集体智慧编程学习笔记（1）推荐系统

1、协作型过滤
   对一大群人搜索，从中选择与我们品味相近的一小群人,算法会对这小群人其他内容进行考察，并将他们组合起来构造一个经过排名的推荐列表
2、搜集偏好
recommendations.py
# A dictionary of movie critics and their ratings of a small
# set of movies
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}}

3、寻找相近的用户
将每个人与其他人进行对比，并计算它们的相似度评价值
1)欧几里得距离
toby和lasalle之间的距离
在recommendations.py中添加代码

from math import sqrt
# Returns a distance-based similarity score for person1 and person2
def sim_distance(prefs,person1,person2):
  # Get the list of shared_items
  si={}
  for item in prefs[person1]:
    if item in prefs[person2]:
       si[item]=1
  # if they have no ratings in common, return 0
  if len(si)==0: return 0
  # Add up the squares of all the differences
  sum_of_squares=sum([pow(prefs[person1][item]-prefs[person2][item],2)
                      for item in prefs[person1] if item in prefs[person2]])
  return 1/(1+sum_of_squares)

>>> reload(recommendations)
>>> recommendations.sim_distance(recommendations.critics,
...   'Lisa Rose','Gene Seymour')
0.148148148148

2) 皮尔逊相关度评价

def sim_pearson(prefs,p1,p2):
  # Get the list of mutually rated items
  si={}
  for item in prefs[p1]:
    if item in prefs[p2]: si[item]=1
  # Find the number of elements
  n=len(si)
  # if they are no ratings in common, return 0
  if n==0: return 0
  # Add up all the preferences
  sum1=sum([prefs[p1][it] for it in si])
  sum2=sum([prefs[p2][it] for it in si])
  # Sum up the squares
  sum1Sq=sum([pow(prefs[p1][it],2) for it in si])
  sum2Sq=sum([pow(prefs[p2][it],2) for it in si])
  # Sum up the products
  pSum=sum([prefs[p1][it]*prefs[p2][it] for it in si])
  # Calculate Pearson score
  num=pSum-(sum1*sum2/n)
  den=sqrt((sum1Sq-pow(sum1,2)/n)*(sum2Sq-pow(sum2,2)/n))
  if den==0: return 0
  r=num/den
  return r

>>> reload(recommendations)
>>> print recommendations.sim_pearson(recommendations.critics,
...  'Lisa Rose','Gene Seymour')
0.396059017191