Hi
Sorry about the late reply...
fre, 29 10 2010 kl. 11:46 +0200, skrev [email protected]:
> I've been searching for a function that give all permutations of c elements
> in n, as "combnk" Matlab function.
> I fund in this forum
> http://octave.1599824.n4.nabble.com/combinations-td1636272.html a solution
> using "perms" function.
> However, "perms" function sets a matrix of prod(2:n) line, which limits the
> use to n=10;
>
> I tried an empiric algorithme, that can replace the use of "unique(sort
> (perms (1:n)(:,1:c), 2), 'rows')"
> I am not a mathematician, and not even a programmer, but this code may
> inspire someone for a "clean" code.
How about the attached implementation of 'combnk' ? I've added this to
the 'statistics' package.
Søren
## Copyright (C) 2010 Soren Hauberg
##
## 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 <http://www.gnu.org/licenses/>.
## -*- texinfo -*-
## @deftypefn {Function File} {...@var{c} =} combnk (@var{data}, @var{k})
## Return all combinations of @var{k} elements in @var{data}.
## @end deftypefn
function retval = combnk (data, k)
## Check input
if (! isvector (data))
error ("combnk: first input argument must be a vector");
endif
if (!isreal (k) || k != round (k) || k < 0)
error ("combnk: second input argument must be a non-negative integer");
endif
## Simple checks
n = numel (data);
if (k == 0 || k > n)
retval = resize (data, 0, k);
elseif (k == n)
retval = data (:).';
else
retval = __combnk__ (data, k);
endif
## For some odd reason Matlab seems to treat strings differently compared to other data-types...
if (ischar (data))
retval = flipud (retval);
endif
endfunction
function retval = __combnk__ (data, k)
## Recursion stopping criteria
if (k == 1)
retval = data (:);
else
## Process data
n = numel (data);
retval = [];
for j = 1:n
C = __combnk__ (data ((j+1):end), k-1);
C = cat (2, repmat (data (j), rows (C), 1), C);
retval = [retval; C];
endfor
endif
endfunction
%!demo
%! c = combnk (1:5, 2);
%! disp ("All pairs of integers between 1 and 5:");
%! disp (c);
%!test
%! c = combnk (1:3, 2);
%! assert (c, [1, 2; 1, 3; 2, 3]);
%!test
%! c = combnk (1:3, 6);
%! assert (isempty (c));
%!test
%! c = combnk ({1, 2, 3}, 2);
%! assert (c, {1, 2; 1, 3; 2, 3});
%!test
%! c = combnk ("hello", 2);
%! assert (c, ["lo"; "lo"; "ll"; "eo"; "el"; "el"; "ho"; "hl"; "hl"; "he"]);
------------------------------------------------------------------------------
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