function r = cut_Y(r, valid_idx)
% disables the invalid parts of the ranking structure Y    

    % get indices not mentioned
    u = find(~valid_idx);
    
    % clear bad data
    for i = 1 : numel(u)
        
        r{u(i), 1} = [];
        r{u(i), 2} = [];
    end
    
    % also clear weights if applicable
    if size(r,2) == 3
        for i = 1 : numel(u)
            r{u(i), 3} = 0;
        end
    end
end