cstrong has asked for the wisdom of the Perl Monks concerning the following question:

I'm only a Perl beginner and I know there are lots and lots of pages out there that deal with searching through arrays, but I cannot find anything relevant to my problem.

I have two arrays and I want to find if one of the arrays is contained within the other (with the order preserved). Moreover I want to then be able to delete the contents of one array from the other

 @bigarray qw(this is the main array from which to delete elements)

 @smallarray qw(delete elements)

An array such as  @other qw(elements delete) should not match against @bigarray.

I need to somehow remove "delete elements" from @bigarray. I have tried using nested loops, trying to keep track of indexs so I can use splice, but I just can't figure out how to do it properly with loops

@bigarray will probably be <1K in size and this delete action will only be called perhaps once or twice during my script, so efficiency is not an issue.

Any help appreciated :-)

Just for clarity, the exact sequence from @smallarray should be removed from @bigarray i.e.

 @bigarray qw(delete this is elements the main array from which to delete elements)

should be transformed to

 @bigarray qw(delete this is elements the main array from which to)

Thanks!

Replies are listed 'Best First'.
Re: Check if one array is a subset of another
by tobyink (Canon) on Jan 04, 2012 at 17:06 UTC

    Use Set::Scalar.

    use Set::Scalar;

my $bigset   = Set::Scalar->new(@bigarray);
my $smallset = Set::Scalar->new(@smallarray);

my $difference = $bigset - $smallset;

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    This assumes though that you don't care about array order, and don't care about duplicates - you're treating them as sets.

    Preserving order, and assuming you're on Perl 5.10+, you can just use smart match instead...

    my $smallarray_ref = \@smallarray;
my @difference = grep { !($_ ~~ $smallarray_ref) } @bigarray;

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      Unfortunately I'm on Perl 5.8, otherwise that solution looks perfect (and simple)! I'm afraid that I will need to preserve the order too.
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Re: Check if one array is a subset of another
by BrowserUk (Patriarch) on Jan 04, 2012 at 17:08 UTC

    If, as suggested by your sample code & data, your array elements contain no spaces, then a very simple, and relatively efficient mechanism would be:

    @bigarray = qw(this is the main array from which to delete elements);;
@smallarray = qw(delete elements);;

($temp = "@bigarray") =~ s[@smallarray][] and  @bigarray = split ' ', 
+$temp;;

print for @bigarray;;
this
is
the
main
array
from
which
to

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    If your elements can contain spaces, then the same technique could be utilised, by using a different separator char -- eg. $;. It is only slightly more complex;

    @bigarray = qw(this is the main array from which to delete elements);;
@smallarray = qw(delete elements);;

$b = join $;, @bigarray;;
$s = join $;, @smallarray;;

$b =~ s[$s][] and @bigarray = split $;, $b;;

print for @bigarray;;
this
is
the
main
array
from
which
to

    [download]
    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.

    The start of some sanity?
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Re: Check if one array is a subset of another
by Anonymous Monk on Jan 04, 2012 at 17:18 UTC
    I recognise this to be a typical example of the Longest Common Subsequence problem. 
    use Algorithm::Diff qw(LCSidx);

my @bigarray 
    = qw(this is the main array from which to delete elements);
my @smallarray = qw(delete elements);

my ($index_range) = LCSidx \@bigarray, \@smallarray;
splice @bigarray, $index_range->[0], @$index_range;

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Re: Check if one array is a subset of another
by Khen1950fx (Canon) on Jan 04, 2012 at 17:52 UTC
    Another way to do it: 
    #!/usr/bin/perl -l

use strict;
use warnings;

my @big = qw(this is the main array from which to delete elements);
my @small = qw(delete elements);
my @new = grep { my $x = $_; not grep { $x =~/\Q$_/i } @small } @big;
print "@new";

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Re: Check if one array is a subset of another
by choroba (Cardinal) on Jan 04, 2012 at 17:00 UTC
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      If the big array is a X b c Y d Y e

      and the small array is

       c Y

      then the resulting array would be

      a X b d Y e

      Thanks

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