深度Q网络DQN(Deep Q-Network)强化学习的原理与实战
作者:wx62088446a1f70
DQN(Deep Q-Network)是一种基于深度学习和强化学习的算法,由DeepMind提出,用于解决离散动作空间下的马尔科夫决策过程(MDP)问题。它是首个成功将深度学习应用于解决强化学习任务的算法之一。DQN,即深度Q网络(Deep Q-network),是指基于深度学习的Q-Learing算法。
一、强化学习基础
强化学习(Reinforcement Learning)是机器学习的一个重要分支,其核心思想是通过与环境的交互学习最优策略。与监督学习不同,强化学习不需要预先准备好的输入-输出对,而是通过试错机制获得奖励信号来指导学习。
1.1 核心概念
• 智能体(Agent):学习的执行者 • 环境(Environment):智能体交互的对象 • 状态(State):环境的当前情况 • 动作(Action):智能体的行为 • 奖励(Reward):环境对动作的反馈 • 策略(Policy):状态到动作的映射
1.2 马尔可夫决策过程
强化学习问题通常建模为马尔可夫决策过程(MDP),由五元组(S, A, P, R, γ)组成: • S:状态集合 • A:动作集合 • P:状态转移概率 • R:奖励函数 • γ:折扣因子(0≤γ<1)
二、Q学习与深度Q网络
2.1 Q学习算法
Q学习是一种经典的强化学习算法,通过维护一个Q值表来估计在给定状态下采取某个动作的长期回报:
import numpy as np # 初始化Q表 q_table = np.zeros((state_space_size, action_space_size)) # Q学习更新公式 alpha = 0.1 # 学习率 gamma = 0.99 # 折扣因子 for episode in range(total_episodes): state = env.reset() done = False while not done: action = select_action(state) # ε-greedy策略 next_state, reward, done, _ = env.step(action) # Q值更新 q_table[state, action] = q_table[state, action] + alpha * ( reward + gamma * np.max(q_table[next_state]) - q_table[state, action] ) state = next_state
2.2 深度Q网络(DQN)
当状态空间较大时,Q表变得不切实际。DQN使用神经网络近似Q函数:
import torch import torch.nn as nn import torch.optim as optim class DQN(nn.Module): def __init__(self, input_dim, output_dim): super(DQN, self).__init__() self.fc1 = nn.Linear(input_dim, 128) self.fc2 = nn.Linear(128, 128) self.fc3 = nn.Linear(128, output_dim) def forward(self, x): x = torch.relu(self.fc1(x)) x = torch.relu(self.fc2(x)) return self.fc3(x)
三、DQN的改进技术
3.1 经验回放(Experience Replay)
解决样本相关性和非平稳分布问题:
from collections import deque import random class ReplayBuffer: def __init__(self, capacity): self.buffer = deque(maxlen=capacity) def push(self, state, action, reward, next_state, done): self.buffer.append((state, action, reward, next_state, done)) def sample(self, batch_size): return random.sample(self.buffer, batch_size) def __len__(self): return len(self.buffer)
3.2 目标网络(Target Network)
稳定训练过程:
target_net = DQN(input_dim, output_dim).to(device) target_net.load_state_dict(policy_net.state_dict()) target_net.eval() # 定期更新目标网络 if steps_done % TARGET_UPDATE == 0: target_net.load_state_dict(policy_net.state_dict())
四、完整DQN实现(CartPole环境)
import gym import numpy as np import torch import random from collections import deque import matplotlib.pyplot as plt # 超参数 BATCH_SIZE = 128 GAMMA = 0.99 EPS_START = 0.9 EPS_END = 0.05 EPS_DECAY = 200 TARGET_UPDATE = 10 LR = 0.001 # 初始化环境 env = gym.make('CartPole-v1') state_dim = env.observation_space.shape[0] action_dim = env.action_space.n # 神经网络定义 class DQN(nn.Module): def __init__(self, input_dim, output_dim): super(DQN, self).__init__() self.fc1 = nn.Linear(input_dim, 64) self.fc2 = nn.Linear(64, 64) self.fc3 = nn.Linear(64, output_dim) def forward(self, x): x = torch.relu(self.fc1(x)) x = torch.relu(self.fc2(x)) return self.fc3(x) # 初始化网络 policy_net = DQN(state_dim, action_dim).to(device) target_net = DQN(state_dim, action_dim).to(device) target_net.load_state_dict(policy_net.state_dict()) optimizer = optim.Adam(policy_net.parameters(), lr=LR) memory = ReplayBuffer(10000) # 训练过程 def train(): if len(memory) < BATCH_SIZE: return transitions = memory.sample(BATCH_SIZE) batch = list(zip(*transitions)) state_batch = torch.FloatTensor(np.array(batch[0])) action_batch = torch.LongTensor(np.array(batch[1])) reward_batch = torch.FloatTensor(np.array(batch[2])) next_state_batch = torch.FloatTensor(np.array(batch[3])) done_batch = torch.FloatTensor(np.array(batch[4])) current_q = policy_net(state_batch).gather(1, action_batch.unsqueeze(1)) next_q = target_net(next_state_batch).max(1)[0].detach() expected_q = reward_batch + (1 - done_batch) * GAMMA * next_q loss = nn.MSELoss()(current_q.squeeze(), expected_q) optimizer.zero_grad() loss.backward() optimizer.step() # 主训练循环 episode_rewards = [] for episode in range(500): state = env.reset() total_reward = 0 done = False while not done: # ε-greedy动作选择 eps_threshold = EPS_END + (EPS_START - EPS_END) * \ np.exp(-1. * episode / EPS_DECAY) if random.random() > eps_threshold: with torch.no_grad(): action = policy_net(torch.FloatTensor(state)).argmax().item() else: action = random.randint(0, action_dim-1) next_state, reward, done, _ = env.step(action) memory.push(state, action, reward, next_state, done) state = next_state total_reward += reward train() episode_rewards.append(total_reward) if episode % 10 == 0: print(f"Episode {episode}, Total Reward: {total_reward}") # 绘制训练曲线 plt.plot(episode_rewards) plt.xlabel('Episode') plt.ylabel('Total Reward') plt.title('DQN Training Progress') plt.show()
五、DQN的局限性与发展
- 过估计问题:Double DQN通过解耦动作选择和Q值评估来解决
- 优先经验回放:给重要的转移更高采样概率
- 竞争网络架构:Dueling DQN分离价值函数和优势函数
- 分布式强化学习:学习价值分布而不仅是期望值
六、总结
深度Q学习将深度神经网络与强化学习相结合,解决了传统Q学习在高维状态空间下的局限性。通过经验回放和目标网络等技术,DQN能够在复杂环境中学习有效的策略。本文通过CartPole环境的完整实现,展示了DQN的核心思想和实现细节。未来,结合改进技术和更强大的网络架构,深度强化学习将在机器人控制、游戏AI等领域发挥更大作用。
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