I am still a newbie, but / therefore - what is wrong with using hash here (with keys as line numbers and values as bases)? 
#!/usr/bin/perl -l
use strict;
use warnings;

print "If an array element is a line number and a base together as a s
+tring:";

my %positions;

for my $line (split /\n/, <<'END') 
1 ACAC
2 AGAC
3 AGTC
4 ACCA
END

{
    next if $line =~ /[GT]/;
    my ($number, $bases ) = split / /, $line;
    $positions{$number} = $bases;    
}

for my $number ( sort keys %positions )
{
    print "$number => $positions{$number}";
    print "Just print a number $number";
}

print "If there is an array element for a line number and for a base:"
+;

my @array = ( qw (1 ACAC 2 AGAC 3 AGTC 4 ACCA) );
%positions = ();
for (my $i = 0; $i < @array; $i +=2 )
{
    my ($number, $bases ) = @array[$i, $i+1];
    next if $bases =~ /[GT]/;
    $positions{$number} = $bases;
}

for my $number (sort keys %positions)
{
    print "$number => $positions{$number}";
    print "Just print a number $number";
}

[download]
It prints: 
If an array element is a line number and a base together as a string:
1 => ACAC
Just print a number 1
4 => ACCA
Just print a number 4
If there is an array element for a line number and for a base:
1 => ACAC
Just print a number 1
4 => ACCA
Just print a number 4

[download]

In reply to Re: filtering an array by vagabonding electron
in thread filtering an array by prbndr

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.