Making Recommendations

Now that the similarity algorithms are in place, it is time to move on to making recommendations any one of the previously covered similarity scores would work,

• Pearson Correlation Score
• Euclidean Distance Score

We begin with calculating similarity scores for every critic, against the person we are looking for, discard anyone whose similarity is below

1. We can plug either one of the similarity scores.

(defn similarities [prefs person algo]
  (filter 
   #(<= 0 (second %))
   (reduce 
    (fn[h p] (assoc h (first p) (algo (prefs person) (second p)))) 
    {} (dissoc prefs person))))

user=> (similarities critics "Toby" pearson)
(["Jack Matthews" 0.66284898035987] ["Mick LaSalle" 0.9244734516419049] 
 ["Claudia Puig" 0.8934051474415647] ["Gene Seymour" 0.38124642583151164]
 ["Lisa Rose" 0.9912407071619299])

Next we filter preferences, remove entries that we already ranked and multiple remaining entries with the users similarity score, that way their ranks only contribute by how much they are similar to the user we are looking for.

(defn weight-prefs [prefs similarity person]
  (reduce 
   (fn [h v]
     (let [other (first v) score (second v)
	   diff (filter #(not (contains? (prefs person) (key %))) (prefs other))
	   weighted-pref (apply hash-map
				(interleave (keys diff) 
					    (map #(* % score) (vals diff))))]
       (assoc h other weighted-pref))) {} similarity))

user=> (weight-prefs critics (similarities critics "Toby" pearson)  "Toby")
{"Lisa Rose" {"Lady in the Water" 2.4781017679048247, 
              "The Night Listener" 2.97372212148579, 
              "Just My Luck" 2.97372212148579}, 
 "Gene Seymour" {"Lady in the Water" 1.143739277494535, 
                 "The Night Listener" 1.143739277494535, 
                 "Just My Luck" 0.5718696387472675}, 
 "Claudia Puig" {"The Night Listener" 4.020323163487041, 
                 "Just My Luck" 2.680215442324694}, 
 "Mick LaSalle" {"Lady in the Water" 2.7734203549257144, 
                 "The Night Listener" 2.7734203549257144, 
                 "Just My Luck" 1.8489469032838097}, 
 "Jack Matthews" {"Lady in the Water" 1.9885469410796102, 
                  "The Night Listener" 1.9885469410796102}}

Using the weighted preferences we calculated, we can build a list of movies to recommend by adding all the ranks for the movies,

(defn sum-scrs [prefs]
  (reduce (fn [h m] (merge-with #(+ %1 %2) h m)) {} (vals prefs)))

user=> (sum-scrs (weight-prefs critics (similarities critics "Toby" pearson)  "Toby"))
{"Just My Luck" 8.074754105841562, 
 "The Night Listener" 12.899751858472692, 
 "Lady in the Water" 8.383808341404684}

In order not to give any advantage to movies that are ranked the most, we need to divide rank, to the sum of similarity of all the critics that ranked the movie,

(defn sum-sims [weighted-pref scores sim-users]
  (reduce (fn [h m]
	    (let [movie (first m)
		  rated-users (reduce 
			       (fn [h m] (if (contains? (val m) movie) 
					  (conj h (key m)) h)) 
			       [] weighted-pref)
		  similarities (apply + (map #(sim-users %) rated-users))]
	      (assoc h movie similarities) ) ) {} scores))

{Lady in the Water 2.9598095649952163, 
 The Night Listener 3.853214712436781, 
 Just My Luck 3.190365732076911}

Now we have everything to make a recommendation to a user, final score for a movie is calculated by diving its total score to the total of the similarities,

(defn recommend [prefs person algo]
  (let [similar-users (into {} (similarities prefs person algo))
	weighted-prefs (weight-prefs prefs similar-users  person)
	scores (sum-scrs weighted-prefs)
	sims (sum-sims weighted-prefs scores similar-users)]
    (interleave (keys scores) (map #(/ (second %) (sims (first %))) scores))))

user=> (recommend critics "Toby" pearson)
("Just My Luck" 2.5309807037655645 
 "The Night Listener" 3.3477895267131013 
 "Lady in the Water" 2.832549918264162)