Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!elroy.jpl.nasa.gov!sdd.hp.com!wuarchive!udel!haven.umd.edu!uvaarpa!mmdf
From: bjaspan@ATHENA.MIT.EDU (Barr3y Jaspan)
Newsgroups: comp.lang.perl
Subject: Add 'continue' to 'foreach'?
Message-ID: <1991May8.015608.19707@uvaarpa.Virginia.EDU>
Date: 8 May 91 01:56:08 GMT
Sender: mmdf@uvaarpa.Virginia.EDU (Uvaarpa Mail System)
Reply-To: bjaspan@ATHENA.MIT.EDU
Organization: The Internet
Lines: 72


I have a perl program in which I am doing a select() on a large number
of sockets and then iterating over those that get set.  Initially, I
was doing this

	for ($i=0; $i < $maxfn; $i++) {
		next unless vec($rdset, $i, 1);

		# stuff
	}

where $maxfn happens to be the largest filedescriptor number in
$rdset.  Since vec() is slow, I am now doing something like this (note
that I have to maintain $i in any event, because I have several arrays
keyed on it):

	@rdset = split(//, unpack('b*', $rdset));
	$i = 0;
	foreach (@rdset) {
		if (! $_) {
			$i++;
			next;
		}

		# stuff
		$i++;
	}

The block attached to the 'if' is ugly, and this is perl, so there
should be a magic way around the problem.  (Also, in a recent posting,
someone said that having a block like that is slower than doing it
with a postfix if/unless.  Clearly, if there is a local() inside the
block there will be overhead; does Perl not optimize if there is no
local()?)  

How about adding an optional continue block to the foreach statement?
Then I could do something like this:

	$i = 0
	foreach (@rdset) {
		next unless $_;

		# stuff
	} continue {
		$i++;
	}

The advantage to doing it this way is that foreach is non-destructive.
Doing it with a while (which has a continue block) would require
shifting the array, which (1) would mean I couldn't use it again, or I
would have to keep a copy, and (2) I would guess is slower than just
iterating over the elements.

-----------

As a separate issue, I just thought of another way to do it (after
all, this is perl):

	$rdstr = unpack('b*', $rdset);
	$i = length($rdstr);
	while (chop $rdstr) {
		next unless $_;
		# stuff
	} continue { $i--; }

This saves a split(), and adds a length() and a bunch of chop()s; it
also iterates backwards (although this doesn't matter in this
particular case).  Is this faster?  (ie: how fast is chop?)

$_ = "\n,rekcah lrep rehtona tsuJ"; print chop while $_;

Barr3y Jaspan, bjaspan@mit.edu