Path: utzoo!attcan!uunet!snorkelwacker!apple!rutgers!mcnc!thorin!oscar!tell
From: tell@oscar.cs.unc.edu (Stephen Tell)
Newsgroups: comp.lang.perl
Subject: Re: Best way to nullify an intersection
Message-ID: <14971@thorin.cs.unc.edu>
Date: 3 Jul 90 14:07:54 GMT
References: <601@inpnms.ROCKVILLE.DG.COM>
Sender: news@thorin.cs.unc.edu
Reply-To: tell@oscar.cs.unc.edu (Stephen Tell)
Organization: University Of North Carolina, Chapel Hill
Lines: 62

In article <601@inpnms.ROCKVILLE.DG.COM> logan@rockville.dg.com (James L. Logan) writes:
>Is there a better or faster way to drop all elements in set B that
>intersect with set A?  Here's what I've got so far:
>	open(FILE1, "file1") || die;
>	@A = <FILE1>;
>	close(FILE1) || die;
>
>	open(FILE2, "file2") || die;
>	@B = <FILE2>;
>	close(FILE2) || die;
>
>	foreach $element (@A) {
>		foreach (@B) {
>			if (/^$element$/) {
>				$_ = '';
>				last;
>			}
>		}
>	}
>	print @B;

>Also, how can I delete the element that was matched in @B, while
>also shortening the array by one element; rather than just
>setting the element to the null string?  

Its called splice(@array, offset, length, [replace])
"Delete length elements from @array starting at offset and return them,
optionaly replacing them with new elements."

>			Thanks,
>			-Jim

This is my first comp.lang.perl posting with a coding suggestion, so be
gentle, guys!
It seems that using an associative array to say "did we see this anywhere"
would help a lot over the O(n**2) approach above.  You said these are
sets, if the order in which B gets printed matters, this goes out the window.
But try:

	open(FILE1, "file1") || die;
	@A=<FILE1>;
	close(FILE1) || die;

	open(FILE2, "file2") || die;
	while(<FILE2>) {
		$B{$_} = 1;
 	}
	close(FILE2) || die;

	foreach $element (@A) {
		if ($B{$element} {
			delete($B{$element});
		}
	}
	print keys(%B);

Is there a better way to read FILE2 into an associative array like this?
Perhaps the ability to say
keys(%B) = <FILE2>;
--------------------------------------------------------------------
Steve Tell      e-mail: tell@wsmail.cs.unc.edu usmail:  #5L Estes Park apts
CS Grad Student, UNC Chapel Hill.                       Carrboro NC 27510