Parse the data

Since the data will all fit in the main memory for convenience, we'll define several functions that will load the ratings into Clojure data structures. The line->rating function takes a line, splits it into fields where a tab character is found, converts each field to a long datatype, then uses zipmap to convert the sequence into a map with the supplied keys:

(defn to-long [s]
  (Long/parseLong s))

(defn line->rating [line]
  (->> (s/split line #"\t")
       (map to-long)
       (zipmap [:user :item :rating])))

(defn load-ratings [file]
  (with-open [rdr (io/reader (io/resource file))]
    (->> (line-seq rdr)
         (map line->rating)
         (into []))))

(defn ex-7-3 []
  (->> (load-ratings "ua.base")
       (first)))

;; {:rating 5, :item 1, :user 1}

Let's write a function to parse the u.items file as well, so that we know what the movie names are:

(defn line->item-tuple [line]
  (let [[id name] (s/split line...