how to create input file for collaborative filtering
This data set is in a sparse format. Most ratings are unknown, which is a common scenario in recommender systems. This data format is much smaller due to leaving out all the unknowns. You can interpret this data as follows:

user, item, rating


What this means is that user U has rated item I with rating R. Every UxI combination not in your dataset is unknown and has to be predicted using collaborative filtering.