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