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From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Best way to nullify an intersection
Message-ID: <8593@jpl-devvax.JPL.NASA.GOV>
Date: 4 Jul 90 00:16:25 GMT
References: <601@inpnms.ROCKVILLE.DG.COM> <15631@bfmny0.BFM.COM> <604@inpnms.ROCKVILLE.DG.COM>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 27

In article <604@inpnms.ROCKVILLE.DG.COM> logan@rockville.dg.com (James L. Logan) writes:
: In article <15631@bfmny0.BFM.COM> tneff@bfmny0.BFM.COM (Tom Neff) writes:
: # 
: # 	@B=grep(!do {$x=$_; grep($x eq $_,@A);},@B);
: # 
: 
: Could someone explain this construct?  I've only been using perl
: for a week now...  

That's just a funny way of writing two nested loops.  It's more or less
equivalent to

    @outer_result = ();
    foreach $_ (@A) {
	$x = $_;
	@inner_result = ();
	foreach $_ (@B) {
	    push(@inner_result,$_) if $x eq $_;
	}
	push(@outer_result, $_) if @inner_result;
    }
    @B = @outer_result;

As such, it's just another O(n**2) algorithm.  Better to use the associative
array method mentioned earlier, which is basically a linear algorithm.

Larry