in reply to Re^3: Doubly link list implementation
in thread Doubly link list implementation

Hi punitpawar,

It seems that you've found a solution by now, but if you really want to build a doubly-linked list, you should most probably create separate subs to insert new nodes (and also subs to delete nodes).

Subs to insert nodes could go like this: 

sub insert_after {
    my ($previous, $val) = @_;
    my $curr_next = $previous->{next};
    $previous->{next} = {
                "data" => $val , 
                "next" => $curr_next,
                "prev" => $previous,
            };
    my $new_next = $previous->{next};
    $curr_next->{prev} = $new_next if defined $curr_next;
    return $new_next;
}    

sub insert_before {
    # simply call insert_after on the previous node
    my ($previous, $val) = @_;
    return insert_after($previous->{prev}, $val);
}

[download]
While we are at it, I think we should also move the displaying of the content into a separate sub: 
sub display {
    my $curr = shift;
    while (1) {
        last unless defined $curr->{next};
        print "Current: ", $curr->{data}, 
              "\tPrevious: ", $curr->{prev}{data} // "", 
              "\tNext: ", $curr->{next}{data} // "", "\n";
        $curr = $curr->{next};
    }
    print "\n";
}

[download]
I now change the program of my previous post to add two nodes after the original construction of the data structure, and display the structure before and after these insertions: 
my $head = {data => 0, prev => undef, next => undef};
my $previous = $head;

while (<DATA>){
    # ... same as in previous post
}

display($head);
my $new_node = insert_after ($head, 0.5);
$new_node = insert_before ($new_node->{next}, 0.7);
display($head);

[download]
This prints out the following: 
$ perl  doubly_linked.pl
Current: 0      Previous:       Next: 1
Current: 1      Previous: 0     Next: 2
Current: 2      Previous: 1     Next: 3
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 5

Current: 0      Previous:       Next: 0.5
Current: 0.5    Previous: 0     Next: 0.7
Current: 0.7    Previous: 0.5   Next: 1
Current: 1      Previous: 0.7   Next: 2
Current: 2      Previous: 1     Next: 3
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 5

[download]
But, there is more to it.

Now that I have created subs to insert nodes, I can use them to build the original structure, rather than doing it manually in the while loop: 
my $head = {data => 0, prev => undef, next => undef};
my $tail = $head;

while (<DATA>){
    chomp;
    $tail = insert_after($tail, $_);
}

[download]
As per your request, I also add the insertion of a node just before the tail, and the overall program (without the subs already listed above) now looks like this: 
#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my $head = {data => 0, prev => undef, next => undef};
my $tail = $head;

while (<DATA>){
    chomp;
    $tail = insert_after($tail, $_); # new tail returned each time thr
+ough
}

display($head);
my $new_node = insert_after ($head, 0.5);
$new_node = insert_before ($new_node->{next}, 0.7);
# adding a new node before the tail
$new_node = insert_before ($tail, 4.5);
display($head);

[download]
Which duly prints: 

$ perl  doubly_linked.pl
Current: 0      Previous:       Next: 1
Current: 1      Previous: 0     Next: 2
Current: 2      Previous: 1     Next: 3
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 5

Current: 0      Previous:       Next: 0.5
Current: 0.5    Previous: 0     Next: 0.7
Current: 0.7    Previous: 0.5   Next: 1
Current: 1      Previous: 0.7   Next: 2
Current: 2      Previous: 1     Next: 3
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 4.5
Current: 4.5    Previous: 4     Next: 5

[download]
This is just a quick illustration, there may be some edge cases not handled correctly (such as trying to add a node before the head, etc.). And I'll leave it to you to create the subs to remove nodes from the linked list, I guess it should be more or less obvious how to do it by now in general, you'll just have to get the details right.

I hope this helps.

In Section Seekers of Perl Wisdom