Daniel@0: function c = nchoose2(v, f)
Daniel@0: %NCHOOSE2 All combinations of N elements taken two at a time.
Daniel@0: %
Daniel@0: %   NCHOOSE2(1:N) or NCHOOSEK(V) where V is a vector of length N,
Daniel@0: %   produces a matrix with N*(N-1)/2 rows and K columns. Each row of
Daniel@0: %   the result has K of the elements in the vector V.
Daniel@0: %
Daniel@0: %   NCHOOSE2(N,FLAG) is the same as NCHOOSE2(1:N) but faster.
Daniel@0: %
Daniel@0: %   NCHOOSE2(V) is much faster than NCHOOSEK(V,2).
Daniel@0: %
Daniel@0: %   See also NCHOOSEK, PERMS.
Daniel@0: 
Daniel@0: %   Author:      Peter J. Acklam
Daniel@0: %   Time-stamp:  2000-03-03 13:03:59
Daniel@0: %   E-mail:      jacklam@math.uio.no
Daniel@0: %   URL:         http://www.math.uio.no/~jacklam
Daniel@0: 
Daniel@0:    nargs = nargin;
Daniel@0:    if nargs < 1
Daniel@0:       error('Not enough input arguments.');
Daniel@0:    elseif nargs == 1
Daniel@0:       v = v(:);
Daniel@0:       n = length(v);
Daniel@0:    elseif nargs == 2
Daniel@0:       n = v;
Daniel@0:    else
Daniel@0:       error('Too many input arguments.');
Daniel@0:    end
Daniel@0: 
Daniel@0:    [ c(:,2), c(:,1) ] = find( tril( ones(n), -1 ) );
Daniel@0: 
Daniel@0:    if nargs == 1
Daniel@0:       c = v(c);
Daniel@0:    end