B(1,:) = sum(A);
B(2,:) = B(1,:)/2;
B(3,:) = sum(cumsum(A)<=B(2,:));
% sanity check
% Checks that the cumulative sums of each column in A up until the
% row numbers identified in B(3,:) are <= B(2,:) and that the
% cumulative sum of each column in A up until the rows B(3,:)+1 exceed
% the values in B(2,:)
Ac = cumsum(A);
allTrue = all(Ac(sub2ind(size(A),B(3,:),1:size(A,2))) <= B(2,:));
allFalse = ~any(Ac(sub2ind(size(A),B(3,:)+1,1:size(A,2))) <= B(2,:));
if ~allTrue || ~allFalse
error('Something''s wrong.')
end
