function  color_selected_elements (L_1, L_2, L_inc, mesh)
%         default                  1, nt, (nt-1)/7, 1
% Copyright 2000, J.E. Akin.  All rights reserved.
%  ------------------------------------------------------
%  Matlab plot 2-D FE elements from L_1 to L_2 in increments L_inc
%  and include full mesh if mesh=1
%  ------------------------------------------------------

% c_x    = x coordinates of nod_per_el line polygon
% c_y    = y coordinates of nod_per_el line polygon
% inc_e  = increment in element %-error on plot, if > 0
  inc_e  = 1 ;
% inc_p  = increment in node numbers on plot, if > 0
% msh_typ_nodes   = connectivity list for elements, nt x nod_per_el
% loop   = corners for nod_per_el line polygon
% lab_p  = 1, if node points are circled
  lab_p  = 0;
% nod_per_el = Nodes per element
% np     = Number of Points
% nt     = Number of elements
% pre_e  = Element items before connectivity list
  pre_e  = 0 ;
% pre_p  = Nodal items before coordinates
  pre_p = 1;
% msh_bc_xyz = Nodal coordinates (with preceeding data)
% t_x    = x coordinates of nod_per_el corners
% t_y    = y coordinates of nod_per_el corners
% x_bar  = x-centroid of each element
% xy     = Coordinates of points, np x 2
% y_bar  = y-centroid of each element

%    Read coordinate file and connectivity file
%    integer bc code, real xy pairs for np points (pre_p = 1)

%    Set control data: number of points
  load msh_bc_xyz.tmp ;  
  np = size (msh_bc_xyz,1) ;   % number of nodal points  
  fprintf ('Read %g mesh coordinate pairs \n', np)
  % fprintf ('       x        y \n')
  
%    Set control data: number elements & nodes per element
  load  msh_typ_nodes.tmp ; % nod_per_el nodes per element
  nt = size (msh_typ_nodes,1) ;          % number of elements in mesh 
  nod_per_el = size (msh_typ_nodes,2) - pre_e -1 ; % nodes per element
  fprintf ('Read %g elements connections \n', nt)
  
  if ( nargin == 0)
    L_1=1; L_2=nt ; L_inc= ceil((L_2 - L_1)/7) ; mesh = 1 ;
  elseif (  nargin == 1 )
    L_2=L_1 ; L_inc=1 ; mesh = 1 ;
  elseif (  nargin == 2 )
    L_inc= ceil((L_2 - L_1)/7) ; mesh = 1 ;
  elseif (  nargin == 3 )
    mesh = 1 ;
  end % if
  if ( L_inc < 1 ) 
    L_inc = 1
  end % if single element

  x (np)      = 0.       ;       % pre-allocate array x
  y (np)      = 0.       ;       % pre-allocate array y
  % x_bar (nt)  = 0.       ;       % pre-allocate array x_bar
  % y_bar (nt)  = 0.       ;       % pre-allocate array y_bar
  t_nodes (nod_per_el) = 0 ;    % Optional pre-allocation
  t_x (nod_per_el) = 0 ;        % Optional pre-allocation
  t_y (nod_per_el) = 0 ;        % Optional pre-allocation
  c_x (nod_per_el + 1) = 0 ;    % Optional pre-allocation
  c_y (nod_per_el + 1) = 0 ;    % Optional pre-allocation
  loop (nod_per_el + 1) = 0 ;   % Optional pre-allocation

%                set constants
  [loop] = get_El_Loop (nod_per_el) ;

  % msh_bc_xyz has: pre_p items then: x, y
  x = msh_bc_xyz (1:np, (pre_p+1)) ; % extract x column of xy
  y = msh_bc_xyz (1:np, (pre_p+2)) ; % extract y column of xy
 
%    Initialize plots
  fprintf ('Begin element error value plots: \n')
  xmax = max (x)      ; xmin = min (x) ;
  ymax = max (y)      ; ymin = min (y) ;
  %b xdiff = xmax - xmin ; ydiff = ymax - ymin ;
  %b if ( ydiff == 0.0 )
    %b ydiff = 0.5 ; % allow for 1-D mesh (with y == 0)
  %b end % if no y coordinates
  %b xmax = xmax + xdiff/10; ymax = ymax + ydiff/10;
  %b xmin = xmin - xdiff/10; ymin = ymin - ydiff/10;
  clf                               % clear graphics
  axis ([xmin, xmax, ymin, ymax])   % set axes
  axis ('equal')                    % true shape style
  hold on                           % hold image for plots
  grid                              % add grid dots
  xlabel (['X: ', int2str(nt),' Elements (with ', ...
          int2str(nod_per_el), ' nodes)'])
  ylabel (['Y: ', int2str(np),' Nodes'])
 title(['Selected Elements ', int2str(L_1),' : ', ...
         int2str(L_inc),' : ', int2str(L_2)])    
  format short
 
%      Plot input mesh points 
  if (lab_p == 1)                   % plot all points
    plot (x, y, 'b.')               % mark each node
  end % if show labels

%    Show 20 nodes and 50 elements
  inc_p = floor(np/20) ; inc_e = floor(nt/50) ;
  if (inc_p == 0 ) 
    inc_p = np - 1 ;
  end % if inc_p
  if (inc_e == 0 ) 
    inc_e = nt - 1 ;
  end % if inc_e
%    Show all if a small mesh
  if ( np <= 20 )
    inc_p = 1 ;
  end % if np
  if ( nt <= 50 )
    inc_e = 1 ;
  end % if nt

%    Loop over all elements
  Rank = 8 ;
  for it = L_1:L_inc:L_2 ;
%      Rank the colors
    Rank = Rank - 1 ;
    if ( Rank == 0 )
      Rank = 7 ;
    end % if
  
% scale colors as one set of 7
    if     ( Rank == 7) ; color = 'r' ; % red
    elseif ( Rank == 6) ; color = 'y' ; % yellow
    elseif ( Rank == 5) ; color = 'g' ; % green
    elseif ( Rank == 4 ) ; color = 'b' ; % blue 
    elseif ( Rank == 3 ) ; color = 'c' ; % cyan  
    elseif ( Rank == 2 ) ; color = 'm' ; % magenta
    elseif ( Rank == 1 ) ; color = 'w' ; % white
    else                 ; color = 'k' ; % black (impossible ??)
    end % if for rank color
    % disp(color)

%      Extract corner connectivity 
    t_nodes = msh_typ_nodes (it, (pre_e+2):(nod_per_el+pre_e+1)); 

%      Extract corner coordinates
    t_x  = x (t_nodes) ;         % x at those nodes, only
    t_y  = y (t_nodes) ;         % y at those nodes, only

%      Get the centroid
    x_bar  = sum (t_x' )/nod_per_el ;
    y_bar  = sum (t_y' )/nod_per_el ;

%      Plot this polygon
    c_x  = t_x (loop) ;       % x for nod_per_el line polygon
    c_y  = t_y (loop) ;       % y for nod_per_el line polygon
    fill (c_x, c_y, color)    % fill element color

% plot elem number, inclined
    t_text = sprintf ('%g', it);  % offset # from pt
    text (x_bar, y_bar, t_text, 'Rotation', 45) % incline
  end % for            

if ( mesh == 1 ) % add all element outlines
  for it = 1:nt          ;
%      Extract corner connectivity 
    t_nodes = msh_typ_nodes (it, (pre_e+2):(nod_per_el+pre_e+1)); 

%      Extract corner coordinates
    t_x  = x (t_nodes) ;         % x at those nodes, only
    t_y  = y (t_nodes) ;         % y at those nodes, only

%      Plot this polygon
    c_x  = t_x (loop) ;       % x for nod_per_el line polygon
    c_y  = t_y (loop) ;       % y for nod_per_el line polygon
    plot (c_x, c_y)    % plot element color
  end % for full mesh
end % if mesh

%       -depsc -tiff            % for an eps version
  %bprint -dpsc color_selected_elements
  hold off
%bfprintf ('Created file color_selected_elements.ps \n')
% end of color_selected_elements