（数据挖掘-入门-1）基于用户的协同过滤之最近邻

主要内容：

1、什么是基于用户的协同过滤

2、python实现

1、什么是基于用户协同过滤：

协同过滤：Collaborative Filtering，一般用于推荐系统，如京东，亚马逊等电商网站上的“购买该物品的用户还喜欢/购买”之类的栏目都是根据协同过滤推荐出来的。

基于用户的协同过滤：User-based CF，通过不同用户对item（物品）的评分来评测用户之间的相似性，基于用户之间的相似性做出推荐。

这里介绍一种最简单的过滤方法：最近邻，即找到与某用户最相似的用户，将该用户喜欢的物品（而某用户并未评分的物品）推荐给某用户。

缺点：

1、用户少，物品多，并不是每个用户都对每个物品进行过评分，因此存在缺失值；

2、如果相似的用户和被推荐的用户评分的物品都相同，会出现无物品推荐的情况；

细节：

衡量相似性：曼哈顿距离，欧几里得距离等（简单，后续介绍其他相似度的计算方法）

当r=1，为曼哈顿距离；当r=2，为欧几里得距离。

2、Python实现

场景：基于用户对一些书籍的评分，来为某些用户推荐书籍；

数据：如下表

实现：

Python（有关python的语法就不介绍了，直接贴出代码）

#
#  FILTERINGDATA.py
#
#  Code file for the book Programmer's Guide to Data Mining
#  http://guidetodatamining.com
#  Ron Zacharski
#

from math import sqrt

users = {"Angelica": {"Blues Traveler": 3.5, "Broken Bells": 2.0, "Norah Jones": 4.5, "Phoenix": 5.0, "Slightly Stoopid": 1.5, "The Strokes": 2.5, "Vampire Weekend": 2.0},
         "Bill":{"Blues Traveler": 2.0, "Broken Bells": 3.5, "Deadmau5": 4.0, "Phoenix": 2.0, "Slightly Stoopid": 3.5, "Vampire Weekend": 3.0},
         "Chan": {"Blues Traveler": 5.0, "Broken Bells": 1.0, "Deadmau5": 1.0, "Norah Jones": 3.0, "Phoenix": 5, "Slightly Stoopid": 1.0},
         "Dan": {"Blues Traveler": 3.0, "Broken Bells": 4.0, "Deadmau5": 4.5, "Phoenix": 3.0, "Slightly Stoopid": 4.5, "The Strokes": 4.0, "Vampire Weekend": 2.0},
         "Hailey": {"Broken Bells": 4.0, "Deadmau5": 1.0, "Norah Jones": 4.0, "The Strokes": 4.0, "Vampire Weekend": 1.0},
         "Jordyn":  {"Broken Bells": 4.5, "Deadmau5": 4.0, "Norah Jones": 5.0, "Phoenix": 5.0, "Slightly Stoopid": 4.5, "The Strokes": 4.0, "Vampire Weekend": 4.0},
         "Sam": {"Blues Traveler": 5.0, "Broken Bells": 2.0, "Norah Jones": 3.0, "Phoenix": 5.0, "Slightly Stoopid": 4.0, "The Strokes": 5.0},
         "Veronica": {"Blues Traveler": 3.0, "Norah Jones": 5.0, "Phoenix": 4.0, "Slightly Stoopid": 2.5, "The Strokes": 3.0}
        }


def manhattan(rating1, rating2):
    """Computes the Manhattan distance. Both rating1 and rating2 are dictionaries
       of the form {'The Strokes': 3.0, 'Slightly Stoopid': 2.5}"""
    distance = 0
    commonRatings = False 
    for key in rating1:
        if key in rating2:
            distance += abs(rating1[key] - rating2[key])
            commonRatings = True
    if commonRatings:
        return distance
    else:
        return -1 #Indicates no ratings in common

def minskowski(rating1,rating2):
    distance=0
    commonRatings=Flase
    for key in rating1:
        for key in rating2:
            distance+=pow(abs(rating1[key]-rating2[key]),r)
            commonRatings=True
    if commonRatings:
        return pow(distance,1/r)
    else:
        return 0 #indicates no ratings in common

def computeNearestNeighbor(username, users):
    """creates a sorted list of users based on their distance to username"""
    distances = []
    for user in users:
        if user != username:
            distance = manhattan(users[user], users[username])
            #distance = minskowski(users[user], users[username], 2)
            distances.append((distance, user))
    # sort based on distance -- closest first
    distances.sort()
    return distances

def recommend(username, users):
    """Give list of recommendations"""
    # first find nearest neighbor
    nearest = computeNearestNeighbor(username, users)[0][1]

    recommendations = []
    # now find bands neighbor rated that user didn't
    neighborRatings = users[nearest]
    userRatings = users[username]
    for artist in neighborRatings:
        if not artist in userRatings:
            recommendations.append((artist, neighborRatings[artist]))
    # using the fn sorted for variety - sort is more efficient
    return sorted(recommendations, key=lambda artistTuple: artistTuple[1], reverse = True)

# examples - uncomment to run

print( recommend('Hailey', users))
#print( recommend('Chan', users))

3、参考文献：

http://www.guidetodatamining.com/chapter2/