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wkeep.m

## Copyright (C) 2008   Sylvain Pelissier   <sylvain.pelissier@gmail.com>
##
## This program is free software; you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 3 of the License, or
## (at your option) any later version.
##
## This program is distributed in the hope that it will be useful,
## but WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
## GNU General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with this program; if not, see 

## -*- texinfo -*-
## @deftypefn {Function File} {[@var{y}] =} wkeep(@var{x,l,opt})
##    Extract the elements of x of size l from the center, the right or the left.
## @end deftypefn


function y = wkeep(x,l,opt)

      if (nargin < 2|| nargin > 3); usage("\nwkeep(x,l)\nwkeep(x,l,opt)\n"); end
      if(nargin==2)
            opt='c';
      end
      if(isvector(x))
            if(nargin==2)
                  opt='c';
            end
            
            if(l > length(x))
                  error('l must be or equal the size of x');
            end
            
            if(opt=='c')
                  s = (length(x)-l)./2;
                  y = x(1+floor(s):end-ceil(s));
      
            elseif(opt=='l')
                  y=x(1:l);
      
            elseif(opt=='r')
                  y = x(end-l+1:end);
            
            else
                  error('opt must be equal to c, l or r');
            end
      else
            if(length(l) == 2)
                  s1 = (length(x)-l(1))./2;
                  s2 = (length(x)-l(2))./2;
            else
                  error('For a matrix l must be a 1x2 vector');
            end
            
            if(nargin==2)
                  y = x(1+floor(s1):end-ceil(s1),1+floor(s2):end-ceil(s2));
            else
                  if(length(opt) == 2)
                        firstr=opt(1);
                        firstc=opt(2);
                  else
                        error('For a matrix l must be a 1x2 vector');
                  end
                  
                  y=x(firstr:firstr+l(1)-1,firstc:firstc+l(2)-1);
            end
            
            
      end

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