comment on

Here's some basic logic which may achieve what you want.

#!/usr/bin/env perl

use strict;
use warnings;

my $COUNT = $ARGV[0];
my %seen;
my @data;

unshift @data, split while <DATA>;
@data = grep !$seen{$_}++, @data;
print "@data[0 .. $COUNT - 1]\n";

__DATA__
1 1 2 2 3 3 4
5 6 7 8
5 6 6 7 8
5 6 7 8
9 10 11 12
13 14 15 16
17 18 19 20
17 18 19 20
21 22 23 24
[download]

Some sample runs:

$ ./pm_11109149_reverse_file_extract_with_conds.pl 24
21 22 23 24 17 18 19 20 13 14 15 16 9 10 11 12 5 6 7 8 1 2 3 4

$ ./pm_11109149_reverse_file_extract_with_conds.pl 12
21 22 23 24 17 18 19 20 13 14 15 16

$ ./pm_11109149_reverse_file_extract_with_conds.pl 7
21 22 23 24 17 18 19
[download]

Notes:

This solution is intended for small files (such as the sample input you show). Some of the techniques shown may also be valid for larger files — such as using %seen for duplicate checking — but you probably won't want to read millions of numbers into an array if you only want seven.
I've modified your data to include duplicates in various places. For future reference, it's useful for us if you provide sample input with examples of this sort of thing.
I've just used the DATA filehandle; you'll need to add your own I/O handling.
I haven't included any exception handling; you'll need to do this. Areas to look at would be I/O errors, $COUNT not being an integer, and so on.

"I am just setting out learning PERL, ..."

That's fine: none of us left the womb knowing any Perl.

[Aside: PERL is wrong: the language is Perl and the executable is perl (which may have an extension on some systems, e.g. perl.exe on MSWin).]

If there's any code that I've used that you don't understand, try the Online Perl Documentation in the first instance — if still in doubt, ask another question here.

— Ken

In reply to Re: Reading the contents of a file from the bottom up by kcott
in thread Reading the contents of a file from the bottom up by Scotmonk

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