Set::Scalar(3) User Contributed Perl Documentation Set::Scalar(3)NAMESet::Scalar - basic set operations
SYNOPSIS
use Set::Scalar;
$s = Set::Scalar->new;
$s->insert('a', 'b');
$s->delete('b');
$t = Set::Scalar->new('x', 'y', $z);
DESCRIPTION
Creating
$s = Set::Scalar->new;
$s = Set::Scalar->new(@members);
$t = $s->clone;
$t = $s->copy; # Clone of clone.
$t = $s->empty_clone; # Like clone() but with no members.
Modifying
$s->insert(@members);
$s->delete(@members);
$s->invert(@members); # Insert if hasn't, delete if has.
$s->clear; # Removes all the elements.
Note that clear() only releases the memory used by the set to be reused
by Perl; it will not reduce the overall memory use.
Displaying
print $s, "\n";
The display format of a set is the members of the set separated by
spaces and enclosed in parentheses ().
You can even display recursive sets.
See "Customising Display" for customising the set display.
Querying
Assuming a set $s:
@members = $s->members;
@elements = $s->elements; # Alias for members.
@$s # Overloaded alias for members.
$size = $s->size; # The number of members.
$s->has($m) # Return true if has that member.
$s->contains($m) # Alias for has().
if ($s->has($member)) { ... }
$s->member($m) # Returns the member if has that member.
$s->element($m) # Alias for member.
$s->is_null # Returns true if the set is empty.
$s->is_empty # Alias for is_null.
$s->is_universal # Returns true if the set is universal.
$s->null # The null set.
$s->empty # Alias for null.
$s->universe # The universe of the set.
Deriving
$u = $s->union($t);
$i = $s->intersection($t);
$d = $s->difference($t);
$e = $s->symmetric_difference($t);
$v = $s->unique($t);
$c = $s->complement;
These methods have operator overloads:
$u = $s + $t; # union
$i = $s * $t; # intersection
$d = $s - $t; # difference
$e = $s % $t; # symmetric_difference
$v = $s / $t; # unique
$c = -$s; # complement
Both the "symmetric_difference" and "unique" are symmetric on all their
arguments. For two sets they are identical but for more than two sets
beware: "symmetric_difference" returns true for elements that are in an
odd number (1, 3, 5, ...) of sets, "unique" returns true for elements
that are in one set.
Some examples of the various set differences:
set or difference value
$a (a b c d e)
$b (c d e f g)
$c (e f g h i)
$a->difference($b) (a b)
$a->symmetric_difference($b) (a b f g)
$a->unique($b) (a b f g)
$b->difference($a) (f g)
$b->symmetric_difference($a) (a b f g)
$b->unique($a) (a b f g)
$a->difference($b, $c) (a b)
$a->symmetric_difference($b, $c) (a b e h i)
$a->unique($b, $c) (a b h i)
Comparing
$eq = $s->is_equal($t);
$dj = $s->is_disjoint($t);
$pi = $s->is_properly_intersecting($t);
$ps = $s->is_proper_subset($t);
$pS = $s->is_proper_superset($t);
$is = $s->is_subset($t);
$iS = $s->is_superset($t);
$cmp = $s->compare($t);
The "compare" method returns a string from the following list: "equal",
"disjoint", "proper subset", "proper superset", "proper intersect", and
in future (once I get around implementing it), "disjoint universes".
These methods have operator overloads:
$eq = $s == $t; # is_equal
$dj = $s != $t; # is_disjoint
# No operator overload for is_properly_intersecting.
$ps = $s < $t; # is_proper_subset
$pS = $s > $t; # is_proper_superset
$is = $s <= $t; # is_subset
$iS = $s >= $t; # is_superset
$cmp = $s <=> $t;
Boolean contexts
In Boolean contexts such as
if ($set) { ... }
while ($set1 && $set2) { ... }
the size of the $set is tested, so empty sets test as false, and non-
empty sets as true.
Iterating
while (defined(my $e = $s->each)) { ... }
This is more memory-friendly than
for my $e ($s->elements) { ... }
which would first construct the full list of elements and then walk
through it: the "$s->each" handles one element at a time.
Analogously to using normal "each(%hash)" in scalar context, using
"$s->each" has the following caveats:
· The elements are returned in (apparently) random order. So don't
expect any particular order.
· When no more elements remain "undef" is returned. Since you may
one day have elements named 0 don't test just like this
while (my $e = $s->each) { ... } # WRONG!
but instead like this
while (defined(my $e = $s->each)) { ... } # Right.
(An "undef" as a set element doesn't really work, you get "".)
· There is one iterator per one set which is shared by many element-
accessing interfaces-- using the following will reset the iterator:
elements(), insert(), members(), size(), unique(). insert() causes
the iterator of the set being inserted (not the set being the
target of insertion) becoming reset. unique() causes the iterators
of all the participant sets becoming reset. The iterator getting
reset most probably causes an endless loop. So avoid doing that.
· Modifying the set during the iteration may cause elements to be
missed or duplicated, or in the worst case, an endless loop; so
don't do that, either.
Cartesian Product and Power Set
· Cartesian product is a product of two or more sets. For two sets,
it is the set consisting of ordered pairs of members from each set.
For example for the sets
(a b)
(c d e)
The Cartesian product of the above is the set
([a, c] [a, d] [a, e] [b, c] [b, d] [b, e])
The [,] notation is for the ordered pairs, which sets are are not.
This means two things: firstly, that [e, b] is not in the above
Cartesian product, and secondly, [b, b] is a possibility:
(a b)
(b c e)
([a, b] [a, c] [a, e] [b, b] [b, c] [b, d])
For example:
my $a = Set::Scalar->new(1..2);
my $b = Set::Scalar->new(3..5);
my $c = $a->cartesian_product($b); # As an object method.
my $d = Set::Scalar->cartesian_product($a, $b); # As a class method.
The $c and $d will be of the same class as $a. The members of $c
and $c in the above will be anonymous arrays (array references),
not sets, since sets wouldn't be able to represent the ordering or
that a member can be present more than once. Also note that since
the members of the input sets are unordered, the ordered pairs
themselves are unlikely to be in any particular order.
If you don't want to construct the Cartesian product set, you can
construct an iterator and call it while it returns more members:
my $iter = Set::Scalar->cartesian_product_iterator($a, $b, $c);
while (my @m = $iter->()) {
process(@m);
}
· Power set is the set of all the subsets of a set. If the set has N
members, its power set has 2**N members. For example for the set
(a b c)
size 3, its power set is
(() (a) (b) (c) (a b) (a c) (b c) (a b c))
size 8. Note that since the elements of the power set are sets,
they are unordered, and therefore (b c) is equal to (c b). For
example:
my $a = Set::Scalar->new(1..3);
my $b = $a->power_set; # As an object method.
my $c = Set::Scalar->power_set($a); # As a class method.
Even the empty set has a power set, of size one.
If you don't want to construct the power set, you can construct an
iterator and call it until it returns no more members:
my $iter = Set::Scalar->power_set($a);
my @m;
do {
@m = $iter->();
process(@m);
} while (@m);
Customising Display
If you want to customise the display routine you will have to modify
the "as_string" callback. You can modify it either for all sets by
using "as_string_callback()" as a class method:
my $class_callback = sub { ... };
Set::Scalar->as_string_callback($class_callback);
or for specific sets by using "as_string_callback()" as an object
method:
my $callback = sub { ... };
$s1->as_string_callback($callback);
$s2->as_string_callback($callback);
The anonymous subroutine gets as its first (and only) argument the set
to display as a string. For example to display the set $s as
"a-b-c-d-e" instead of "(a b c d e)"
$s->as_string_callback(sub{join("-",sort $_[0]->elements)});
If called without an argument, the current callback is returned.
If called as a class method with undef as the only argument, the
original callback (the one returning "(a b c d e)") for all the sets is
restored, or if called for a single set the callback is removed (and
the callback for all the sets will be used).
CAVEATS
The first priority of Set::Scalar is to be a convenient interface to
sets. While not designed to be slow or big, neither has it been
designed to be fast or compact.
Using references (or objects) as set members has not been extensively
tested. The desired semantics are not always clear: what should happen
when the elements behind the references change? Especially unclear is
what should happen when the objects start having their own
stringification overloads.
SEE ALSO
Set::Bag for bags (multisets, counted sets), and Bit::Vector for fast
set operations (you have to take care of the element name to bit number
and back mappings yourself), or Set::Infinite for sets of intervals,
and many more. CPAN is your friend.
AUTHOR
Jarkko Hietaniemi <jhi@iki.fi>
COPYRIGHT AND LICENSE
Copyright 2001,2002,2003,2004,2005,2007,2009 by Jarkko Hietaniemi
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.14.0 2009-12-27 Set::Scalar(3)
