vec=nCtwo(dat)

function vec=nCtwo(dat)
%NCTWO - All combinations of N elements taken two at a time.
%   nCtwo(dat) where dat is a vector of length N, produces a matrix with
%   N*(N-1)/2 rows and 2 columns.  Each row of the result has two of the
%   elements in the vector dat. Similar to the MATLAB function
%   NCHOOSEK(dat,2) but much faster.
% 
%   See also NCHOOSEK, PERMS.
%
%   Authors: Simone Scaringi{1} & Antonis Loizou{2}
%            {1}School of Physics and Astronomy, University of Southampton
%	     {2}School of Electronics and Computer Science, University of Southampton
%   Date:    28-05-2008
%   E-mail:  simo(at)astro.soton.ac.uk, al05r(at)ecs.soton.ac.uk

if ~isvector(dat) || isscalar(dat)
  error('ERROR:nCtwo:InvalidArg1',...
        'The input has to be a vector.');
else 
    combo=length(dat)*(length(dat)-1)/2;

    arr1=zeros(1,combo);
    arr2ix=zeros(1,combo);
    n=length(dat);
    pointold=0;
    pointnu=0;

    for i=1:n-1
       pointold=pointnu;
       pointnu=pointold+n-i;  
       arr1(pointold+1:pointnu)=dat(i);
       arr2ix(pointold+1:pointnu)=i+1:n;
    end

    vec=[arr1; dat(arr2ix)]';
end