基于显式评分

def ItemSimilarity(self):

    # 一个字典，记录 rui*ruj
    # 一个字典 记录 rui^2
    # 建立物品-物品的共现矩阵
    item2item = dict()  # 物品-物品的共现矩阵。记录rui*ruj，分子
    buy = dict()  # 物品被多少个不同用户进行过评分。记录rui*rui分母
    for user, items in self.data.items():
        for i in items.keys():
            buy.setdefault(i, 0)
            temp = items[i]*items[i]
            buy[i] += temp
            item2item.setdefault(i, {})
            for j in items.keys():
                if i == j: continue
                item2item[i].setdefault(j, 0)
                item2item[i][j] += items[i]*items[j]
    # 计算相似度矩阵
    self.similar_matrix = dict()
    for i, related_items in item2item.items():
        self.similar_matrix.setdefault(i, {})
        for j, cij in related_items.items():
            self.similar_matrix[i][j] = cij / (math.sqrt(buy[i]) * math.sqrt(buy[j]))
    return self.similar_matrix