struct::tree - Create and manipulate tree objects
A tree is a collection of named elements, called nodes, one of which is distinguished as a root, along with a relation ("parenthood") that places a hierarchical structure on the nodes. (Data Structures and Algorithms; Aho, Hopcroft and Ullman; Addison-Wesley, 1987). In addition to maintaining the node relationships, this tree implementation allows any number of keyed values to be associated with each node.
The element names can be arbitrary strings.
A tree is thus similar to an array, but with three important differences:
Trees are accessed through an object command, whereas arrays are accessed as variables. (This means trees cannot be local to a procedure.)
Trees have a hierarchical structure, whereas an array is just an unordered collection.
Each node of a tree has a separate collection of attributes and values. This is like an array where every value is a dictionary.
Note: The major version of the package struct has been changed to version 2.0, due to backward incompatible changes in the API of this module. Please read the section Changes for 2.0 for a full list of all changes, incompatible and otherwise.
The main commands of the package are:
The command creates a new tree object with an associated global Tcl command whose name is treeName. This command may be used to invoke various operations on the tree. It has the following general form:
Option and the args determine the exact behavior of the command.
If treeName is not specified a unique name will be generated by the package itself. If a source is specified the new tree will be initialized to it. For the operators =, :=, and as source is interpreted as the name of another tree object, and the assignment operator = will be executed. For deserialize the source is a serialized tree object and deserialize will be executed.
In other words
::struct::tree mytree = b
is equivalent to
::struct::tree mytree
mytree = b
and
::struct::tree mytree deserialize $b
is equivalent to
::struct::tree mytree
mytree deserialize $b
This command is provided outside of the tree methods, as it is not a tree method per se. It however interacts tightly with the method walk. When used in the walk script it causes the traversal to ignore the children of the node we are currently at. This command cannot be used with the traversal modes which look at children before their parent, i.e. post and in. The only applicable orders of traversal are pre and both. An error is thrown if the command and chosen order of traversal do not fit.
Two general observations beforehand:
The root node of the tree can be used in most places where a node is asked for. The default name of the rootnode is "root", but this can be changed with the method rename (see below). Whatever the current name for the root node of the tree is, it can be retrieved by calling the method rootname.
The method insert is the only way to create new nodes, and they are automatically added to a parent. A tree object cannot have nodes without a parent, save the root node.
And now the methods supported by tree objects created by this package:
This is the assignment operator for tree objects. It copies the tree contained in the tree object sourcetree over the tree data in treeName. The old contents of treeName are deleted by this operation.
This operation is in effect equivalent to
treeName deserialize [sourcetree serialize]
This is the reverse assignment operator for tree objects. It copies the tree contained in the tree object treeName over the tree data in the object desttree. The old contents of desttree are deleted by this operation.
This operation is in effect equivalent to
desttree deserialize [treeName serialize]
This method extends the method parent and returns a list containing all ancestor nodes to the specified node. The immediate ancestor, in other words, parent node, is the first element in that list, its parent the second element, and so on until the root node is reached, making it the last element of the returned list.
Appends a value to one of the keyed values associated with an node. Returns the new value given to the attribute key.
This method retrieves the value of the attribute named key, for all nodes in the tree (matching the restriction specified via one of the possible options) and having the specified attribute.
The result is a dictionary mapping from node names to the value of attribute key at that node. Nodes not having the attribute key, or not passing a specified restriction, are not listed in the result.
The possible restrictions are:
Return a list of the children of node. If the option -all is specified, then not only the direct children, but their children, and so on are returned in the result. If a filter command is specified only those nodes are listed in the final result which pass the test. The command in cmdprefix is called with two arguments, the name of the tree object, and the name of the node in question. It is executed in the context of the caller and has to return a boolean value. Nodes for which the command returns false are removed from the result list before it is returned to the caller.
Some examples:
mytree insert root end 0 ; mytree set 0 volume 30
mytree insert root end 1
mytree insert root end 2
mytree insert 0 end 3
mytree insert 0 end 4
mytree insert 4 end 5 ; mytree set 5 volume 50
mytree insert 4 end 6
proc vol {t n} {
$t keyexists $n volume
}
proc vgt40 {t n} {
if {![$t keyexists $n volume]} {return 0}
expr {[$t get $n volume] > 40}
}
tclsh> lsort [mytree children -all root filter vol]
0 5
tclsh> lsort [mytree children -all root filter vgt40]
5
tclsh> lsort [mytree children root filter vol]
0
tclsh> puts ([lsort [mytree children root filter vgt40]])
()
Removes the node specified by node from the tree, but not its children. The children of node are made children of the parent of the node, at the index at which node was located.
Remove the specified nodes from the tree. All of the nodes' children will be removed as well to prevent orphaned nodes.
Return the number of steps from node node to the root node.
This method extends the method children and returns a list containing all nodes descending from node, and passing the filter, if such was specified.
This is actually the same as "treeName children -all". descendants should be prefered, and "children -all" will be deprecated sometime in the future.
This is the complement to serialize. It replaces tree data in treeName with the tree described by the serialization value. The old contents of treeName are deleted by this operation.
Destroy the tree, including its storage space and associated command.
Remove true if the specified node exists in the tree.
Returns the value associated with the key key for the node node.
Returns a dictionary (suitable for use with [array set]) containing the attribute data for the node. If the glob pattern is specified only the attributes whose names match the pattern will be part of the dictionary.
Returns a list of keys for the node. If the pattern is specified only the attributes whose names match the pattern will be part of the returned list. The pattern is a glob pattern.
Return true if the specified key exists for the node.
Returns the index of node in its parent's list of children. For example, if a node has nodeFoo, nodeBar, and nodeBaz as children, in that order, the index of nodeBar is 1.
Insert one or more nodes into the tree as children of the node parent. The nodes will be added in the order they are given. If parent is root, it refers to the root of the tree. The new nodes will be added to the parent node's child list at the index given by index. The index can be end in which case the new nodes will be added after the current last child. Indices of the form "end-n" are accepted as well.
If any of the specified children already exist in treeName, those nodes will be moved from their original location to the new location indicated by this command.
If no child is specified, a single node will be added, and a name will be generated for the new node. The generated name is of the form nodex, where x is a number. If names are specified they must neither contain whitespace nor colons (":").
The return result from this command is a list of nodes added.
Returns true if node is a leaf of the tree (if node has no children), false otherwise.
Appends a value (as a list) to one of the keyed values associated with an node. Returns the new value given to the attribute key.
Return a list containing all leaf nodes known to the tree.
Make the specified nodes children of parent, inserting them into the parent's child list at the index given by index. Note that the command will take all nodes out of the tree before inserting them under the new parent, and that it determines the position to place them into after the removal, before the re-insertion. This behaviour is important when it comes to moving one or more nodes to a different index without changing their parent node.
Return the right sibling of node, or the empty string if node was the last child of its parent.
Return the number of immediate children of node.
Return a list containing all nodes known to the tree.
Return the parent of node.
Return the left sibling of node, or the empty string if node was the first child of its parent.
Renames the node node to newname. An error is thrown if either the node does not exist, or a node with name newname does exist. The result of the command is the new name of the node.
Returns the name of the root node of the tree.
This method serializes the sub-tree starting at node. In other words it returns a tcl value completely describing the tree starting at node. This allows, for example, the transfer of tree objects (or parts thereof) over arbitrary channels, persistence, etc. This method is also the basis for both the copy constructor and the assignment operator.
The result of this method has to be semantically identical over all implementations of the tree interface. This is what will enable us to copy tree data between different implementations of the same interface.
The result is a list containing containing a multiple of three elements. It is like a serialized array except that there are two values following each key. They are the names of the nodes in the serialized tree. The two values are a reference to the parent node and the attribute data, in this order.
The reference to the parent node is the empty string for the root node of the tree. For all other nodes it is the index of the parent node in the list. This means that they are integers, greater than or equal to zero, less than the length of the list, and multiples of three. The order of the nodes in the list is important insofar as it is used to reconstruct the lists of children for each node. The children of a node have to be listed in the serialization in the same order as they are listed in their parent in the tree.
The attribute data of a node is a dictionary, i.e. a list of even length containing a serialized array. For a node without attribute data the dictionary is the empty list.
Note: While the current implementation returns the root node as the first element of the list, followed by its children and their children in a depth-first traversal this is not necessarily true for other implementations. The only information a reader of the serialized data can rely on for the structure of the tree is that the root node is signaled by the empty string for the parent reference, that all other nodes refer to their parent through the index in the list, and that children occur in the same order as in their parent.
A possible serialization for the tree structure
+- d
+- a -+
root -+- b +- e
+- c
is
{root {} {} a 0 {} d 3 {} e 3 {} b 0 {} c 0 {}}
The above assumes that none of the nodes have attributes.
Set or get one of the keyed values associated with a node. A node may have any number of keyed values associated with it. If value is not specified, this command returns the current value assigned to the key; if value is specified, this command assigns that value to the key, and returns it.
Return a count of the number of descendants of the node node; if no node is specified, root is assumed.
Insert a node named child into the tree as a child of the node parent. If parent is root, it refers to the root of the tree. The new node will be added to the parent node's child list at the index given by from. The children of parent which are in the range of the indices from and to are made children of child. If the value of to is not specified it defaults to end. If no name is given for child, a name will be generated for the new node. The generated name is of the form nodex, where x is a number. The return result from this command is the name of the new node.
The arguments from and to are regular list indices, i.e. the form "end-n" is accepted as well.
Swap the position of node1 and node2 in the tree.
Remove a keyed value from the node node. The method will do nothing if the key does not exist.
Perform a breadth-first or depth-first walk of the tree starting at the node node. The type of walk, breadth-first or depth-first, is determined by the value of type; bfs indicates breadth-first, dfs indicates depth-first. Depth-first is the default. The order of the walk, pre-, post-, both- or in-order is determined by the value of order; pre indicates pre-order, post indicates post-order, both indicates both-order and in indicates in-order. Pre-order is the default.
Pre-order walking means that a parent node is visited before any of its children. For example, a breadth-first search starting from the root will visit the root, followed by all of the root's children, followed by all of the root's grandchildren. Post-order walking means that a parent node is visited after any of its children. Both-order walking means that a parent node is visited before and after any of its children. In-order walking means that a parent node is visited after its first child and before the second. This is a generalization of in-order walking for binary trees and will do the right thing if a binary tree is walked. The combination of a breadth-first walk with in-order is illegal.
As the walk progresses, the script will be evaluated at each node. The evaluation takes place in the context of the caller of the method. Regarding loop variables, these are listed in loopvar. If one only one variable is specified it will be set to the id of the node. When two variables are specified, i.e. loopvar is a true list, then the first variable will be set to the action performed at the node, and the other to the id of the node itself. All loop variables are created in the context of the caller.
There are three possible actions: enter, leave, or visit. enter actions occur during pre-order walks; leave actions occur during post-order walks; visit actions occur during in-order walks. In a both-order walk, the command will be evaluated twice for each node; the action is enter for the first evaluation, and leave for the second.
Note: The enter action for a node is always performed before the walker will look at the children of that node. This means that changes made by the script to the children of the node will immediately influence the walker and the steps it will take.
Any other manipulation, for example of nodes higher in the tree (i.e already visited), or upon leaving will have undefined results. They may succeed, error out, silently compute the wrong result, or anything in between.
At last a small table showing the relationship between the various options and the possible actions.
order type actions notes ----- ---- ----- ----- pre dfs enter parent before children post dfs leave parent after children in dfs visit parent between first and second child. both dfs enter, leave parent before and after children ----- ---- ----- ----- pre bfs enter parent before children post bfs leave parent after children in bfs -- illegal -- both bfs enter, leave parent before and after children ----- ---- ----- -----
Note the command ::struct::tree::prune. This command can be used in the walk script to force the command to ignore the children of the node we are currently at. It will throw an error if the order of traversal is either post or in as these modes visit the children before their parent, making pruning non-sensical.
This method is like method walk in all essentials, except the interface to the user code. This method invokes a command prefix with three additional arguments (tree, node, and action), instead of evaluating a script and passing the node via a loop variable.
The following noteworthy changes have occurred:
The API for accessing attributes and their values has been simplified.
All functionality regarding the default attribute "data" has been removed. This default attribute does not exist anymore. All accesses to attributes have to specify the name of the attribute in question. This backward incompatible change allowed us to simplify the signature of all methods handling attributes.
Especially the flag -key is not required anymore, even more, its use is now forbidden. Please read the documentation for the methods set, get, getall, unset, append, lappend, keyexists and keys for a description of the new API's.
The methods keys and getall now take an optional pattern argument and will return only attribute data for keys matching this pattern.
Nodes can now be renamed. See the documentation for the method rename.
The structure has been extended with API's for the serialization and deserialization of tree objects, and a number of operations based on them (tree assignment, copy construction).
Please read the documentation for the methods serialize, deserialize, =, and -->, and the documentation on the construction of tree objects.
Beyond the copying of whole tree objects these new API's also enable the transfer of tree objects over arbitrary channels and for easy persistence.
The walker API has been streamlined and made more similar to the command foreach. In detail:
The superfluous option -command has been removed.
Ditto for the place holders. Instead of the placeholders two loop variables have to be specified to contain node and action information.
The old command argument has been documented as a script now, which it was in the past too.
The fact that enter actions are called before the walker looks at the children of a node has been documented now. In other words it is now officially allowed to manipulate the list of children for a node under these circumstances. It has been made clear that changes under any other circumstances will have undefined results, from silently computing the wrong result to erroring out.
A new method, attr, was added allowing the query and retrieval of attribute data without regard to the node relationship.
The method children has been extended with the ability to select from the children of the node based on an arbitrary filtering criterium. Another extension is the ability to look not only at the immediate children of the node, but the whole tree below it.
The following example demonstrates the creation of new nodes:
mytree insert root end 0 ; # Create node 0, as child of the root
mytree insert root end 1 2 ; # Ditto nodes 1 & 2
mytree insert 0 end 3 ; # Now create node 3 as child of node 0
mytree insert 0 end ; # Create another child of 0, with a
# generated name. The name is returned
# as the result of the command.
This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category struct :: tree of the Tcllib SF Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.
Data structures
Copyright © 2002-2004 Andreas Kupries <andreas_kupries@users.sourceforge.net>