in reply to Help with hash/array data structure

References are a big spicy meatball! And, of course, there's more than one way to do it.

The following snippet might help:

use strict;

my %packet_fields = (
  'Format_X' => ['ident', 'flags', 'x', 'y', 'mode3'],
  'Format_Y' => ['ident', 'RDP_Ident', 'Radar_Site_Ident', 'site_x']
);

foreach my $format (keys %packet_fields) {
  foreach my $field (@{$packet_fields{$format}}) {    #line 9
    print "$format -> $field\n";
  }
}

foreach my $format (keys %packet_fields) {
  print "$format's 2nd field = ", $packet_fields{$format}->[1], "\n"; 
+#line 15
  print "$format's 3rd field = ", $packet_fields{$format}[2], "\n"; #l
+ine 16
}

[download]

In this sample, %packet_fields is a regular old hash. The keys for this hash are garden-variety scalars. The value associated w/ each key is a reference to an anonymous (unnamed) array. These two anon. arrays are the only references that we have to deal with, and the only things that require any dereferencing syntax.

The @{} in line 9 is using a form of dereferencing notation to give me an array.

There's another form of dereferencing that I can use, too, as seen in lines 15 and 16.

Hanlon's Razor - "Never attribute to malice that which can be adequately explained by stupidity"

Replies are listed 'Best First'.
Re: Re: Help with hash/array data structure
by robinbowes (Beadle) on Nov 13, 2003 at 10:07 UTC
    Hi, Thanks for all the replies - most informative!

    I've now got working code, something like this: 

    # Declaration of the data structure
my %packet_fields = (
   $Idents{ Format_7 } => [qw(    ident
                x
                    y
                    flight_level
                chksum
                    )],
   $Idents{ Rotation_Marker } => [qw(    ident
                            site_x
                            site_y
                            chksum
                      )],
);

# This structure listed for completeness of this example!
my %packet_template = (
    $Idents{ Rotation_Marker } => "C3 n n n n n n n n n n n n C3",
    $Idents{ Format_7 }                => "C B16 n2 A4 A8 A2 A A A3 C2
+",
);
# Referencing the data structure
# code here to read $buffer from STDIN
# $ident is extracted from $buffer
    @packet{ @{$packet_fields{ $ident }} } =
        unpack ( $packet_template{ $ident }, $buffer);

    [download]
    I've got one further question about creating and accessing a hash of hashes.

    I'm currently using the following structures: 

    my %ARP_GLA = (
    x    =>    247866.9,
    y    =>    666999.7,
);

my %ARP_EDI = (
    x    =>    314382.0,
    y    =>    673841.9,

    [download]
    I'd like to combine these into one data structure which I guess is a hash of hashes.

    I believe I know enough to create the hash. This should do the trick: 

    my %ARP = (
    GLA => {
        x    =>    247866.9,
        y    =>    666999.7
        },
    EDI => {
        x    =>    314382.0,
        y    =>    673841.9
        }
);

    [download]
    How do I now access the elements in this structure?

    Is it something like: 

    my $edi_arp = $ARP{EDI}->{x};

    [download]
    Thanks.

    R.

    --
    Robin Bowes | http://robinbowes.com

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