机器学习算法之交替最小二乘法(ALS)

我不是女神ヾ 2023-08-17 17:06 125阅读 0赞

# 引入 #

我们知道很多豆瓣的用户都会对电影进行打分，根据这个评分，我们可能知道这些用户除了自己评过分之外的电影还偏好哪些电影？这是一个推荐问题，解决这类问题的算法被称作推荐算法。

# 算法原理 #

## 协同过滤 ##

协同过滤简单来说是利用某兴趣相投、拥有共同经验之群体的喜好来推荐用户感兴趣的信息，个人通过合作的机制给予信息相当程度的回应（如评分）并记录下来以达到过滤的目的进而帮助别人筛选信息，回应不一定局限于特别感兴趣的，特别不感兴趣信息的纪录也相当重要。一般来说协同过滤分为三种类型：1.基于用户(user-based)的协同过滤，，通过计算用户和用户的相似度找到跟用户A相似的用户B, C, D…再把这些用户喜欢的内容推荐给A；2.基于物品(item-based)的协同过滤，通过计算物品和物品的相似度找到跟物品1相似的物品2, 3, 4…再把这些物品推荐给看过物品1的用户们；3.基于模型(model-based)的协同过滤。主流方法分为矩阵分解，关联算法，聚类算法，分类算法，回归算法，神经网络。

## ALS ##

本文要介绍的算法ALS是基于矩阵分解，所谓矩阵分解就是将矩阵拆解为多个矩阵的乘积。如果想通过矩阵分解的方法实现基于模型的协同过滤，ALS算法是一个不错的选择，ALS的全称为Alternating Least Square，翻译过来为交替最小二乘法。这里假设用户为a，物品为b，评分矩阵为R(m, n)可以分解为用户矩阵U(k,m)和物品矩阵I(k,n)，其中m,n,k代表矩阵的维度。

*  根据矩阵分解的定义，有  
     R = U T ∗ I R = U^\{T\} \* I R=UT∗I
 *  用MSE作为损失函数，为了方便化简，加法符号左侧的常数改为-1/2
 *   L = − 1 2 ∑ i = 0 n ( R a i − U a T ∗ I i ) 2 L = -\\large\\frac\{1\}\{2\}\\normalsize\\sum\_\{i=0\}^\{n\}(R\_\{ai\} - U\_a^\{T\} \* I\_\{i\})^2 L=−21∑i=0n(Rai−UaT∗Ii)2
 *  对损失函数求U\_a的一阶偏导数，那么  
     d L d U a = ( R a − U a T ∗ I ) ∗ I T \\large\\frac\{dL\}\{dU\_\{a\}\}\\normalsize=(R\_\{a\} - U\_a^\{T\} \* I) \* I^\{T\} dUadL=(Ra−UaT∗I)∗IT  
     d L d U a = I ∗ ( R a T − I T ∗ U a ) \\large\\frac\{dL\}\{dU\_\{a\}\}\\normalsize=I \* (R\_a^\{T\} - I^\{T\} \* U\_\{a\}) dUadL=I∗(RaT−IT∗Ua)
 *  令一阶偏导数等于0  
     I ∗ R a T = I ∗ I T ∗ U a I \* R\_a^\{T\} = I \* I^\{T\} \* U\_\{a\} I∗RaT=I∗IT∗Ua  
     U a = ( I ∗ I T ) − 1 ∗ I ∗ R a T U\_\{a\} = (I \* I^\{T\})^\{-1\} \* I \* R^\{T\}\_\{a\} Ua=(I∗IT)−1∗I∗RaT…(1)
 *  同理，可求  
     I b = ( U ∗ U T ) − 1 ∗ U ∗ R b I\_\{b\} = (U \* U^\{T\})^\{-1\} \* U \* R\_\{b\} Ib=(U∗UT)−1∗U∗Rb…(2)

ALS算法的目标是在已知R的情况下，求出用户矩阵U和物品矩阵I。  
1.我们先随机生成用户矩阵U，利用(2)，R，U求出I。  
2.利用(2)式子，R和I求出U。  
如此交替地执行步骤1和2，直到算法收敛或者迭代次数超过了最大限制，最终我们用RMSE来评价模型的表现。

# 代码实现 #

#coding=utf-8
    
    from collections import defaultdict
    from random import random
    from itertools import product, chain
    from time import time
    
    def load_movie_ratings():
    
        f = open("boston/movie_ratings.csv")
        lines = iter(f)
        col_names = ", ".join(next(lines)[:-1].split(",")[:-1])
        print("The column names are: %s." % col_names)
        data = [[float(x) if i == 2 else int(x)
                 for i, x in enumerate(line[:-1].split(",")[:-1])]
                for line in lines]
        f.close()
        return data
    
    class Matrix(object):
        def __init__(self, data):
            self.data = data
            self.shape = (len(data), len(data[0]))
    
        def row(self, row_no):
            return Matrix([self.data[row_no]])
    
        def col(self, col_no):
            m = self.shape[0]
            return Matrix([[self.data[i][col_no]] for i in range(m)])
    
        @property
        def is_square(self):
            return self.shape[0] == self.shape[1]
    
        @property
        def transpose(self):
            data = list(map(list, zip(*self.data)))
            return Matrix(data)
    
        # 生成一个长度为n的单位阵
        def _eye(self, n):
            return [[0 if i != j else 1 for j in range(n)] for i in range(n)]
    
        @property
        def eye(self):
            assert self.is_squre, "The matrix has to be squre"
            data = self._eye(self.shape[0])
            return Matrix(data)
    
        # 高斯消元
        def gaussian_elimination(self, aug_matrix):
            n = len(aug_matrix)
            m = len(aug_matrix[0])
    
            # From top to bottom.
            for col_idx in range(n):
                # Check if element on the diagonal is zero.
                if aug_matrix[col_idx][col_idx] == 0:
                    row_idx = col_idx
                    # Find a row whose element has same column index with
                    # the element on the diagonal is not zero.
                    while row_idx < n and aug_matrix[row_idx][col_idx] == 0:
                        row_idx += 1
                    # Add this row to the row of the element on the diagonal.
                    for i in range(col_idx, m):
                        aug_matrix[col_idx][i] += aug_matrix[row_idx][i]
    
                # Elimiate the non-zero element.
                for i in range(col_idx + 1, n):
                    # Skip the zero element.
                    if aug_matrix[i][col_idx] == 0:
                        continue
                    # Elimiate the non-zero element.
                    k = aug_matrix[i][col_idx] / aug_matrix[col_idx][col_idx]
                    for j in range(col_idx, m):
                        aug_matrix[i][j] -= k * aug_matrix[col_idx][j]
    
            # From bottom to top.
            for col_idx in range(n - 1, -1, -1):
                # Elimiate the non-zero element.
                for i in range(col_idx):
                    # Skip the zero element.
                    if aug_matrix[i][col_idx] == 0:
                        continue
                    # Elimiate the non-zero element.
                    k = aug_matrix[i][col_idx] / aug_matrix[col_idx][col_idx]
                    for j in chain(range(i, col_idx + 1), range(n, m)):
                        aug_matrix[i][j] -= k * aug_matrix[col_idx][j]
    
            # Iterate the element on the diagonal.
            for i in range(n):
                k = 1 / aug_matrix[i][i]
                aug_matrix[i][i] *= k
                for j in range(n, m):
                    aug_matrix[i][j] *= k
    
            return aug_matrix
    
        # 矩阵求逆
        def _inverse(self, data):
            n = len(data)
            unit_matrix = self._eye(n)
            aug_matrix = [a + b for a, b in zip(self.data, unit_matrix)]
            ret = self.gaussian_elimination(aug_matrix)
    
            return list(map(lambda x: x[n:], ret))
    
        # 矩阵求逆，原理：https://baike.baidu.com/item/%E9%AB%98%E6%96%AF%E6%B6%88%E5%85%83%E6%B3%95/619561?fr=aladdin
        @property
        def inverse(self):
            assert self.is_square, "The matrix has to be square!"
            data = self._inverse(self.data)
    
            return Matrix(data)
    
        def row_mul(self, row_A, row_B):
            return sum(x[0] * x[1] for x in zip(row_A, row_B))
    
        def _mat_mul(self, row_A, B):
            row_pairs = product([row_A], B.transpose.data)
            return [self.row_mul(*row_pair) for row_pair in row_pairs]
    
        def mat_mul(self, B):
            assert self.shape[1] == B.shape[0], "A's column count does not match B's row count!"
            return Matrix([self._mat_mul(row_A, B) for row_A in self.data])
    
        def _mean(self, data):
            m = len(data)
            n = len(data[0])
            ret = [0 for _ in range(n)]
            for row in data:
                for j in range(n):
                    ret[j] += row[j] / m
            return ret
        def mean(self, data):
            return Matrix(self._mean(self.data))
    
    # 统计程序运行时间函数
    # fn代表运行的函数
    def run_time(fn):
        def fun():
            start = time()
            fn()
            ret = time() - start
            if ret < 1e-6:
                unit = "ns"
                ret *= 1e9
            elif ret < 1e-3:
                unit = "us"
                ret *= 1e6
            elif ret < 1:
                unit = "ms"
                ret *= 1e3
            else:
                unit = "s"
            print("Total run time is %.1f %s\n" % (ret, unit))
        return fun()
    
    
    class ALS(object):
        # 初始化，存储用户ID、物品ID、用户ID与用户矩阵列号的对应关系、物品ID
        # 与物品矩阵列号的对应关系、用户已经看过哪些物品、评分矩阵的Shape以及RMSE
        def __init__(self):
            self.user_ids = None
            self.item_ids = None
            self.user_ids_dict = None
            self.item_ids_dict = None
            self.user_matrix = None
            self.item_matrix = None
            self.user_items = None
            self.shape = None
            self.rmse = None
    
        # 对训练数据进行处理，得到用户ID、物品ID、用户ID与用户矩阵列号的对应关系、物
        # 品ID与物品矩阵列号的对应关系、评分矩阵的Shape、评分矩阵及评分矩阵的转置。
        def process_data(self, X):
            self.user_ids = tuple((set(map(lambda x: x[0], X))))
            self.user_ids_dict = dict(map(lambda x: x[::-1], enumerate(self.user_ids)))
            self.item_ids = tuple((set(map(lambda x: x[1], X))))
            self.item_ids_dict = dict(map(lambda x: x[::-1], enumerate(self.item_ids)))
            self.shape = (len(self.user_ids), len(self.item_ids))
            ratings = defaultdict(lambda : defaultdict(int))
            ratings_T = defaultdict(lambda : defaultdict(int))
            for row in X:
                user_id, item_id, rating = row
                ratings[user_id][item_id] = rating
                ratings_T[item_id][user_id] = rating
            err_msg = "Length of user_ids %d and ratings %d not match!" % (
                len(self.user_ids), len(ratings))
            assert len(self.user_ids) == len(ratings), err_msg
            err_msg = "Length of item_ids %d and ratings_T %d not match!" % (
                len(self.item_ids), len(ratings_T))
            assert len(self.item_ids) == len(ratings_T), err_msg
            return ratings, ratings_T
    
        # 用户矩阵乘以评分矩阵，实现稠密矩阵与稀疏矩阵的矩阵乘法，得到用户矩阵与评分矩阵的乘积。
        def users_mul_ratings(self, users, ratings_T):
            def f(users_row, item_id):
                user_ids = iter(ratings_T[item_id].keys())
                scores = iter(ratings_T[item_id].values())
                col_nos = map(lambda x: self.user_ids_dict[x], user_ids)
                _users_row = map(lambda x: users_row[x], col_nos)
                return sum(a * b for a, b in zip(_users_row, scores))
    
            ret = [[f(users_row, item_id) for item_id in self.item_ids]
                   for users_row in users.data]
            return Matrix(ret)
    
        # 物品矩阵乘以评分矩阵，实现稠密矩阵与稀疏矩阵的矩阵乘法，得到物品矩阵与评分矩阵的乘积。
        def items_mul_ratings(self, items, ratings):
            def f(items_row, user_id):
                item_ids = iter(ratings[user_id].keys())
                scores = iter(ratings[user_id].values())
                col_nos = map(lambda x: self.item_ids_dict[x], item_ids)
                _items_row = map(lambda x: items_row[x], col_nos)
                return sum(a * b for a, b in zip(_items_row, scores))
    
            ret = [[f(items_row, user_id) for user_id in self.user_ids]
                   for items_row in items.data]
            return Matrix(ret)
    
        # 生成随机矩阵
        def gen_random_matrix(self, n_rows, n_colums):
            data = [[random() for _ in range(n_colums)] for _ in range(n_rows)]
            return Matrix(data)
    
        # 计算RMSE
        def get_rmse(self, ratings):
            m, n = self.shape
            mse = 0.0
            n_elements = sum(map(len, ratings.values()))
            for i in range(m):
                for j in range(n):
                    user_id = self.user_ids[i]
                    item_id = self.item_ids[j]
                    rating = ratings[user_id][item_id]
                    if rating > 0:
                        user_row = self.user_matrix.col(i).transpose
                        item_col = self.item_matrix.col(j)
                        rating_hat = user_row.mat_mul(item_col).data[0][0]
                        square_error = (rating - rating_hat) ** 2
                        mse += square_error / n_elements
            return mse ** 0.5
    
        # 训练模型
        # 1.数据预处理
        # 2.变量k合法性检查
        # 3.生成随机矩阵U
        # 4.交替计算矩阵U和矩阵I，并打印RMSE信息，直到迭代次数达到max_iter
        # 5.保存最终的RMSE
        def fit(self, X, k, max_iter=10):
            ratings, ratings_T = self.process_data(X)
            self.user_items = {k: set(v.keys()) for k,v in ratings.items()}
            m, n = self.shape
            error_msg = "Parameter k must be less than the rank of original matrix"
            assert k < min(m, n), error_msg
            self.user_matrix = self.gen_random_matrix(k, m)
            for i in range(max_iter):
                if i % 2:
                    items = self.item_matrix
                    self.user_matrix = self.items_mul_ratings(
                        items.mat_mul(items.transpose).inverse.mat_mul(items),
                        ratings
                    )
                else:
                    users = self.user_matrix
                    self.item_matrix = self.users_mul_ratings(
                        users.mat_mul(users.transpose).inverse.mat_mul(users),
                        ratings_T
                    )
                rmse = self.get_rmse(ratings)
                print("Iterations: %d, RMSE: %.6f" % (i + 1, rmse))
            self.rmse = rmse
        # 预测一个用户
        def _predict(self, user_id, n_items):
            users_col =  self.user_matrix.col(self.user_ids_dict[user_id])
            users_col = users_col.transpose
    
            items_col = enumerate(users_col.mat_mul(self.item_matrix).data[0])
            items_scores = map(lambda x: (self.item_ids[x[0]], x[1]), items_col)
            viewed_items = self.user_items[user_id]
            items_scores = filter(lambda x: x[0] not in viewed_items, items_scores)
    
            return sorted(items_scores, key=lambda x: x[1], reverse=True)[:n_items]
    
        # 预测多个用户
        def predict(self, user_ids, n_items=10):
            return [self._predict(user_id, n_items) for user_id in user_ids]
    
    def format_prediction(item_id, score):
        return "item_id:%d score:%.2f" % (item_id, score)
    
    @run_time
    def main():
        print("Tesing the accuracy of ALS...")
    
        X = load_movie_ratings()
    
        model = ALS()
        model.fit(X, k=3, max_iter=5)
        print("Showing the predictions of users...")
    
        user_ids = range(1, 5)
        predictions = model.predict(user_ids, n_items=2)
        for user_id, prediction in zip(user_ids, predictions):
            _prediction = [format_prediction(item_id, score)
                           for item_id, score in prediction]
            print("User id:%d recommedation: %s" % (user_id, _prediction))

# 在影评数据集上测试结果如下 #

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