这章主要讲了如何做推荐,现在推荐最常用的几种算法:Collaborative Filtering、Cluster Models、Search-Based Methods、Item-to-Item Collaborative Filtering.前两种是通过找相似的Customer,后两种通过找相似的Item.论文Amazon.com Recommendations Item-to-Item Collaborative Filtering 对这几种算法都有介绍。这章主要提了Collaborative Filtering和tem-to-Item Collaborative Filtering。 Collaborative Filtering:通过搜索大量的Customer数据集来找到那一小撮和你口味相似的。书中举了一个电影评论的例子,每个人都对一些电影进行评等级,通过这些数据来找到和你口味相似的人,以及对你没有看过的电影做推荐,并以这个例子演示了如何做推荐。
准备数据:(本笔记的代码使用ruby实现,python代码的实现见原书)
- critics={
- 'Lisa Rose' => {'Lady in the Water' => 2.5, 'Snakes on a Plane' => 3.5,
- 'Just My Luck' => 3.0, 'Superman Returns' => 3.5, 'You, Me and Dupree' => 2.5,
- 'The Night Listener' => 3.0},
-
- 'Gene Seymour' => {'Lady in the Water' => 3.0, 'Snakes on a Plane' => 3.5,
- 'Just My Luck' => 1.5, 'Superman Returns' => 5.0, 'The Night Listener'=> 3.0,
- 'You, Me and Dupree' => 3.5},
-
- 'Michael Phillips' => {'Lady in the Water' => 2.5, 'Snakes on a Plane' => 3.0,
- 'Superman Returns' => 3.5, 'The Night Listener' => 4.0},
-
- 'Claudia Puig' => {'Snakes on a Plane' => 3.5, 'Just My Luck' => 3.0,
- 'The Night Listener' => 4.5, 'Superman Returns' => 4.0,
- 'You, Me and Dupree' => 2.5},
-
- 'Mick LaSalle'=> {'Lady in the Water' => 3.0, 'Snakes on a Plane' => 4.0,
- 'Just My Luck' => 2.0, 'Superman Returns' => 3.0, 'The Night Listener' => 3.0,
- 'You, Me and Dupree' => 2.0},
-
- 'Jack Matthews'=> {'Lady in the Water' => 3.0, 'Snakes on a Plane' => 4.0,
- 'The Night Listener'=> 3.0, 'Superman Returns'=> 5.0, 'You, Me and Dupree' => 3.5},
-
- 'Toby' => {'Snakes on a Plane' =>4.5,'You, Me and Dupree' =>1.0,'Superman Returns' => 4.0}
- }
定义相似度:
欧拉距离:
代码实现:
- def sim_distance(prefs,person1,person2)
- si = {}
- prefs[person1].each_key do |item|
- si[item] = 1 if prefs[person2][item]
- end
-
- return 0 if si.empty?
-
- sum_of_squares = si.keys.inject(0) do |sum,item|
- sum + (prefs[person1][item] - prefs[person2][item]) ** 2
- end
-
- return 1 / (1 + sum_of_squares)
- end
Pearson Correlation Score:
代码实现:
- def sim_pearson(prefs,person1,person2)
- si = {}
- prefs[person1].each_key do |item|
- si[item] = 1 if prefs[person2][item]
- end
-
- return 0 if si.empty?
-
- sum1 = si.keys.inject(0){|sum,item| sum + prefs[person1][item]}
- sum2 = si.keys.inject(0){|sum,item| sum + prefs[person2][item]}
-
- sum1Sq = si.keys.inject(0){|sum,item| sum + prefs[person1][item] ** 2}
- sum2Sq = si.keys.inject(0){|sum,item| sum + prefs[person2][item] ** 2}
-
- pSum = si.keys.inject(0){|sum,item| sum + prefs[person1][item] * prefs[person2][item]}
- num = pSum - (sum1 * sum2 / si.size)
- den = Math.sqrt((sum1Sq - sum1 ** 2 / si.size) * (sum2Sq - sum2 ** 2 / si.size))
- return (if den == 0 then 0 else num/den end)
- end
根据前面的两个相似度的函数,我们可以计算和你相同电影的口味的top N了:
- def top_matches(prefs,person,n=5,similarity="sim_pearson")
- scores = []
-
- prefs.each_key{|other| scores << eval("[#{similarity}(prefs,person,other),other]") if other != person}
-
- return scores.sort.reverse[0...n]
- end
下面我们看看如何推荐你没有看过的电影,我们平时的想法是,如果这部电影
大家评论很好,我们就认为值得我们看,但是你的口味可能和这些评论很高的
的人不同,所以和你口味相似的人评论很高的电影,推荐给你效果会很好。
我们这样虽然一个人对一部电影的评价很高,但是由于他和你的口味不同,那么
这个评价对于你的贡献也不会太多。结合相似度和评价的一种方法是:
相似度与评价的成绩作为这个电影评论的一个贡献,同时为了避免评论的人越多
最终的总分越高,可以用这个公式:
所有人(相似度与评论分的成绩) 之和 / 相似度之和,于是我们可以得到如下
代码:
- def get_recommendations(prefs,person,similarity='sim_pearson')
- totals = {}
- simSums = {}
- prefs.each_key do |other|
-
- next if person == other
- sim = eval("#{similarity}(prefs,person,other)")
-
- next if sim <= 0
-
- prefs[other].each_key do |item|
- if (not prefs[person][item]) or (prefs[person][item] == 0) then
-
- totals[item] = if totals[item] then
- totals[item] + prefs[other][item] * sim
- else
- prefs[other][item] * sim
- end
-
- simSums[item] = if simSums[item] then
- simSums[item] + sim
- else
- sim
- end
- end