Hi ovedpo15,

The only problem to keep the original order of the input is that the data structure used in my solution, a hash, does not have a defined order. However, the solution is fairly easy: we only need to keep track of the original order one way or another.

One possible way to do that is to add an array (@ranks) as an auxiliary data structure when reading the file and then to loop over the array when preparing the output: 

use strict;
use warnings;
use feature 'say';

my (%tree, @ranks);
while (<DATA>) {
    chomp;
    my ($id, $pid, $name) = split /,/;
    $tree{$id} = {pid => $pid, name => $name};
    push @ranks, $id;
}
for my $id (@ranks) {
    my @parent_list;
    my $temp_id = $id;
    while (exists $tree{$temp_id}) {
        push @parent_list, $tree{$temp_id}{name};
        $temp_id = $tree{$temp_id}{pid};
    }
    say join ",", @parent_list;
}

__DATA__
15,10,name3
10,#,name1
12,10,name2
5,12,name4
8,5,name5

[download]
which produces the following output: 
$ perl test_ranks.pl
name3,name1
name1
name2,name1
name4,name2,name1
name5,name4,name2,name1

[download]
The solution you contemplated in your question is also relatively easy, except for the syntax used to sort the records according to the ranks, which may be regarded as a bit complicated for a Perl beginner. Here, I have added a rank field into the records and then sort the id's in accordance to the rank: 
use strict;
use warnings;
use feature 'say';

my (%tree);
while (<DATA>) {
    chomp;
    my ($id, $pid, $name) = split /,/;
    $tree{$id} = {pid => $pid, name => $name, rank => $.};
}
for my $id (sort { $tree{$a}{rank} <=> $tree{$b}{rank} } keys %tree) {
    my @parent_list;
    my $temp_id = $id;
    while (exists $tree{$temp_id}) {
        push @parent_list, $tree{$temp_id}{name};
        $temp_id = $tree{$temp_id}{pid};
    }
    say join ",", @parent_list;
}

__DATA__
15,10,name3
10,#,name1
12,10,name2
5,12,name4
8,5,name5

[download]
which produces the same output as before.

Another possible way might be to read the input file again to get the id's in the proper order, but I wouldn't recommend that as that would be less efficient.

You may also take a look at the Hash::Ordered module on the CPAN, which implements a hash data structure with a builtin order.

Update: Fixed a typo: s/reparing the output/preparing the output/.

In reply to Re^5: Reorganizing the content of a file by Laurent_R
in thread Reorganizing the content of a file by ovedpo15

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.