【TensorFlow深度学习】使用TensorFlow实现双DQN与优先级经验回放

使用TensorFlow实现双DQN与优先级经验回放：强化学习的高级策略探索

在深度强化学习领域，双深度Q网络(Double Deep Q-Network, DDQN)与优先级经验回放(Perse Experience Replay)机制是提升学习效率与稳定性的两项关键技术。本文将深入解析双DQN的原理，介绍优先级经验回放的重要性，并通过TensorFlow的代码实例，展现如何结合两者实现高效的学习系统，为复杂决策问题提供解决方案。

双DQN算法简介

双DQN旨在解决标准DQN中的过估计问题，通过分离动作选择与动作评价过程，提高学习的准确性。具体而言，它引入了两个网络：一个用于决策（选择动作），另一个用于评估（计算Q值）。更新时，动作由决策网络选择，但其Q值由评价网络评估，减少了过估计倾向。

优先级经验回放

优先级经验回放通过赋予重要经验（导致高收益或意外结果的事件）更高的采样概率，提高学习效率。它基于每个经验的TD误差（或重要性）建立优先级，使得学习过程聚焦于更有价值的信息。

代码实现

假设使用TensorFlow 2.x版本，环境为OpenAI Gym的CartPole-v0。

import numpy as np
import tensorflow as tf
from collections import deque
import gym
from tensorflow.keras.layers import Dense
from tensorflow.keras.models import Sequential
from tensorflow.keras.optimizers import Adam

# 环境与超参数设置
env = gym.make('CartPole-v0')
state_dim = env.observation_space.shape[0]
action_dim = env.action_space.n
buffer_size = 10000
batch_size = 32
gamma = 0.95
lr = 0.001
epsilon = 1.0
epsilon_decay = 0.995
epsilon_min = 0.01
alpha = 0.6  # 优先级经验回放的α参数
beta = 0.4  # 重要性采样β参数
prioritized_replay = True

# 经验回放缓冲
class PrioritizedReplayBuffer:
    def __init__(self, buffer_size, alpha, beta):
        self.buffer = deque(maxlen=buffer_size)
        self.alpha = alpha
        self.beta = beta
        self.pos = 0
        self.priorities = np.zeros((buffer_size,), dtype=np.float32)

    def store(self, transition):
        max_prio = self.priorities.max() if self.buffer else 1.
        self.priorities[self.pos] = max_prio
        self.buffer.append(transition)
        self.pos = (self.pos + 1) % self.buffer_size

    def sample(self, batch_size):
        if len(self.buffer) < batch_size:
            return None
        
        priorities = self.priorities[:len(self.buffer)]
        probs = priorities ** self.alpha
        probs /= probs.sum()
        
        indices = np.random.choice(len(self.buffer), size=batch_size, replace=False, p=probs)
        samples = [self.buffer[idx] for idx in indices]
        weights = (len(self.buffer) * probs[indices]) ** (-self.beta)
        weights /= weights.max()
        return samples, indices, np.array(weights, dtype=np.float32)

    def update_priorities(self, indices, new_priorities):
        for idx, prio in zip(indices, new_priorities):
            self.priorities[idx] = prio

# 网络结构
def build_model():
    model = Sequential()
    model.add(Dense(24, input_dim=state_dim, activation='relu'))
    model.add(Dense(24, activation='relu'))
    model.add(Dense(action_dim, activation='linear'))
    return model

# 主网络与目标网络
main_model = build_model()
target_model = build_model()
target_model.set_weights(main_model.get_weights())

# 训练习函数
def train(model, target_model, states, actions, rewards, next_states, dones, weights, indices=None):
    next_q_values = target_model.predict_on_batch(next_states)
    max_next_q = np.amax(next_q_values, axis=1)
    targets = rewards + gamma * (1 - dones) * max_next_q

    q_values = model.predict_on_batch(states)
    q_values[range(batch_size), actions] = targets

    if prioritized_replay:
        errors = np.abs(targets - q_values[range(batch_size), actions])
        replay_buffer.update_priorities(indices, errors + 1e-6)  # 避尾部加小量以避免优先级为0

    model.train_on_batch(states, q_values, sample_weight=weights)

# 主循环
replay_buffer = PrioritizedReplay(buffer_size, alpha)
optimizer = Adam(lr)

for episode in range(num_episodes):
    state = env.reset()
    done = False
    episode_reward = 0
    
    while not done:
        if np.random.rand() < epsilon:
            action = env.action_space.sample()
        else:
            q_values = main_model.predict(np.expand_dims(state, axis=0))
            action = np.argmax(q_values)
        
        next_state, reward, done, _ = env.step(action)
        replay_buffer.store((state, action, reward, next_state, done))
        
        state = next_state
        episode_reward += reward
        
        # 学习更新
        if prioritized_replay:
            experience, indices, weights = replay_buffer.sample(batch_size)
            if experience is not None:
                states, actions, rewards, next_states, dones = zip(*experience)
                states, next_states = np.vstack(states), np.vstack(next_states)
                train(main_model, target_model, states, actions, rewards, next_states, dones, weights, indices)
        else:
            # 非优先级经验回放的简化处理
            pass
            
        # ε衰减
        if epsilon > epsilon_min:
            epsilon *= epsilon_decay

    # 定期更新目标网络
    if episode % target_update_freq == 0:
        target_model.set_weights(main_model.get_weights())
        
    print(f"Episode {episode}: Reward: {episode_reward}")

env.close()

结语

通过上述代码，我们不仅理解了双DQN与优先级经验回放在理论上的优势，还实践了如何在TensorFlow框架下实现这一高级强化学习系统。结合两者，不仅提升了学习效率，还增强了模型的稳定性，这对于解决复杂、高维度的现实世界问题至关重要。随着技术的持续发展，双DQN与优先级经验回放等策略将继续在强化学习领域发挥核心作用，推动智能决策系统的前沿探索。