists, is defined, or is true :

1.print "Value EXISTS, but may be undefined.\n" if exists $hash{ $key };
2.print "Value is DEFINED, but may be false.\n" if defined $hash{ $key };
3.print "Value is TRUE at hash key $key.\n" if $hash{ $key };

Hash of Hashes

Create a hash of hashes via references:

The following two solutions are equivalent, except for the way the look.

Solution 1 :

$requiredPatches_href->{ $patch }->{ os } = $os;
$requiredPatches_href->{ $patch }->{ arch } = $arch;
$requiredPatches_href->{ $patch }->{ info } = $info;

Solution 2 : ( It is much clearer)

$requiredPatches_href->{ $patch } = {
os => $os,
arch => $arch,
info => $info,

};


Here is a small program to extract users their login name, uid,homedir and gid from /etc/passwd using HashOfHashes and hash reference:

#!/usr/bin/perl  -w

  sub func
  {
      my ( $login, $p, $uid, $gid, $gecos, $dir, $s );
      my %HoH = ();
      my $file = '/etc/passwd';
      open( PASSWD, "lt; $file" ) or die "Can't open $file : $!";
      while( <passwd> )
      {
          ( $login, $p, $uid, $gid, $gecos, $dir, $s ) = split( ':' );
#writing a hash of hash using reference
          $HoH{$login} =  {
                           'uid' => $uid,
                           'gid' => $gid,
                           'dir' => $dir,
                          };
      }
      close PASSWD;
      return \%HoH;

  }
  my $hash_ref = &func;

# Print the keys and values of a hash, given a hash reference

  while( my ($k, $v) = each %$hash_ref )
  {
# Print Outer hash HOH keys and values
      print "login: $k ";
# Print Inner hash keys and values for each outer hash key i.e login name
      while ( my ($key,$val) = each %$v )
      {
          print "$key:$val ";
      }
          print "\n";
  }

Output looks like this:

login: bin uid:2 dir:/bin gid:2
login: clamav uid:110 dir:/var/lib/clamav gid:120
login: nobody uid:65534 dir:/nonexistent gid:65534
login: lp uid:7 dir:/var/spool/lpd gid:7

Perl Hash

A hash is a simple collection of keys and their corresponding values. The values can be homogeneous or heterogeneous data types.


Example :

%hash = (
      key1 => 'value1',
      key2 => 'value2',
      key3 => 'value3',
  );

Add a key/value pair to a hash :

In the solutions below, quotes around the keys can be omitted when the keys are identifiers.

Hash:

  $hash{ 'key' } = 'value';    # hash
  $hash{ $key } = $value;      # hash, using variables

Hash reference:

  $href->{ 'key' } = 'value';  # hash ref
  $href->{ $key } = $value;    # hash ref, using variables

You can also store references to functions in your data structures, just as you can store references to arrays or hashes:

%HoF = (                           # Compose a hash of functions
exit    =>  sub { exit },
help    =>  \&show_help,
watch   =>  sub { $watch = 1 },
mail    =>  sub { mail_msg($msg) },
edit    =>  sub { $edited++; editmsg($msg); },
delete  =>  \&confirm_kill,
);

if   ($HoF{lc $cmd}) { $HoF{lc $cmd}->() }   # Call function
else { warn "Unknown command: `$cmd'; Try `help' next time\n" }

In the second to last line, we check whether the specified command name (in lowercase) exists in our "dispatch table", %HoF. If so, we invoke the appropriate command by dereferencing the hash value as a function and pass that function an empty argument list. We could also have dereferenced it as &{ $HoF{lc $cmd} }(), or simply $HoF{lc $cmd}().