function [ undx, xdnu ] = r8col_sorted_undex ( m, n, a, unique_num )
%*****************************************************************************80
%
%% R8COL_SORTED_UNDEX returns unique sorted indexes for a sorted R8COL.
%
% Discussion:
%
% An R8COL is an M by N array of R8's, regarded as an array of N columns,
% each of length M.
%
% The goal of this routine is to determine a vector UNDX,
% which points, to the unique elements of A, in sorted order,
% and a vector XDNU, which identifies, for each entry of A, the index of
% the unique sorted element of A.
%
% This is all done with index vectors, so that the elements of
% A are never moved.
%
% Assuming A is already sorted, we examine the entries of A in order,
% noting the unique entries, creating the entries of XDNU and
% UNDX as we go.
%
% Once this process has been completed, the vector A could be
% replaced by a compressed vector XU, containing the unique entries
% of A in sorted order, using the formula
%
% XU(1:X_UNIQUE_NUM) = A(UNDX(1:X_UNIQUE_NUM)).
%
% We could then, if we wished, reconstruct the entire vector A, or
% any element of it, by index, as follows:
%
% A(I) = XU(XDNU(I)).
%
% We could then replace A by the combination of XU and XDNU.
%
% Later, when we need the I-th entry of A, we can locate it as
% the XDNU(I)-th entry of XU.
%
% Here is an example of a vector A, the unique sort and
% inverse unique sort vectors and the compressed unique sorted vector.
%
% I A XU Undx Xdnu
% ----+------+------+-----+-----+
% 1 | 11.0 | 11.0 1 1
% 2 | 11.0 | 22.0 5 1
% 3 | 11.0 | 33.0 8 1
% 4 | 11.0 | 55.0 9 1
% 5 | 22.0 | 2
% 6 | 22.0 | 2
% 7 | 22.0 | 2
% 8 | 33.0 | 3
% 9 | 55.0 | 4
%
% Licensing:
%
% This code is distributed under the GNU LGPL license.
%
% Modified:
%
% 17 July 2010
%
% Author:
%
% John Burkardt
%
% Input:
%
% integer M, the dimension of the data values.
%
% integer N, the number of data values.
%
% real A(M,N), the data values.
%
% integer UNIQUE_NUM, the number of unique values in A.
% This value is only required for languages in which the size of
% UNDX must be known in advance.
%
% Output:
%
% integer UNDX(UNIQUE_NUM), the UNDX vector.
%
% integer XDNU(N), the XDNU vector.
%
%
% Walk through the sorted array.
%
i = 1;
j = 1;
undx(j) = i;
xdnu(i) = j;
for i = 2 : n
diff = max ( abs ( a(1:m,i) - a(1:m,undx(j)) ) );
if ( 0.0 < diff )
j = j + 1;
undx(j) = i;
end
xdnu(i) = j;
end
return
end