Question

So I am trying to learn Linked Lists using Perl. I am reading Mastering Algorithms with Perl by Jon Orwant. In the book he explains how to create a linked list.
I understand most of it, but I just simply fail to understand the command/index/key NEXT in the second last line of the code snippet.

$list=undef;
$tail=\$list;

foreach (1..5){
  my $node = [undef, $_ * $_];
  $$tail = $node;
  $tail = \${$node->[NEXT]}; # The NEXT on this line? 
}

What is he trying to do there?

Is $node a scalar, which stores the address of the unnamed array? Also even if we are dereferencing $node, should we not refer to the individual elements by an index number, such as (0,1). If we do use NEXT as a key, is $node a reference to a hash?
I am very confused.

Something in plain English will be highly appreciated.

Answer 1

NEXT is a constant, declared earlier in the script. It contains an integer value representing the index of the current node’s member element that refers to the next node.

Under this scheme, each node is a small anonymous array. One element of this anonymous array contains the payload, and the other contains a reference pointing to the next node.

If you look at some of the earlier examples in that chapter you will see the following declarations:

use constant NEXT    => 0;
use constant VAL     => 1;

So $node->[NEXT] is synonymous to $node->[0], which contains a reference to the next node in the linked list chain, while $node->[VAL] is synonymous with $node->[1]; the value (or payload) stored in the current node.

I’ll comment on the code snippet you provided:

foreach (1..5){
  my $node = [undef, $_ * $_];    # Create a new node as an anon array.

  # Set the previous node's "next node reference" to point to this new node.
  $$tail = $node;                 

  # Remember a reference to the new node's "next node reference" element.
  # So that it can be updated when another new element is added on next iteraton.
  $tail = \${$node->[NEXT]}; # The NEXT on this line? 
}

Excellent book, by the way. I’ve got several algorithms books, and that one continues to be among my favorites after all these years.

Update: I do agree that the book isn’t a model of current idiomatic Perl, or current “best practices” Perl, but do feel it is a nice resource for gaining an understanding of the application of classic algorithms with Perl. I still refer back to it from time to time.