my @b = grep EXPR, @a;
[download]

is equivalent to

my @b;
foreach (@a) {
   if (EXPR) {
      push @b, $_;
   }
}
[download]

In this case:

my @uniqueIPs;
foreach (@uniqueIPs) {
   if (!$counts{$_}++) {
      push @ipAddresses, $_;
   }
}
[download]

As for post-incrementing,

$x++
[download]

is equivalent to

my $orig_x = $x;
++$x;
$orig_x
[download]

In this case:

my @uniqueIPs;
foreach (@uniqueIPs) {
   my $old_count = $counts{$_};
   $counts{$_}++;             # Add one to count.
   if (!$old_count) {         # If it's the first time we've seen it,
      push @ipAddresses, $_;  #    save it
   }
}
[download]

So,

my %counts;
my @uniqueIPs = grep !$counts{$_}++, @ipAddresses;
[download]

is short for

my @uniqueIPs;
foreach (@uniqueIPs) {
   $counts{$_}++;             # Add one to count.
   if ($counts{$_} == 1) {    # If it's the first time we've seen it,
      push @ipAddresses, $_;  #    save it
   }
}
[download]

It is dense, but it's tried and true. Just add a comment for the less learned readers.

# Remove duplicates IP addresses by counting
# the number of times each address occurs.
my %counts;
my @uniqueIPs = grep !$counts{$_}++, @ipAddresses;
[download]

Oh by the way, you could also write it as follows if it's less confusing:

# Remove duplicates IP addresses by counting
# the number of times each address occurs.
my %counts;
my @uniqueIPs = grep ++$counts{$_} == 1, @ipAddresses;
[download]

---

---

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! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

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:
	&		&
	<		&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.