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## distribution or <https://octave.org/copyright/>.
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## -*- texinfo -*-
## @deftypefn  {} {} spy (@var{x})
## @deftypefnx {} {} spy (@dots{}, @var{markersize})
## @deftypefnx {} {} spy (@dots{}, @var{line_spec})
## Plot the sparsity pattern of the sparse matrix @var{x}.
##
## If the optional numeric argument @var{markersize} is given, it determines
## the size of the markers used in the plot.
##
## If the optional string @var{line_spec} is given it is passed to @code{plot}
## and determines the appearance of the plot.
## @seealso{plot, gplot}
## @end deftypefn

function spy (x, varargin)

  if (nargin < 1 || nargin > 3)
    print_usage ();
  endif

  markersize = NaN;
  if (nnz (x) < 1000)
    line_spec = "*";
  else
    line_spec = ".";
  endif
  for arg = varargin
    arg = arg{1};
    if (ischar (arg))
      if (numel (arg) == 1)
        line_spec = [line_spec, arg];
      else
        line_spec = arg;
      endif
    elseif (isreal (arg) && isscalar (arg))
      markersize = arg;
    else
      error ("spy: expected markersize or linespec");
    endif
  endfor

  [i, j] = find (x);
  [m, n] = size (x);

  if (isnan (markersize))
    plot (j, i, line_spec);
  else
    plot (j, i, line_spec, "markersize", markersize);
  endif

  axis ([0, n+1, 0, m+1], "ij");
  xlabel (sprintf ("nnz = %d", nnz (x)));

endfunction


%!demo
%! clf;
%! spy (sprand (10,10, 0.2));

## Test input validation
%!error <Invalid call> spy ()
%!error <Invalid call> spy (1,2,3,4)