[Perl.sig] Perl Tip: Slicing Arrays and Hashes plus Spring Special

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==== Slicing arrays and hashes ====

    As well as dealing with a single element from an array or hash, Perl
    allows us to refer to multiple elements at the same time. This is called
    a *slice*.


==   Array slices   ==

    When we ask for a single element from an array via a look-up, we precede
    the name of the array with a ``$'' sign. This tells Perl we want a
    scalar.

    When we're slicing an array, we precede the name of the array with a
    ``@'' sign. This tells Perl that we expect to receive multiple items.

            my @big_array = ( 0 .. 200 );

            my @slice = @big_array[ 20 .. 50 ];

    In the above code, @slice will be populated with the 20th through to the
    50th elements from @big_array (which are the numbers 20 - 50).

    We can use slices to change the values in the array:

            @big_array[ 20 .. 50 ] = ( 10 .. 20 );

    or to assign values to variables:

            # assigns last four values from array to variables
            my ($a, $b, $c, $d) = @big_array[ -1 .. -4 ];

    we can even use slices on the results of functions which return lists:

            my ($day, $month, $year) = (localtime())[3, 4, 5];

    although in this case, notice that we don't use a ``@'' character as we
    are not accessing an array.


==   Hash slices   ==

    Hash slices are very similar to array slices. In a hash slice, we
    precede the name of the hash with an ``@'' sign as we expect to be
    dealing with multiple items.

            # simple hash slice
            my @values = @hash{$key1, $key2, $key3, $key4};

    As with array slices we can use hash slices to change the values in the
    hash:

            my %age_of_friend = (
                    James => 30,
                    Ralph => 5,
                    John => 23,
                    Jane => 34,
                    Maria => 26,
                    Bettie => 29
            );

            # James and Ralph just had their birthdays
            @age_of_friend{ qw/James Ralph/ } = (31, 6);

    or assign values to variables:

            my ($maria_age, $bettie_age) = @age_of_friend{ qw/Maria Bettie/
};

    A common use of hash slices is to create a look-up table for data stored
    in an array. As hash look-ups occur in constant time while random
    searches across an array occur in polynomial time, using hash slices
    increases the efficiency of your code.

            # The array of things we'd like to test against
            my @colours = qw/red green yellow orange white mauve blue ochre
                             pink purple gold silver grey brown steel/;

            # A list of things that might be in @colours or not
            my @find = qw/red blue green black/;

            # hashes and arrays can have the same names.
            # hash slices use curly braces {}
            # array slices use square brackets []
            my %colours;

            # set all values in %colours from the keys in @colours to
            # have the undefined value (but exist in the hash).
            @colours{@colours} = ();

            # We now look for @find in %colours rather than @colours.
            foreach my $colour (@find) {
                    if(exists( $colours{$colour} )) {
                            print "$colour exists\n";
                    }
                    else {
                            print "$colour is unknown\n";
                    }
            }

    The uniqueness property of hash keys can also be used to remove
    duplicates from a list.


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