function center_of_mass = center_of_mass(x,y)
% center_of_mass = get_center_of_mass(x,y)
% y can be a matrix or a vector.
if size(x,1)<size(x,2) % number of rows < number of columns
    x = x';% now x is a column vector indicating time
end
if size(x,2)>1
    error('x should be a 1-d array indicating time');
end 

sizey = size(y);
dim_time_y = find(sizey==size(x,1));% which dimension of y equals to time.
if dim_time_y ~=2 % time should be the columns dimension
    y = y';
end
if size(y,2)~=size(x,1)
    error('x and y should have the same length in time');
end
center_of_mass = (y*x)./(y*ones(size(x)));
end