A ''tree'' is an acyclic directed-[graph] [data structure] where one node (the root) has no predecessor,
and all other nodes have exactly one predecessor.
Trees can be used, e.g., for representing the result in syntax parsing.
An [XML] document is structured as a tree.



** See Also **

   [Data Format]:   Includes a list of formats for tree-like data.

   [Decision Trees]:   

   [deep list]:   A standard Tcl list in which each single value is encoded as a list containing only one item, and each sequence of values is a list containing more than one item.  This makes it possible to differentiate e.g., the single value `one two three` from a list containing the values `one`, `two`, and `three`.

   [What's wrong with the tree widgets?]:   

   [https://groups.google.com/forum/#!topic/comp.lang.tcl/7A1BjFVsgJE%|%Best Tree Widget], [comp.lang.tcl], 2009-03-11:   A discussion between Martyn, Ruediger, and Alexander



** Types of Trees **

   [RBTree]:   red-black tree data type



** Implementations **

   [Tcllib struct tree]:   

   [Containers], by Iain B. Findleton:   

   [ODIE]:   Includes tools for data tree maintenance

   [tdom]:   An implementatio of the [DOM].

   [Tree Objects]:   discussed by [Clif Flynt]

   [Various useful Tcl packages (Rempel)]:   Includes some tree operation packages

   [A Tree class using TclOO], by [Tom Krehbiel]:   

   [An experimental tree structure], by [Marco Maggi] :   

   [Simple tree structure], by [Rui Lopes%|%rui]:   

   [ycl%|%ycl list deep], by [PYK]:   An implementation of a [deep list].



** Widgets **

   [BWidget::Tree]:   A tree widget.   More accessible for beginners.  Featurs edit capabilities.

   [TkTreeCtrl]:   A multicolumn, hierarchical listbox widget.  Complex for beginners.

   [Hipp miscellaneous widgets]:   Includes some tree widgets.
   
   [mtree widget]:   
   
   [BLT]:   
   
   [Tix]:   
   
   [ClassyTk]:   
   
   [incr widgets]:   Includes a ''hierarchy'' widget
   
   [mkWidgets]:    
   
   [PRS Open Source Software]:   
   
   [Zinc]:   
   
   [http://oomadness.tuxfamily.org/p-treewidget.php%|%TreeWidget]:   A pure-[Python] tree widget for [Tkinter]

   [Tk Tree Widget in C++]:   
   
   [tkgcv]:   
   
   [hierarchy]:   
   
   [Tree Table]:   
   
   born2net's [Iwidgets] version of Hipp's tree widget:   

   [Tree Widget], by Brighton:   An [Iwidgets] tree widget.

   [Tree widget], by Mutlu:   An [Iwidgets] version tree widget:   
   
   Pryce's itk widget base class:   

   [tktree]:   

   [GRIDPLUS]:   Includes a basic scrollable tree widget

   [A Minimal Tree Widget]:   

   [gnocl::tree]:   




** Browsers **

   [A little RSS browser]:   
   
   [A little XML browser], by RS:   
   
   [Tree nodes in motion]:    Move nodes on a [Bwidget::Tree%|%Bwidget tree].

   [LemonTree]:   

   [Trees as nested lists]:   An exploration of both structural approaches for trees, along with some simple graphical interfaces.


   [Displaying Tree Hierarchy with Incremental Node Numbering]:   



** Techniques **

   [Simple tree layout], by [Richard Suchenwirth]:   How to display a tree structure on a canvas.



** Visualizations **

   [Pythagoras Tree]:   






** description **


[AMG]: A tree is sometimes called a DAG because it is a directed acyclic graph.  I find this to be a useful mnemonic.

[NEM]: Well, all trees are DAGs, but not all DAGs are trees.

[AMG]: Yes, quite right; see the other tree properties listed in the first line of this page.

----

[LV] In the following text, I notice a number of cases where GUI widget
sets have some sort of '''tree''' widget.  In each of these cases, is the
data structure intermingled with the data representation?  Seems a shame
that someone doesn't just build a graphically viewable widget that
depends on a data structure that is separate.  That way, someone could
do work with one piece of code, and then, in other pieces, display
that data.  Seems like a natural method of developing and maintaining
code....


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