What shall we call these? "Ephemeral list elements"? As you know, merlyn, they actually last a little longer then that piece of doc implies. They are not just immediately thrown away. They last long enough for us to grab the value that undef held a place for. Some examples will help here:

  my @w = () = 1;
  print "[", @w, "]\n";               # prints: []

  my @x = (undef) = 2;
  print "[", @x, "]\n";               # prints: [2]

  my @y = (undef) = (3,4);
  print "[", @y, "]\n";               # prints: [3]

  my @z = (undef, undef) = (5,6);
  print "[", @z, "]\n";               # prints: [56]

  my $temp;
  my @z = (undef, $temp, undef) = (7,8,9);
  print "[", @z, "]\n";               # prints: [789]
[download]

All of which is related to but a bit different from the following which (like the first example above) does not return values-in-a-list but still manages to force a list context for what will be returned by the regex (or any other expression that can return scalar or list). That elusive ghost list can then be construed into a scalar count of the list elements:

  my $count = () = "abacadaeafag" =~ /a/g;
  my @empty = () = "abacadaeafag" =~ /a/g;
  print "[", $count, "]\n";           # prints: [6]
  print "[", @empty, "]\n";           # prints: []
[download]

Even more ephemeral. But the "$count = () = m//" construct can be handy occasionally.

Update: a friend asked for an example of 'handy'. Herewith offered:

  $_ = "abacadaeafag";                # or whatever
  if ( scalar( () = /a/g ) > 7 ) {    # more than 7 'hits'?
     # do something...
  }
[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.