How Do I Compare Array A to Array B, removing B elements from A.

fuzzball has asked for the wisdom of the Perl Monks concerning the following question: ⭐ (arrays)

I need to take a array of URL's in @A, compare it to a array of "banned" domain names in @B, and either remove matches from @A, or put non-matches into a new array, @C. Given example data:

 @A = ( 'yahoo.com', 'altavista.com', 'yahoo.com/index.html' );
 @B = ( 'yahoo.com', 'webcrawler.com' );
[download]

@A (or @C) should end up with only 'altavista.com'. Clearly, it is has to match with patterns, not just exact string equality.

Originally posted as a Categorized Question.

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Re: How Do I Compare Array A to Array B, removing B elements from A.⭐ by merlyn (Sage) on Mar 21, 2002 at 21:03 UTC
`my @C; OUTER: for ( @A ) { for my $pat ( @B ) { if ( /\Q$pat/i ) { next OUTER; } } push @C, $_; }` [download]	[reply] [d/l]
Re: Answer: How Do I Compare Array A to Array B, removing B elements from A. by amitbhosale (Acolyte) on Apr 09, 2008 at 07:39 UTC
other strategy for above solution is. `@array_one = qw(one two three four); @array_two = qw(three four five six); %array_one = map {$_, 1} @array_one; @difference = grep {!$array_one {$_}} @array_two; print "@difference\n";` [download]	[reply] [d/l]
Re: How Do I Compare Array A to Array B, removing B elements from A.⭐ by dragonchild (Archbishop) on Mar 21, 2002 at 20:15 UTC
Something like: `my @C = grep { my $x = $_; not grep { $x =~ /\Q$_/i } @B } @A;` [download]	[reply] [d/l]
Re: How Do I Compare Array A to Array B, removing B elements from A. by SavannahLion (Pilgrim) on Jan 16, 2006 at 15:48 UTC
For anybody who comes after me, I'd like to point out in the first above example by dragonchild a minor correction. When I copied the example, I was losing the data pushed into @ArrayC. Adding a pair of ( ) on the line with grep solves the problem. `my @ArrayC = map { my $x = $_; (grep { $x =~ /^$_/ } @ArrayB) ? () : $x } @ArrayA;` [download] I don't know if it's a typo or a change in how Perl handles the expression. I figure this note will help save someone fifteen minutes of frustration. Originally posted as a Categorized Answer.	[reply] [d/l]
Re: How Do I Compare Array A to Array B, removing B elements from A. by knobunc (Pilgrim) on Mar 21, 2002 at 21:06 UTC
It might be more efficient (depending on the sizes of the 2 arrays) to make a hash from one to do a quick lookup: `my %HashB = map { ($_ => 1) } @ArrayB; @ArrayA = grep { $HashB{$_} } @ArrayA;` [download] -ben Originally posted as a Categorized Answer.	[reply] [d/l]


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