function j=findelem(fem_grid_struct,xylist)
%FINDELEM element finding utility
%   FINDELEM finds the element number for the
%   current mouse position.  FINDELEM prompts
%   the user to click on the current axes and returns
%   the element for the "click-on" position, or NaN
%   if the click is outside the domain.
%
%   Alternatively, a list of horizontal points can
%   be passed in and a list of element numbers, one for
%   each input point, will be returned; NaN is returned
%   for each point outside of the FEM domain.
%
%   In determining which element has been selected,
%   FINDELEM needs elemental areas and shape functions.
%   The routines BELINT and EL_AREAS compute these arrays
%   and add them to a previously created fem_grid_struct.
%   These two functions MUST be run before FINDELEM will
%   run.
%   	  BELINT is run as:
%   		 new_struct=belint(fem_grid_struct);
%   	  EL_AREAS is run as:
%   		 [new_struct,ineg]=el_areas(fem_grid_struct);
%
%   INPUT : fem_grid_struct - (from LOADGRID, see FEM_GRID_STRUCT)
%   	    xylist	    - points to find elements for [n x 2 double]
%
%   OUTPUT : an element number(s)
% 
%   CALL : >> j=findelem(fem_grid_struct)   for interactive
%     OR   >> j=findelem(fem_grid_struct,xylist)        
%
%   Written by : Brian O. Blanton 
%   Summer 1997
%

% VERIFY INCOMING STRUCTURE
%
if ~is_valid_struct(fem_grid_struct)
   error('    fem_grid_struct to FINDELEM invalid.')
end

% Make sure additional needed fields of the fem_grid_struct
% have been filled.
if ~is_valid_struct2(fem_grid_struct)
   error('    fem_grid_struct to FINDELEM invalid.')
end

if exist('xylist')
   xp=xylist(:,1);
   yp=xylist(:,2);
%   line(xp,yp,'LineStyle','.','Marker','+')
else
   disp('Click on element ...');
   waitforbuttonpress;
   Pt=gcp;
   xp=Pt(2);yp=Pt(4);
   line(xp,yp,'LineStyle','+')

end
j=findelemex5(xp,yp,fem_grid_struct.ar,...
                    fem_grid_struct.A,...
                    fem_grid_struct.B,...
                    fem_grid_struct.T);

%
%        Brian O. Blanton
%        Department of Marine Sciences
%        Ocean Processes Numerical Modeling Laboratory
%        15-1A Venable Hall
%        CB# 3300
%        Uni. of North Carolina
%        Chapel Hill, NC
%                 27599-3300
%
%        919-962-4466
%        blanton@marine.unc.edu
%
%        Summer 1997
%