Find points on convex hulls                                     [STB-23: gr16]
---------------------------

        ^conhull^ varlist^,^ ^h^ull^(^#^)^ [ ^c^oname^(^str^)^ ^nogra^ph ^g^roup^(^str^)^ ]

finds points on the convex hull of a set of points in two-dimensional space
using an algorithm proposed by R. A. Jarvis.  If ^nograph^ is not selected, the
hull is graphed.  The outer hull is "peeled" and the next hull found and
plotted.  This process continues until all points are assigned to a hull or
until a user-defined number of hulls are assigned.  The variable ^coname^
contains the hull of each point.  If the ^group^ option is in effect a series
of convex hulls is generated for each class of a categorical variable.  If the
^coname^ variable is saved the ^condraw^ ado-file can be used to graph
individual hulls.

Options
-------

^c^oname^(^str^)^ is the variable name for the convex hull number.  The default
   is ^conhull^.  Since ^conhull^ is not a temporary variable the program will
   give a ^conhull already defined^ error message if your data set already
   contains a variable named ^conhull^.

^h^ull^(^#^)^ specifies the number of hulls to be generated for each group.  ^h(^1^)^
   would calculate only the outermost hull.  The default value is 500.

^nogra^ph suppresses the graphing of each hull.  If you expect a large number
   of hulls, it may be best to allow ^conhull^ to assign the hulls without
   graphing and to use ^condraw^ to graph the hulls that interest you.  The
   default is to graph each hull.  Each graph contains all points referenced 
   by their ^group^ value.  All points on the lines of the graph are on the
   hull.   
   
^g^roup(str) is the name of a categorical variable for which convex hulls
   will be generated.  The default is to use the entire sample.


Examples
--------

  .^conhull x y^   [graphs and peels the hulls for points with x and
                    y coordinates]

  .^conhull x y, g(smoke)^  [graphs and peels hulls for the points in
                             each level of variable ^smoke^]

  .^conhull x y,h(1) g(smoke)^   [graphs the outermost hull for each level
                                  of variable ^smoke^]

  .^conhull x y, n c(taste)^  [peels all the hulls in the data set, but does
                               not graph them.  The hull number is saved in
                               variable ^taste^]


Reference
---------
        
	Jarvis, R. A. 1973.  "On the identification of the convex hull of a 
           finite set of points in the plane"  Information Processing Letters.
           2:18-21.


Authors
-------
        J. Patrick Gray
        Department of Anthropology
        University of Wisconsin-Milwaukee
        Milwaukee, WI 53201
	jpgray@@alpha1.csd.uwm.edu

        Tim McGuire
        Pillsbury College
        Owatonna, MN 55060


Also see
--------

    STB:  gr16 (STB-23)
On-line:  ^help^ for @condraw@