Tree::DAG_Node(3pm) User Contributed Perl Documentation Tree::DAG_Node(3pm)NAMETree::DAG_Node - (super)class for representing nodes in a tree
SYNOPSIS
Using as a base class:
package Game::Tree::Node; # or whatever you're doing
use Tree::DAG_Node;
@ISA = qw(Tree::DAG_Node);
...your own methods overriding/extending
the methods in Tree::DAG_Node...
Using as a class of its own:
use Tree::DAG_Node;
my $root = Tree::DAG_Node->new();
$root->name("I'm the tops");
my $new_daughter = $root->new_daughter;
$new_daughter->name("More");
...
DESCRIPTION
This class encapsulates/makes/manipulates objects that represent nodes
in a tree structure. The tree structure is not an object itself, but is
emergent from the linkages you create between nodes. This class
provides the methods for making linkages that can be used to build up a
tree, while preventing you from ever making any kinds of linkages which
are not allowed in a tree (such as having a node be its own mother or
ancestor, or having a node have two mothers).
This is what I mean by a "tree structure", a bit redundantly stated:
* A tree is a special case of an acyclic directed graph.
* A tree is a network of nodes where there's exactly one root node
(i.e., 'the top'), and the only primary relationship between nodes is
the mother-daugher relationship.
* No node can be its own mother, or its mother's mother, etc.
* Each node in the tree has exactly one "parent" (node in the "up"
direction) -- except the root, which is parentless.
* Each node can have any number (0 to any finite number) of daughter
nodes. A given node's daughter nodes constitute an ordered list.
(However, you are free to consider this ordering irrelevant. Some
applications do need daughters to be ordered, so I chose to consider
this the general case.)
* A node can appear in only one tree, and only once in that tree.
Notably (notable because it doesn't follow from the two above points),
a node cannot appear twice in its mother's daughter list.
* In other words, there's an idea of up (toward the root) versus down
(away from the root), and left (i.e., toward the start (index 0) of a
given node's daughter list) versus right (toward the end of a given
node's daughter list).
Trees as described above have various applications, among them:
representing syntactic constituency, in formal linguistics;
representing contingencies in a game tree; representing abstract syntax
in the parsing of any computer language -- whether in expression trees
for programming languages, or constituency in the parse of a markup
language document. (Some of these might not use the fact that
daughters are ordered.)
(Note: B-Trees are a very special case of the above kinds of trees, and
are best treated with their own class. Check CPAN for modules
encapsulating B-Trees; or if you actually want a database, and for some
reason ended up looking here, go look at AnyDBM_File.)
Many base classes are not usable except as such -- but Tree::DAG_Node
can be used as a normal class. You can go ahead and say:
use Tree::DAG_Node;
my $root = Tree::DAG_Node->new();
$root->name("I'm the tops");
$new_daughter = Tree::DAG_Node->new();
$new_daughter->name("More");
$root->add_daughter($new_daughter);
and so on, constructing and linking objects from Tree::DAG_Node and
making useful tree structures out of them.
A NOTE TO THE READER
This class is big and provides lots of methods. If your problem is
simple (say, just representing a simple parse tree), this class might
seem like using an atomic sledgehammer to swat a fly. But the
complexity of this module's bells and whistles shouldn't detract from
the efficiency of using this class for a simple purpose. In fact, I'd
be very surprised if any one user ever had use for more that even a
third of the methods in this class. And remember: an atomic
sledgehammer will kill that fly.
OBJECT CONTENTS
Implementationally, each node in a tree is an object, in the sense of
being an arbitrarily complex data structure that belongs to a class
(presumably Tree::DAG_Node, or ones derived from it) that provides
methods.
The attributes of a node-object are:
mother -- this node's mother. undef if this is a root.
daughters -- the (possibly empty) list of daughters of this node.
name -- the name for this node.
Need not be unique, or even printable. This is printed in some of
the various dumper methods, but it's up to you if you don't put
anything meaningful or printable here.
attributes -- whatever the user wants to use it for.
Presumably a hashref to whatever other attributes the user wants to
store without risk of colliding with the object's real attributes.
(Example usage: attributes to an SGML tag -- you definitely
wouldn't want the existence of a "mother=foo" pair in such a tag to
collide with a node object's 'mother' attribute.)
Aside from (by default) initializing it to {}, and having the
access method called "attributes" (described a ways below), I don't
do anything with the "attributes" in this module. I basically
intended this so that users who don't want/need to bother deriving
a class from Tree::DAG_Node, could still attach whatever data they
wanted in a node.
"mother" and "daughters" are attributes that relate to linkage -- they
are never written to directly, but are changed as appropriate by the
"linkage methods", discussed below.
The other two (and whatever others you may add in derived classes) are
simply accessed thru the same-named methods, discussed further below.
ABOUT THE DOCUMENTED INTERFACE
Stick to the documented interface (and comments in the source --
especially ones saying "undocumented!" and/or "disfavored!" -- do not
count as documentation!), and don't rely on any behavior that's not in
the documented interface.
Specifically, unless the documentation for a particular method says
"this method returns thus-and-such a value", then you should not rely
on it returning anything meaningful.
A passing acquintance with at least the broader details of the source
code for this class is assumed for anyone using this class as a base
class -- especially if you're overriding existing methods, and
definitely if you're overriding linkage methods.
MAIN CONSTRUCTOR, AND INITIALIZER
the constructor CLASS->new() or CLASS->new({...options...})
This creates a new node object, calls
$object->_init({...options...}) to provide it sane defaults (like:
undef name, undef mother, no daughters, 'attributes' setting of a
new empty hashref), and returns the object created. (If you just
said "CLASS->new()" or "CLASS->new", then it pretends you called
"CLASS->new({})".)
Currently no options for putting in {...options...} are part of the
documented interface, but the options is here in case you want to
add such behavior in a derived class.
Read on if you plan on using Tree::DAG_New as a base class.
(Otherwise feel free to skip to the description of _init.)
There are, in my mind, two ways to do object construction:
Way 1: create an object, knowing that it'll have certain
uninteresting sane default values, and then call methods to change
those values to what you want. Example:
$node = Tree::DAG_Node->new;
$node->name('Supahnode!');
$root->add_daughter($node);
$node->add_daughters(@some_others)
Way 2: be able to specify some/most/all the object's attributes in
the call to the constructor. Something like:
$node = Tree::DAG_Node->new({
name => 'Supahnode!',
mother => $root,
daughters => \@some_others
});
After some deliberation, I've decided that the second way is a Bad
Thing. First off, it is not markedly more concise than the first
way. Second off, it often requires subtly different syntax (e.g.,
\@some_others vs @some_others). It just complicates things for the
programmer and the user, without making either appreciably happier.
(This is not to say that options in general for a constructor are
bad -- "random_network", discussed far below, necessarily takes
options. But note that those are not options for the default
values of attributes.)
Anyway, if you use Tree::DAG_Node as a superclass, and you add
attributes that need to be initialized, what you need to do is
provide an _init method that calls $this->SUPER::_init($options) to
use its superclass's _init method, and then initializes the new
attributes:
sub _init {
my($this, $options) = @_[0,1];
$this->SUPER::_init($options); # call my superclass's _init to
# init all the attributes I'm inheriting
# Now init /my/ new attributes:
$this->{'amigos'} = []; # for example
}
...or, as I prefer when I'm being a neat freak:
sub _init {
my($this, $options) = @_[0,1];
$this->SUPER::_init($options);
$this->_init_amigos($options);
}
sub _init_amigos {
my $this = $_[0];
# Or my($this,$options) = @_[0,1]; if I'm using $options
$this->{'amigos'} = [];
}
In other words, I like to have each attribute initialized thru a
method named _init_[attribute], which should expect the object as
$_[0] and the the options hashref (or {} if none was given) as
$_[1]. If you insist on having your _init recognize options for
setting attributes, you might as well have them dealt with by the
appropriate _init_[attribute] method, like this:
sub _init {
my($this, $options) = @_[0,1];
$this->SUPER::_init($options);
$this->_init_amigos($options);
}
sub _init_amigos {
my($this,$options) = @_[0,1]; # I need options this time
$this->{'amigos'} = [];
$this->amigos(@{$options->{'amigos'}}) if $options->{'amigos'};
}
All this bookkeeping looks silly with just one new attribute in a
class derived straight from Tree::DAG_Node, but if there's lots of
new attributes running around, and if you're deriving from a class
derived from a class derived from Tree::DAG_Node, then tidy
stratification/modularization like this can keep you sane.
the constructor $obj->new() or $obj->new({...options...})
Just another way to get at the "new" method. This does not copy
$obj, but merely constructs a new object of the same class as it.
Saves you the bother of going $class = ref $obj; $obj2 =
$class->new;
the method $node->_init({...options...})
Initialize the object's attribute values. See the discussion
above. Presumably this should be called only by the guts of the
"new" constructor -- never by the end user.
Currently there are no documented options for putting in
{...options...}, but (in case you want to disregard the above rant)
the option exists for you to use {...options...} for something
useful in a derived class.
Please see the source for more information.
see also (below) the constructors "new_daughter" and
"new_daughter_left"
LINKAGE-RELATED METHODS
$node->daughters
This returns the (possibly empty) list of daughters for $node.
$node->mother
This returns what node is $node's mother. This is undef if $node
has no mother -- i.e., if it is a root.
$mother->add_daughters( LIST )
This method adds the node objects in LIST to the (right) end of
$mother's "daughter" list. Making a node N1 the daughter of
another node N2 also means that N1's "mother" attribute is
"automatically" set to N2; it also means that N1 stops being
anything else's daughter as it becomes N2's daughter.
If you try to make a node its own mother, a fatal error results.
If you try to take one of a a node N1's ancestors and make it also
a daughter of N1, a fatal error results. A fatal error results if
anything in LIST isn't a node object.
If you try to make N1 a daughter of N2, but it's already a daughter
of N2, then this is a no-operation -- it won't move such nodes to
the end of the list or anything; it just skips doing anything with
them.
$node->add_daughter( LIST )
An exact synonym for $node->add_daughters(LIST)
$mother->add_daughters_left( LIST )
This method is just like "add_daughters", except that it adds the
node objects in LIST to the (left) beginning of $mother's daughter
list, instead of the (right) end of it.
$node->add_daughter_left( LIST )
An exact synonym for $node->add_daughters_left( LIST )
Note:
The above link-making methods perform basically an "unshift" or
"push" on the mother node's daughter list. To get the full range
of list-handling functionality, copy the daughter list, and change
it, and then call "set_daughters" on the result:
@them = $mother->daughters;
@removed = splice(@them, 0,2, @new_nodes);
$mother->set_daughters(@them);
Or consider a structure like:
$mother->set_daughters(
grep($_->name =~ /NP/ ,
$mother->daughters
)
);
the constructor $daughter = $mother->new_daughter, or
the constructor $daughter = $mother->new_daughter({...options...})
This constructs a new node (of the same class as $mother), and adds
it to the (right) end of the daughter list of $mother. This is
essentially the same as going
$daughter = $mother->new;
$mother->add_daughter($daughter);
but is rather more efficient because (since $daughter is guaranteed
new and isn't linked to/from anything), it doesn't have to check
that $daughter isn't an ancestor of $mother, isn't already daughter
to a mother it needs to be unlinked from, isn't already in
$mother's daughter list, etc.
As you'd expect for a constructor, it returns the node-object
created.
the constructor $mother->new_daughter_left, or
$mother->new_daughter_left({...options...})
This is just like $mother->new_daughter, but adds the new daughter
to the left (start) of $mother's daughter list.
$mother->remove_daughters( LIST )
This removes the nodes listed in LIST from $mother's daughter list.
This is a no-operation if LIST is empty. If there are things in
LIST that aren't a current daughter of $mother, they are ignored.
Not to be confused with $mother->clear_daughters.
$node->remove_daughter( LIST )
An exact synonym for $node->remove_daughters( LIST )
$node->unlink_from_mother
This removes node from the daughter list of its mother. If it has
no mother, this is a no-operation.
Returns the mother unlinked from (if any).
$mother->clear_daughters
This unlinks all $mother's daughters. Returns the the list of what
used to be $mother's daughters.
Not to be confused with $mother->remove_daughters( LIST ).
$mother->set_daughters( LIST )
This unlinks all $mother's daughters, and replaces them with the
daughters in LIST.
Currently implemented as just $mother->clear_daughters followed by
$mother->add_daughters( LIST ).
$node->replace_with( LIST )
This replaces $node in its mother's daughter list, by unlinking
$node and replacing it with the items in LIST. This returns a list
consisting of $node followed by LIST, i.e., the nodes that replaced
it.
LIST can include $node itself (presumably at most once). LIST can
also be empty-list. However, if any items in LIST are sisters to
$node, they are ignored, and are not in the copy of LIST passed as
the return value.
As you might expect for any linking operation, the items in LIST
cannot be $node's mother, or any ancestor to it; and items in LIST
are, of course, unlinked from their mothers (if they have any) as
they're linked to $node's mother.
(In the special (and bizarre) case where $node is root, this simply
calls $this->unlink_from_mother on all the items in LIST, making
them roots of their own trees.)
Note that the daughter-list of $node is not necessarily affected;
nor are the daughter-lists of the items in LIST. I mention this in
case you think replace_with switches one node for another, with
respect to its mother list and its daughter list, leaving the rest
of the tree unchanged. If that's what you want, replacing $Old with
$New, then you want:
$New->set_daughters($Old->clear_daughters);
$Old->replace_with($New);
(I can't say $node's and LIST-items' daughter lists are never
affected my replace_with -- they can be affected in this case:
$N1 = ($node->daughters)[0]; # first daughter of $node
$N2 = ($N1->daughters)[0]; # first daughter of $N1;
$N3 = Tree::DAG_Node->random_network; # or whatever
$node->replace_with($N1, $N2, $N3);
As a side affect of attaching $N1 and $N2 to $node's mother,
they're unlinked from their parents ($node, and $N1, replectively).
But N3's daughter list is unaffected.
In other words, this method does what it has to, as you'd expect it
to.
$node->replace_with_daughters
This replaces $node in its mother's daughter list, by unlinking
$node and replacing it with its daughters. In other words, $node
becomes motherless and daughterless as its daughters move up and
take its place. This returns a list consisting of $node followed
by the nodes that were its daughters.
In the special (and bizarre) case where $node is root, this simply
unlinks its daughters from it, making them roots of their own
trees.
Effectively the same as $node->replace_with($node->daughters), but
more efficient, since less checking has to be done. (And I also
think $node->replace_with_daughters is a more common operation in
tree-wrangling than $node->replace_with(LIST), so deserves a named
method of its own, but that's just me.)
$node->add_left_sisters( LIST )
This adds the elements in LIST (in that order) as immediate left
sisters of $node. In other words, given that B's mother's
daughter-list is (A,B,C,D), calling B->add_left_sisters(X,Y) makes
B's mother's daughter-list (A,X,Y,B,C,D).
If LIST is empty, this is a no-op, and returns empty-list.
This is basically implemented as a call to
$node->replace_with(LIST, $node), and so all replace_with's
limitations and caveats apply.
The return value of $node->add_left_sisters( LIST ) is the elements
of LIST that got added, as returned by replace_with -- minus the
copies of $node you'd get from a straight call to
$node->replace_with(LIST, $node).
$node->add_left_sister( LIST )
An exact synonym for $node->add_left_sisters(LIST)
$node->add_right_sisters( LIST )
Just like add_left_sisters (which see), except that the the
elements in LIST (in that order) as immediate right sisters of
$node;
In other words, given that B's mother's daughter-list is (A,B,C,D),
calling B->add_right_sisters(X,Y) makes B's mother's daughter-list
(A,B,X,Y,C,D).
$node->add_right_sister( LIST )
An exact synonym for $node->add_right_sisters(LIST)OTHER ATTRIBUTE METHODS
$node->name or $node->name(SCALAR)
In the first form, returns the value of the node object's "name"
attribute. In the second form, sets it to the value of SCALAR.
$node->attributes or $node->attributes(SCALAR)
In the first form, returns the value of the node object's
"attributes" attribute. In the second form, sets it to the value
of SCALAR. I intend this to be used to store a reference to a
(presumably anonymous) hash the user can use to store whatever
attributes he doesn't want to have to store as object attributes.
In this case, you needn't ever set the value of this. (_init has
already initialized it to {}.) Instead you can just do...
$node->attributes->{'foo'} = 'bar';
...to write foo => bar.
$node->attribute or $node->attribute(SCALAR)
An exact synonym for $node->attributes or $node->attributes(SCALAR)OTHER METHODS TO DO WITH RELATIONSHIPS
$node->is_node
This always returns true. More pertinently,
$object->can('is_node') is true (regardless of what "is_node" would
do if called) for objects belonging to this class or for any class
derived from it.
$node->ancestors
Returns the list of this node's ancestors, starting with its
mother, then grandmother, and ending at the root. It does this by
simply following the 'mother' attributes up as far as it can. So
if $item IS the root, this returns an empty list.
Consider that scalar($node->ancestors) returns the ply of this node
within the tree -- 2 for a granddaughter of the root, etc., and 0
for root itself.
$node->root
Returns the root of whatever tree $node is a member of. If $node
is the root, then the result is $node itself.
$node->is_daughter_of($node2)
Returns true iff $node is a daughter of $node2. Currently
implemented as just a test of ($it->mother eq $node2).
$node->self_and_descendants
Returns a list consisting of itself (as element 0) and all the
descendants of $node. Returns just itself if $node is a
terminal_node.
(Note that it's spelled "descendants", not "descendents".)
$node->descendants
Returns a list consisting of all the descendants of $node. Returns
empty-list if $node is a terminal_node.
(Note that it's spelled "descendants", not "descendents".)
$node->leaves_under
Returns a list (going left-to-right) of all the leaf nodes under
$node. ("Leaf nodes" are also called "terminal nodes" -- i.e.,
nodes that have no daughters.) Returns $node in the degenerate
case of $node being a leaf itself.
$node->depth_under
Returns an integer representing the number of branches between this
$node and the most distant leaf under it. (In other words, this
returns the ply of subtree starting of $node. Consider
scalar($it->ancestors) if you want the ply of a node within the
whole tree.)
$node->generation
Returns a list of all nodes (going left-to-right) that are in
$node's generation -- i.e., that are the some number of nodes down
from the root. $root->generation is just $root.
Of course, $node is always in its own generation.
$node->generation_under(NODE2)
Like $node->generation, but returns only the nodes in $node's
generation that are also descendants of NODE2 -- in other words,
@us = $node->generation_under( $node->mother->mother );
is all $node's first cousins (to borrow yet more kinship
terminology) -- assuming $node does indeed have a grandmother.
Actually "cousins" isn't quite an apt word, because @us ends up
including $node's siblings and $node.
Actually, "generation_under" is just an alias to "generation", but
I figure that this:
@us = $node->generation_under($way_upline);
is a bit more readable than this:
@us = $node->generation($way_upline);
But it's up to you.
$node->generation_under($node) returns just $node.
If you call $node->generation_under($node) but NODE2 is not $node
or an ancestor of $node, it behaves as if you called just
$node->generation().
$node->self_and_sisters
Returns a list of all nodes (going left-to-right) that have the
same mother as $node -- including $node itself. This is just like
$node->mother->daughters, except that that fails where $node is
root, whereas $root->self_and_siblings, as a special case, returns
$root.
(Contrary to how you may interpret how this method is named, "self"
is not (necessarily) the first element of what's returned.)
$node->sisters
Returns a list of all nodes (going left-to-right) that have the
same mother as $node -- not including $node itself. If $node is
root, this returns empty-list.
$node->left_sister
Returns the node that's the immediate left sister of $node. If
$node is the leftmost (or only) daughter of its mother (or has no
mother), then this returns undef.
(See also $node->add_left_sisters(LIST).)
$node->left_sisters
Returns a list of nodes that're sisters to the left of $node. If
$node is the leftmost (or only) daughter of its mother (or has no
mother), then this returns an empty list.
(See also $node->add_left_sisters(LIST).)
$node->right_sister
Returns the node that's the immediate right sister of $node. If
$node is the rightmost (or only) daughter of its mother (or has no
mother), then this returns undef.
(See also $node->add_right_sisters(LIST).)
$node->right_sisters
Returns a list of nodes that're sisters to the right of $node. If
$node is the rightmost (or only) daughter of its mother (or has no
mother), then this returns an empty list.
(See also $node->add_right_sisters(LIST).)
$node->my_daughter_index
Returns what index this daughter is, in its mother's "daughter"
list. In other words, if $node is ($node->mother->daughters)[3],
then $node->my_daughter_index returns 3.
As a special case, returns 0 if $node has no mother.
$node->address or $anynode->address(ADDRESS)
With the first syntax, returns the address of $node within its
tree, based on its position within the tree. An address is formed
by noting the path between the root and $node, and concatenating
the daughter-indices of the nodes this passes thru (starting with 0
for the root, and ending with $node).
For example, if to get from node ROOT to node $node, you pass thru
ROOT, A, B, and $node, then the address is determined as:
* ROOT's my_daughter_index is 0.
* A's my_daughter_index is, suppose, 2. (A is index 2 in ROOT's
daughter list.)
* B's my_daughter_index is, suppose, 0. (B is index 0 in A's
daughter list.)
* $node's my_daughter_index is, suppose, 4. ($node is index 4 in
B's daughter list.)
The address of the above-described $node is, therefore, "0:2:0:4".
(As a somewhat special case, the address of the root is always "0";
and since addresses start from the root, all addresses start with a
"0".)
The second syntax, where you provide an address, starts from the
root of the tree $anynode belongs to, and returns the node
corresponding to that address. Returns undef if no node
corresponds to that address. Note that this routine may be
somewhat liberal in its interpretation of what can constitute an
address; i.e., it accepts "0.2.0.4", besides "0:2:0:4".
Also note that the address of a node in a tree is meaningful only
in that tree as currently structured.
(Consider how ($address1 cmp $address2) may be magically meaningful
to you, if you mant to figure out what nodes are to the right of
what other nodes.)
$node->common(LIST)
Returns the lowest node in the tree that is ancestor-or-self to the
nodes $node and LIST.
If the nodes are far enough apart in the tree, the answer is just
the root.
If the nodes aren't all in the same tree, the answer is undef.
As a degenerate case, if LIST is empty, returns $node.
$node->common_ancestor(LIST)
Returns the lowest node that is ancestor to all the nodes given (in
nodes $node and LIST). In other words, it answers the question:
"What node in the tree, as low as possible, is ancestor to the
nodes given ($node and LIST)?"
If the nodes are far enough apart, the answer is just the root --
except if any of the nodes are the root itself, in which case the
answer is undef (since the root has no ancestor).
If the nodes aren't all in the same tree, the answer is undef.
As a degenerate case, if LIST is empty, returns $node's mother;
that'll be undef if $node is root.
YET MORE METHODS
$node->walk_down({ callback => \&foo, callbackback => \&foo, ... })
Performs a depth-first traversal of the structure at and under
$node. What it does at each node depends on the value of the
options hashref, which you must provide. There are three options,
"callback" and "callbackback" (at least one of which must be
defined, as a sub reference), and "_depth". This is what
"walk_down" does, in pseudocode form:
* Start at the $node given.
* If there's a "callback", call it with $node as the first
argument, and the options hashref as the second argument (which
contains the potentially useful "_depth", remember). This function
must return true or false -- if false, it will block the next step:
* If $node has any daughter nodes, increment "_depth", and call
$daughter->walk_down(options_hashref) for each daughter (in order,
of course), where options_hashref is the same hashref it was called
with. When this returns, decrements "_depth".
* If there's a "callbackback", call just it as with "callback" (but
tossing out the return value). Note that "callback" returning
false blocks traversal below $node, but doesn't block calling
callbackback for $node. (Incidentally, in the unlikely case that
$node has stopped being a node object, "callbackback" won't get
called.)
* Return.
$node->walk_down is the way to recursively do things to a tree (if
you start at the root) or part of a tree; if what you're doing is
best done via pre-pre order traversal, use "callback"; if what
you're doing is best done with post-order traversal, use
"callbackback". "walk_down" is even the basis for plenty of the
methods in this class. See the source code for examples both
simple and horrific.
Note that if you don't specify "_depth", it effectively defaults to
0. You should set it to scalar($node->ancestors) if you want
"_depth" to reflect the true depth-in-the-tree for the nodes
called, instead of just the depth below $node. (If $node is the
root, there's difference, of course.)
And by the way, it's a bad idea to modify the tree from the
callback. Unpredictable things may happen. I instead suggest
having your callback add to a stack of things that need changing,
and then, once "walk_down" is all finished, changing those nodes
from that stack.
Note that the existence of "walk_down" doesn't mean you can't write
you own special-use traversers.
@lines = $node->dump_names({ ...options... });
Dumps, as an indented list, the names of the nodes starting at
$node, and continuing under it. Options are:
* _depth -- A nonnegative number. Indicating the depth to consider
$node as being at (and so the generation under that is that plus
one, etc.). Defaults to 0. You may choose to use set _depth =>
scalar($node->ancestors).
* tick -- a string to preface each entry with, between the
indenting-spacing and the node's name. Defaults to empty-string.
You may prefer "*" or "-> " or someting.
* indent -- the string used to indent with. Defaults to " " (two
spaces). Another sane value might be ". " (period, space).
Setting it to empty-string suppresses indenting.
The dump is not printed, but is returned as a list, where each item
is a line, with a "\n" at the end.
the constructor CLASS->random_network({...options...})
the method $node->random_network({...options...})
In the first case, constructs a randomly arranged network under a
new node, and returns the root node of that tree. In the latter
case, constructs the network under $node.
Currently, this is implemented a bit half-heartedly, and half-
wittedly. I basically needed to make up random-looking networks to
stress-test the various tree-dumper methods, and so wrote this. If
you actually want to rely on this for any application more serious
than that, I suggest examining the source code and seeing if this
does really what you need (say, in reliability of randomness); and
feel totally free to suggest changes to me (especially in the form
of "I rewrote "random_network", here's the code...")
It takes four options:
* max_node_count -- maximum number of nodes this tree will be
allowed to have (counting the root). Defaults to 25.
* min_depth -- minimum depth for the tree. Defaults to 2. Leaves
can be generated only after this depth is reached, so the tree will
be at least this deep -- unless max_node_count is hit first.
* max_depth -- maximum depth for the tree. Defaults to 3 plus
min_depth. The tree will not be deeper than this.
* max_children -- maximum number of children any mother in the tree
can have. Defaults to 4.
the constructor CLASS->lol_to_tree($lol);
Converts something like bracket-notation for "Chomsky trees" (or
rather, the closest you can come with Perl
list-of-lists(-of-lists(-of-lists))) into a tree structure.
Returns the root of the tree converted.
The conversion rules are that: 1) if the last (possibly the only)
item in a given list is a scalar, then that is used as the "name"
attribute for the node based on this list. 2) All other items in
the list represent daughter nodes of the current node --
recursively so, if they are list references; otherwise, (non-
terminal) scalars are considered to denote nodes with that name.
So ['Foo', 'Bar', 'N'] is an alternate way to represent [['Foo'],
['Bar'], 'N'].
An example will illustrate:
use Tree::DAG_Node;
$lol =
[
[
[ [ 'Det:The' ],
[ [ 'dog' ], 'N'], 'NP'],
[ '/with rabies\\', 'PP'],
'NP'
],
[ 'died', 'VP'],
'S'
];
$tree = Tree::DAG_Node->lol_to_tree($lol);
$diagram = $tree->draw_ascii_tree;
print map "$_\n", @$diagram;
...returns this tree:
|
<S>
|
/------------------\
| |
<NP> <VP>
| |
/---------------\ <died>
| |
<NP> <PP>
| |
/-------\ </with rabies\>
| |
<Det:The> <N>
|
<dog>
By the way (and this rather follows from the above rules), when
denoting a LoL tree consisting of just one node, this:
$tree = Tree::DAG_Node->lol_to_tree( 'Lonely' );
is okay, although it'd probably occur to you to denote it only as:
$tree = Tree::DAG_Node->lol_to_tree( ['Lonely'] );
which is of course fine, too.
$node->tree_to_lol_notation({...options...})
Dumps a tree (starting at $node) as the sort of LoL-like bracket
notation you see in the above example code. Returns just one big
block of text. The only option is "multiline" -- if true, it dumps
the text as the sort of indented structure as seen above; if false
(and it defaults to false), dumps it all on one line (with no
indenting, of course).
For example, starting with the tree from the above example, this:
print $tree->tree_to_lol_notation, "\n";
prints the following (which I've broken over two lines for sake of
printablitity of documentation):
[[[['Det:The'], [['dog'], 'N'], 'NP'], [["/with rabies\x5c"],
'PP'], 'NP'], [['died'], 'VP'], 'S'],
Doing this:
print $tree->tree_to_lol_notation({ multiline => 1 });
prints the same content, just spread over many lines, and prettily
indented.
$node->tree_to_lol
Returns that tree (starting at $node) represented as a LoL, like
what $lol, above, holds. (This is as opposed to
"tree_to_lol_notation", which returns the viewable code like what
gets evaluated and stored in $lol, above.)
Lord only knows what you use this for -- maybe for feeding to
Data::Dumper, in case "tree_to_lol_notation" doesn't do just what
you want?
the constructor CLASS->simple_lol_to_tree($simple_lol);
This is like lol_to_tree, except that rule 1 doesn't apply -- i.e.,
all scalars (or really, anything not a listref) in the LoL-
structure end up as named terminal nodes, and only terminal nodes
get names (and, of course, that name comes from that scalar value).
This method is useful for making things like expression trees, or
at least starting them off. Consider that this:
$tree = Tree::DAG_Node->simple_lol_to_tree(
[ 'foo', ['bar', ['baz'], 'quux'], 'zaz', 'pati' ]
);
converts from something like a Lispish or Iconish tree, if you
pretend the brackets are parentheses.
Note that there is a (possibly surprising) degenerate case of what
I'm calling a "simple-LoL", and it's like this:
$tree = Tree::DAG_Node->simple_lol_to_tree('Lonely');
This is the (only) way you can specify a tree consisting of only a
single node, which here gets the name 'Lonely'.
$node->tree_to_simple_lol
Returns that tree (starting at $node) represented as a simple-LoL
-- i.e., one where non-terminal nodes are represented as listrefs,
and terminal nodes are gotten from the contents of those nodes'
"name' attributes.
Note that in the case of $node being terminal, what you get back is
the same as $node->name.
Compare to tree_to_simple_lol_notation.
$node->tree_to_simple_lol_notation({...options...})
A simple-LoL version of tree_to_lol_notation (which see); takes the
same options.
$list_r = $node->draw_ascii_tree({ ... options ... })
Draws a nice ASCII-art representation of the tree structure at-and-
under $node, with $node at the top. Returns a reference to the
list of lines (with no "\n"s or anything at the end of them) that
make up the picture.
Example usage:
print map("$_\n", @{$tree->draw_ascii_tree});
draw_ascii_tree takes parameters you set in the options hashref:
* "no_name" -- if true, "draw_ascii_tree" doesn't print the name of
the node; simply prints a "*". Defaults to 0 (i.e., print the node
name.)
* "h_spacing" -- number 0 or greater. Sets the number of spaces
inserted horizontally between nodes (and groups of nodes) in a
tree. Defaults to 1.
* "h_compact" -- number 0 or 1. Sets the extent to which
"draw_ascii_tree" tries to save horizontal space. Defaults to 1.
If I think of a better scrunching algorithm, there'll be a "2"
setting for this.
* "v_compact" -- number 0, 1, or 2. Sets the degree to which
"draw_ascii_tree" tries to save vertical space. Defaults to 1.
This occasionally returns trees that are a bit cock-eyed in parts;
if anyone can suggest a better drawing algorithm, I'd be
appreciative.
$node->copy_tree or $node->copy_tree({...options...})
This returns the root of a copy of the tree that $node is a member
of. If you pass no options, copy_tree pretends you've passed {}.
This method is currently implemented as just a call to
$this->root->copy_at_and_under({...options...}), but magic may be
added in the future.
Options you specify are passed down to calls to $node->copy.
$node->copy_at_and_under or $node->copy_at_and_under({...options...})
This returns a copy of the subtree consisting of $node and
everything under it.
If you pass no options, copy_at_and_under pretends you've passed
{}.
This works by recursively building up the new tree from the leaves,
duplicating nodes using $orig_node->copy($options_ref) and then
linking them up into a new tree of the same shape.
Options you specify are passed down to calls to $node->copy.
the constructor $node->copy or $node->copy({...options...})
Returns a copy of $node, minus its daughter or mother attributes
(which are set back to default values).
If you pass no options, "copy" pretends you've passed {}.
Magic happens with the 'attributes' attribute: if it's a hashref
(and it usually is), the new node doesn't end up with the same
hashref, but with ref to a hash with the content duplicated from
the original's hashref. If 'attributes' is not a hashref, but
instead an object that belongs to a class that provides a method
called "copy", then that method is called, and the result saved in
the clone's 'attribute' attribute. Both of these kinds of magic
are disabled if the options you pass to "copy" (maybe via
"copy_tree", or "copy_at_and_under") includes ("no_attribute_copy"
=> 1).
The options hashref you pass to "copy" (derictly or indirectly)
gets changed slightly after you call "copy" -- it gets an entry
called "from_to" added to it. Chances are you would never know nor
care, but this is reserved for possible future use. See the source
if you are wildly curious.
Note that if you are using $node->copy (whether directly or via
$node->copy_tree or $node->copy_at_or_under), and it's not properly
copying object attributes containing references, you probably
shouldn't fight it or try to fix it -- simply override copy_tree
with:
sub copy_tree {
use Storable qw(dclone);
my $this = $_[0];
return dclone($this->root);
# d for "deep"
}
or
sub copy_tree {
use Data::Dumper;
my $this = $_[0];
$Data::Dumper::Purity = 1;
return eval(Dumper($this->root));
}
Both of these avoid you having to reinvent the wheel.
How to override copy_at_or_under with something that uses Storable
or Data::Dumper is left as an exercise to the reader.
Consider that if in a derived class, you add attributes with really
bizarre contents (like a unique-for-all-time-ID), you may need to
override "copy". Consider:
sub copy {
my($it, @etc) = @_;
$it->SUPER::copy(@etc);
$it->{'UID'} = &get_new_UID;
}
...or the like. See the source of Tree::DAG_Node::copy for
inspiration.
$node->delete_tree
Destroys the entire tree that $node is a member of (starting at the
root), by nulling out each node-object's attributes (including,
most importantly, its linkage attributes -- hopefully this is more
than sufficient to eliminate all circularity in the data
structure), and then moving it into the class DEADNODE.
Use this when you're finished with the tree in question, and want
to free up its memory. (If you don't do this, it'll get freed up
anyway when your program ends.)
If you try calling any methods on any of the node objects in the
tree you've destroyed, you'll get an error like:
Can't locate object method "leaves_under"
via package "DEADNODE".
So if you see that, that's what you've done wrong. (Actually, the
class DEADNODE does provide one method: a no-op method
"delete_tree". So if you want to delete a tree, but think you may
have deleted it already, it's safe to call $node->delete_tree on it
(again).)
The "delete_tree" method is needed because Perl's garbage collector
would never (as currently implemented) see that it was time to de-
allocate the memory the tree uses -- until either you call
$node->delete_tree, or until the program stops (at "global
destruction" time, when everything is unallocated).
Incidentally, there are better ways to do garbage-collecting on a
tree, ways which don't require the user to explicitly call a method
like "delete_tree" -- they involve dummy classes, as explained at
"http://mox.perl.com/misc/circle-destroy.pod"
However, introducing a dummy class concept into Tree::DAG_Node
would be rather a distraction. If you want to do this with your
derived classes, via a DESTROY in a dummy class (or in a tree-
metainformation class, maybe), then feel free to.
The only case where I can imagine "delete_tree" failing to totally
void the tree, is if you use the hashref in the "attributes"
attribute to store (presumably among other things) references to
other nodes' "attributes" hashrefs -- which 1) is maybe a bit odd,
and 2) is your problem, because it's your hash structure that's
circular, not the tree's. Anyway, consider:
# null out all my "attributes" hashes
$anywhere->root->walk_down({
'callback' => sub {
$hr = $_[0]->attributes; %$hr = (); return 1;
}
});
# And then:
$anywhere->delete_tree;
(I suppose "delete_tree" is a "destructor", or as close as you can
meaningfully come for a circularity-rich data structure in Perl.)
When and How to Destroy
It should be clear to you that if you've built a big parse tree or
something, and then you're finished with it, you should call
$some_node->delete_tree on it if you want the memory back.
But consider this case: you've got this tree:
A
/ | \
B C D
| | \
E X Y
Let's say you decide you don't want D or any of its descendants in the
tree, so you call D->unlink_from_mother. This does NOT automagically
destroy the tree D-X-Y. Instead it merely splits the tree into two:
A D
/ \ / \
B C X Y
|
E
To destroy D and its little tree, you have to explicitly call
delete_tree on it.
Note, however, that if you call C->unlink_from_mother, and if you don't
have a link to C anywhere, then it does magically go away. This is
because nothing links to C -- whereas with the D-X-Y tree, D links to X
and Y, and X and Y each link back to D. Note that calling
C->delete_tree is harmless -- after all, a tree of only one node is
still a tree.
So, this is a surefire way of getting rid of all $node's children and
freeing up the memory associated with them and their descendants:
foreach my $it ($node->clear_daughters) { $it->delete_tree }
Just be sure not to do this:
foreach my $it ($node->daughters) { $it->delete_tree }
$node->clear_daughters;
That's bad; the first call to $_->delete_tree will climb to the root of
$node's tree, and nuke the whole tree, not just the bits under $node.
You might as well have just called $node->delete_tree. (Moreavor, once
$node is dead, you can't call clear_daughters on it, so you'll get an
error there.)
BUG REPORTS
If you find a bug in this library, report it to me as soon as possible,
at the address listed in the MAINTAINER section, below. Please try to
be as specific as possible about how you got the bug to occur.
HELP!
If you develop a given routine for dealing with trees in some way, and
use it a lot, then if you think it'd be of use to anyone else, do email
me about it; it might be helpful to others to include that routine, or
something based on it, in a later version of this module.
It's occurred to me that you might like to (and might yourself develop
routines to) draw trees in something other than ASCII art. If you do
so -- say, for PostScript output, or for output interpretable by some
external plotting program -- I'd be most interested in the results.
RAMBLINGS
This module uses "strict", but I never wrote it with -w warnings in
mind -- so if you use -w, do not be surprised if you see complaints
from the guts of DAG_Node. As long as there is no way to turn off -w
for a given module (instead of having to do it in every single
subroutine with a "local $^W"), I'm not going to change this. However,
I do, at points, get bursts of ambition, and I try to fix code in
DAG_Node that generates warnings, as I come across them -- which is
only occasionally. Feel free to email me any patches for any such
fixes you come up with, tho.
Currently I don't assume (or enforce) anything about the class
membership of nodes being manipulated, other than by testing whether
each one provides a method "is_node", a la:
die "Not a node!!!" unless UNIVERSAL::can($node, "is_node");
So, as far as I'm concerned, a given tree's nodes are free to belong to
different classes, just so long as they provide/inherit "is_node", the
few methods that this class relies on to navigate the tree, and have
the same internal object structure, or a superset of it. Presumably
this would be the case for any object belonging to a class derived from
"Tree::DAG_Node", or belonging to "Tree::DAG_Node" itself.
When routines in this class access a node's "mother" attribute, or its
"daughters" attribute, they (generally) do so directly (via
$node->{'mother'}, etc.), for sake of efficiency. But classes derived
from this class should probably do this instead thru a method (via
$node->mother, etc.), for sake of portability, abstraction, and general
goodness.
However, no routines in this class (aside from, necessarily, "_init",
"_init_name", and "name") access the "name" attribute directly;
routines (like the various tree draw/dump methods) get the "name" value
thru a call to $obj->name(). So if you want the object's name to not
be a real attribute, but instead have it derived dynamically from some
feature of the object (say, based on some of its other attributes, or
based on its address), you can to override the "name" method, without
causing problems. (Be sure to consider the case of $obj->name as a
write method, as it's used in "lol_to_tree" and "random_network".)
SEE ALSO
HTML::Element
Wirth, Niklaus. 1976. Algorithms + Data Structures = Programs
Prentice-Hall, Englewood Cliffs, NJ.
Knuth, Donald Ervin. 1997. Art of Computer Programming, Volume 1,
Third Edition: Fundamental Algorithms. Addison-Wesley, Reading, MA.
Wirth's classic, currently and lamentably out of print, has a good
section on trees. I find it clearer than Knuth's (if not quite as
encyclopedic), probably because Wirth's example code is in a block-
structured high-level language (basically Pascal), instead of in
assembler (MIX).
Until some kind publisher brings out a new printing of Wirth's book,
try poking around used bookstores (or "www.abebooks.com") for a copy.
I think it was also republished in the 1980s under the title Algorithms
and Data Structures, and in a German edition called Algorithmen und
Datenstrukturen. (That is, I'm sure books by Knuth were published
under those titles, but I'm assuming that they're just later
printings/editions of Algorithms + Data Structures = Programs.)
MAINTAINER
David Hand, "<cogent@cpan.org>"
AUTHOR
Sean M. Burke, "<sburke@cpan.org>"
COPYRIGHT, LICENSE, AND DISCLAIMER
Copyright 1998-2001, 2004, 2007 by Sean M. Burke and David Hand.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
This program is distributed in the hope that it will be useful, but
without any warranty; without even the implied warranty of
merchantability or fitness for a particular purpose.
perl v5.10.1 2007-12-09 Tree::DAG_Node(3pm)
