One can well imagine that split-ing each line read from the input file and creating some sort of hash table might be involved in the solution of the problem presented in the OP. Unfortunately, I, for one, do not now understand the basic problem well enough to want to make the effort to attempt a solution. (Update: davido, a more patient man than I (at least today), has made a determined effort to provide a helpful solution in the face of a vague specification. ++davido.) Please see this and thanos1983's reply.