Item-Based Collaborative Filtering Recommendation Algorithms

【參考文獻】：Sarwar B M . Item-based collaborative filtering recommendation algorithms[C]// International Conference on World Wide Web. ACM, 2001.
背景：推薦領域必讀文獻之一，經典之做，本博客主要記錄了該文章的主要思想和相關實現代碼，歡迎觀摩！

前提或假設

1. 用戶對項目的評分值，可以反應用戶對項目某種程度上的偏好。
2. 用戶過去的偏好極可能展現或者反應將來的興趣偏好。

數據集

咱們選用MovieLens 100K Dataset，=> 100,000 ratings from 1000 users on 1700 movies.
下載地址：movielens數據集

算法理論
算法框架：如圖，輸入是user-item的評分矩陣，該矩陣很是稀疏。算法的任務是預測特定用戶對特定項目的評分，填補矩陣中空白單元格，接着根據預測評分從高到低爲特定用戶進行top-N推薦

算法預測：算法認爲某用戶喜歡某項目，在很大程度上也會對和該項目較類似的項目產生興趣。因此預測分兩步進行：計算項目之間的類似性和根據類似性進行預測評分。
文章提供了三個類似性計算公式：
Cosine-based Similarity
$$ sim(i,j)= cos(\vec{i},\vec{j})= \frac{\vec{i}\cdot \vec{j}}{\left \| \vec{i} \right \|_{2}*\left \| \vec{j} \right \|_{2}} $$
Correlation-based Similarity
$$ sim(i,j)= \frac{\sum _{u\in U}(R_{u,i}-\bar{R}_{i})(R_{u,j}-\bar{R}_{j})}{\sqrt{\sum _{u\in U}(R_{u,i}-\bar{R}_{i})^{2}}\sqrt{\sum _{u\in U}(R_{u,j}-\bar{R}_{j})^{2}}} $$
Adjusted Cosine Similarity
$$ sim(i,j)= \frac{\sum _{u\in U}(R_{u,i}-\bar{R}_{u})(R_{u,j}-\bar{R}_{u})}{\sqrt{\sum _{u\in U}(R_{u,i}-\bar{R}_{u})^{2}}\sqrt{\sum _{u\in U}(R_{u,j}-\bar{R}_{u})^{2}}} $$
可是全部的類似性計算公式必須在共同評分項上進行，即同時評價過i和j的歷史評分

算法選取和該項目最類似的前N個項目做爲預測基礎，預測公式以下：
$$ P_{u,i}=\frac{\sum _{all similar items,N}(S_{i,N}*R_{u,N})}{\sum _{all similar items,N}(\left | S_{i,N} \right |)} $$
算法最後一步，根據預測評分值從高到低進行推薦

實驗度量
文章採用MAE進行偏差度量，公式以下：
$$ MAE = \frac{\sum_{i=1}^{N}\left | p_{i}-q_{i} \right |}{N} $$

Python 代碼

# !usr/bin/python
# -*- coding=utf-8 -*-
import math
import operator
#加載數據
def loadData():
    # trainSet格式爲： testSet格式一致
    # {
    #   userid:{
    #       itemid1: rating,
    #       itemid2: rating
    #   }
    # }
    # movieUser格式爲：看過某一部電影的全部用戶集合
    # {
    #   itemid: {
    #       userid1: rating,
    #       userid2: rating
    #   }
    # }
    # 
    # 
    # 
    trainSet = {}
    testSet = {}
    movieUser = {}

    TrainFile = './dataset/u1.base'  # 指定訓練集
    TestFile = './dataset/u1.test'  # 指定測試集

    # 讀取訓練集
    f = open(TrainFile,'r')
    lines = f.readlines()
    for line in lines:
        arr = line.strip().split('\t')
        userId = arr[0]
        itemId = arr[1]
        rating = arr[2]
        trainSet.setdefault(userId, {})
        trainSet[userId].setdefault(itemId, float(rating))
        movieUser.setdefault(itemId, {})
        movieUser[itemId].setdefault(userId, float(rating))

    # 讀取測試集
    f1 = open(TestFile,'r')
    lines1 = f1.readlines()
    for line1 in lines1:
        arr1 = line1.strip().split('\t')
        userId1 = arr1[0]
        itemId1 = arr1[1]
        rating1 = arr1[2]
        testSet.setdefault(userId1, {})
        testSet[userId1].setdefault(itemId1, float(rating1))

    arr = [trainSet,movieUser]
    return arr


# 生成電影電影共有用戶矩陣
def i_j_users(i_id,j_id,movieUser):
    # ij_users格式爲：
    # {
    #   (i_id,j_id):{userid1:None,userid2:None,....}
    # }
    if i_id in movieUser.keys():
        i_users = movieUser[i_id]
    else:
        i_users = {}
    if j_id in movieUser.keys():
        j_users = movieUser[j_id]
    else:
        j_users = {}
    
    inter = dict.fromkeys([x for x in i_users if x in j_users])
    i_j_users = {(i_id,j_id):inter}
    return i_j_users


#計算一個用戶的平均分數
def getAverageRating(trainSet,userid):
    average = (sum(trainSet[userid].values()) * 1.0) / len(trainSet[userid].keys())
    return average

#計算項目類似度
def getItemSim(i_j_users,i_id,j_id,trainSet):
    # 分子 sumtop
    # 分母 sumbot1  sumbot2
    sumtop = 0
    sumbot1 = 0
    sumbot2 = 0
    ij_users = i_j_users[(i_id,j_id)]
    if not ij_users:
        ij_sim = -9999  # 疑問？ 爲0 或者爲None
    else:
        for user in ij_users.keys():
            avr_user = getAverageRating(trainSet,user)
            # 求分子
            left = trainSet[user][i_id] - avr_user
            right = trainSet[user][j_id] - avr_user
            sumtop += left*right
            # 求分母
            sumbot1 += left*left
            sumbot2 += right*right      
    if sumbot1 == 0 or sumbot2 == 0:
        ij_sim = 1
    else:
        ij_sim = sumtop*1.0 / (math.sqrt(sumbot1)*math.sqrt(sumbot2))
    
    return ij_sim

# 計算項目i和其她全部項目的類似度並排序
# i_allitem_sim格式爲：
# {
#     j_id1:s1,
#     j_id2:s2
# }
def i_allitem_sort(i_id,movieUser,trainSet,N):
    i_allitem = {}
    for j in movieUser.keys():
        if j != i_id:
            i_j_user = i_j_users(i_id,j,movieUser)
            s = getItemSim(i_j_user,i_id,j,trainSet)
            i_allitem.setdefault(j, s)

    i_allitem_sort1 = sorted(i_allitem.items(), key = operator.itemgetter(1), reverse = True)[0:N]
    i_allitem_sort_dict = {}
    for n in range(len(i_allitem_sort1)):
        j1 = i_allitem_sort1[n][0]
        s = i_allitem_sort1[n][1]
        i_allitem_sort_dict.setdefault(j1, s)
    return i_allitem_sort_dict

# 預測評分
def prediction(userid,itemid,moviUser,trainSet,N):
    # predict 格式爲：
    # {
    #   (userid,itemid): pui
    # }
    predict = 0
    sumtop = 0
    sumbot = 0
    nsets = i_allitem_sort(itemid,movieUser,trainSet,N)
    for j in nsets.keys():
        # 防止用戶對i的領域集合內的j沒評分
        if j not in trainSet[userid].keys():
            ruj = 0
            mid = 0
        else:
            ruj = trainSet[userid][j]
            mid = abs(nsets[j])
        sumtop += nsets[j]*ruj
        sumbot += mid
    # 防止分母爲0 
    if sumbot == 0:
        predict = 0
    else:
        predict = sumtop * 1.0 / sumbot
    return predict

def saveFile(moviUser,trainSet,N):
    # 讀取用戶
    string = ""
    # 正在讀取
    f = open("../Collaborative Filtering/dataset/u1.test")
    fw = open("../Collaborative Filtering/predict",'w')
    fl = f.readlines()
    for i in fl:
        arr = i.split('\t')
        uid = str(arr[0].strip())
        item = str(arr[1].strip())
        rating = float(arr[2].strip())
        predictScore = prediction(str(uid),str(item),moviUser,trainSet,N)
        string = string + str(uid) + "\t" + str(item) + "\t" + str(rating) + "\t" + str(predictScore) + "\n"
    fw.write(string)
    f.close()
    fw.close()
    
# 計算預測分析準確度
def getMAE():
    f = open("../Collaborative Filtering/predict")
    fl = f.readlines()
    mae = 0.0
    s = 0
    counttest = 0# 測試集的個數
    for i in fl:
        arr = i.split('\t')
        uid = str(arr[0].strip())
        item = str(arr[1].strip())
        rating = float(arr[2].strip())
        predictScore = float(arr[3].strip())
        if predictScore == 0:
            mid = 0
        else:
            mid = abs((predictScore-rating))
            counttest = counttest + 1
        s = s + mid 
    mae = s/counttest
    print(mae)        


if __name__ == '__main__':

    N = 30
    arr = loadData()
    trainSet = arr[0]
    movieUser = arr[1]
    saveFile(movieUser,trainSet,N)
    # getMAE()