function triangulation_boundary_nodes ( input_node_filename, ...
  input_triangulation_filename )

%% MAIN is the main program for TRIANGULATION_BOUNDARY_NODES.
%
%  Discussion:
%
%    TRIANGULATION_BOUNDARY_NODES outputs boundary nodes of a triangulation.
%
%    Each connected segment of the boundary is written to a separate file.
%
%  Modified:
%
%    01 April 2008
%
%  Author:
%
%    John Burkardt
%
%  Usage:
%
%    triangulation_boundary ( 'node_file.txt'. 'triangle_file.txt' )
%
  timestamp;

  fprintf ( 1, '\n' );
  fprintf ( 1, 'TRIANGULATION_BOUNDARY_NODES\n' );
  fprintf ( 1, '  MATLAB version\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  Identify triangulation nodes on the boundary.\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '* Read a dataset of NODE_NUM points in 2 dimensions;\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '* Read an associated triangulation dataset of  \n' );
  fprintf ( 1, '  triangles using 3 or 6 nodes;\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '* Determine which nodes are on the boundary;\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '* Print a list of the node coordinates and indices;\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '* Write a file of the boundary node indices.\n' );
  fprintf ( 1, '* Write a file of the boundary node coordinates.\n' );
%
%  Get the number of command line arguments.
%
  if ( nargin < 1 )

    fprintf ( 1, '\n' );
    fprintf ( 1, 'TRIANGULATION_BOUNDARY_NODES:\n' );
    input_node_filename = input( ...
      '  Please enter the name of the node file (in ''quotes''!).\n' );

  end
%
%  If at least two command line arguments, 
%  the second is the triangulation file.
%
  if ( nargin < 2 )

    fprintf ( 1, '\n' );
    fprintf ( 1, 'TRIANGULATION_BOUNDARY_NODES:\n' );
    input_triangulation_filename = input ( ...
      '  Please enter the name of the triangle file (in ''quotes''!).\n' );

  end

  [ dim_num, node_num ] = dtable_header_read ( input_node_filename );

  fprintf ( 1, '\n' );
  fprintf ( 1, '  Read the header of "%s"\n.', input_node_filename );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  Spatial dimension DIM_NUM = %d\n', dim_num );
  fprintf ( 1, '  Number of nodes NODE_NUM  = %d\n', node_num );

  node_xy = dtable_data_read ( input_node_filename, dim_num, node_num );

  fprintf ( 1, '\n' );
  fprintf ( 1, '  Read the data in "%s"\n', input_node_filename );

  r8mat_transpose_print_some ( dim_num, node_num, node_xy, 1, 1, ...
    dim_num, 5, '  Portion of coordinate data from file:' );
%
%  Read the triangulation data.
%
  [ triangle_order, triangle_num ] = itable_header_read ( ...
    input_triangulation_filename );

  fprintf ( 1, '\n' );
  fprintf ( 1, '  Read the header of "%s"\n', ...
    input_triangulation_filename );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  Triangle order = %d\n', triangle_order );
  fprintf ( 1, '  Number of triangles = %d\n', triangle_num );

  triangle_node = itable_data_read ( input_triangulation_filename, ...
    triangle_order, triangle_num );

  fprintf ( 1, '\n' );
  fprintf ( 1, '  Read the data in "%s".\n', ...
    input_triangulation_filename );

  i4mat_transpose_print_some ( triangle_order, triangle_num, ...
    triangle_node, 1, 1, triangle_order, 5, ...
    '  Portion of triangle file:' );
%
%  Determine which nodes lie on the boundary.
%
  if ( triangle_order == 3 )

    node_boundary = triangulation_order3_boundary_node ( node_num, ...
      triangle_num, triangle_node );

  elseif ( triangle_order == 6 )

    node_boundary = triangulation_order6_boundary_node ( node_num, ...
      triangle_num, triangle_node );

  end
%
%  Write the output file.
%
  node_boundary_num = 0;

  for node = 1 : node_num
    if ( node_boundary(node) )
      node_boundary_num = node_boundary_num + 1;
    end
  end

  fprintf ( 1, '\n' );
  fprintf ( 1, '  Boundary nodes:\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '         #     Index          X               Y\n' );
  fprintf ( 1, '\n' );
  node2 = 0;
  for node = 1 : node_num
    if ( node_boundary(node) )
      node2 = node2 + 1;
      node_boundary_xy(1:2,node2) = node_xy(1:2,node);
      fprintf ( 1, '  %8d  %8d  %14f  %14f\n', ...
        node2, node, node_xy(1:2,node) );
    end
  end
%
%  Write a file containing the boundary node indices.
%
  node2 = 0;
  for node = 1 : node_num
    if ( node_boundary(node) )
      node2 = node2 + 1;
      node_indices(node2) = node;
    end
  end 

  node_indices_filename = 'boundary_node_indices.txt';
  header = 0;

  itable_write ( node_indices_filename, 1, node_boundary_num, ...
    node_indices, header );

  fprintf ( 1, '\n' );
  fprintf ( 1, '  The file "%s" lists the boundary node indices.\n', ...
    node_indices_filename );

%
%  Write a file containing the boundary node coordinates.
%
  output_filename = 'boundary_nodes.txt';
  header = 0;

  dtable_write ( output_filename, dim_num, node_boundary_num, ...
    node_boundary_xy, header );

  fprintf ( 1, '\n' );
  fprintf ( 1, '  The file "%s" lists the boundary node coordinates.\n', ...
    output_filename );

  fprintf ( 1, '\n' );
  fprintf ( 1, 'TRIANGULATION_BOUNDARY_NODES\n' );
  fprintf ( 1, '  Normal end of execution.\n' );

  fprintf ( 1, '\n' );
  timestamp;