Julia un-initialize array at particular index

Im writing a Neural Network in Julia that tests random topologies. I've left all indices in an array of nodes that are not occupied by a node (but which may be under a future topology) undefined as it saves memory. When a node in an old topology is no longer connected to other nodes in a new topology, is there a way to un initialize the index to which the node belongs. Also, are there any reasons not to do it this way.