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Title: Gymnasium Cart Pole 环境与 REINFORCE 算法 —— 强化学习入门 2

文章目录

I. Gymnasium Cart Pole 环境
II. REINFORCE 算法
- 1. 原理说明
- 2. REINFORCE 算法实现

I. Gymnasium Cart Pole 环境

Gymnasium Cart Pole 环境是一个倒立摆的动力学仿真环境.

状态空间:

0: Cart Position

1: Cart Velocity

2: Pole Angle

3: Pole Angular Velocity

动作空间:

0: Push cart to the left

1: Push cart to the right

即时激励:

为了更长时间地保持倒立摆呈倒立状态, 每一时间步都是获得即时激励 +1.

回合结束判据:

Termination: Pole Angle is greater than ±12°

Termination: Cart Position is greater than ±2.4 (center of the cart reaches the edge of the display)

Truncation: Episode length is greater than 200

II. REINFORCE 算法

1. 原理说明

REINFORCE 算法原理及 Python实现, 我们参考了 Foundations of Deep Reinforcement Learning: Theory and Practice in Python.
需要说明的是, 我们此处采用了 Improving REINFORCE
$\nabla_{\theta} J(\pi_\theta) \approx \sum_{t=0}^{T} \left(R_t(\tau)-b\right) \nabla_{\theta}\log\pi_\theta(a_t|s_t)$
其中 $b$ 是整个轨迹上的回报均值, 是每条轨迹的常值基线.
$b=\frac{1}{T} \sum_{t=0}^{T} R_t(\tau)$
另外, 我们设定连续 15 次倒立摆控制成功后, 结束 REINFORCE 算法训练, 并保存策略映射神经网络.

测试的时候, 加载已保存的策略映射神经网络, 加长测试时间步, 也都能较好控制倒立摆.

2. REINFORCE 算法实现

REINFORCE 算法的策略映射网络:

class Pi(nn.Module):# a policy network to be optimized in reinforcement learning# 待优化的策略网络def __init__(self, in_dim, out_dim): # in_dim = 4, out_dim = 2# super(Pi, self).__init__()super().__init__()# a policy networklayers = [nn.Linear(in_dim, 64), # 4 -> 64nn.ReLU(), # activation functionnn.Linear(64, out_dim), # 64 -> 2]self.model = nn.Sequential(*layers) self.onpolicy_reset()  # initialize memoryself.train()  # Set the model to training modedef onpolicy_reset(self):self.log_probs = []self.rewards = []def forward(self, x): # x -> statepdparam = self.model(x) # forward passreturn pdparam# pdparam -> probability distribution# such as the logits of a categorical distributiondef act(self, state):# Convert the state from a NumPy array to a PyTorch tensor# 由策略网络输出的采样动作和对数概率分布x = torch.from_numpy(state.astype(np.float32)) # print("state: {}".format(state))pdparam = self.forward(x)     # Perform a forward pass through the neural network   # print("pdparam: {}".format(pdparam))# to obtain the probability distribution parameterspd = torch.distributions.Categorical(logits=pdparam) # probability distribution# print("pd.probs: {}\t pd.logits: {}".format(pd.probs, pd.logits))action = pd.sample()            # pi(a|s) in action via pd#calculates the log probability of the sampled action action under the probability distribution pd#$\log(\pi_{\theta}(a_t|s_t))$#where $\pi_{\theta}$ is the policy network,#	$a_t$ is the action at time step $t$,#	$s_t$ is the state at time step $t$log_prob = pd.log_prob(action)  # log_prob of pi(a|s), log_prob = pd.logitsself.log_probs.append(log_prob) # store for trainingreturn action.item()  # extracts the value of a single-element tensor as a scalar

对策略映射网络的方向传播训练:

def train(pi, optimizer):# 以下利用蒙特卡洛法计算损失函数值，并利用梯度上升法更新策略网络参数# 蒙特卡洛法需要采样多条轨迹来求损失函数的均值，但是为了简化只采样了一条轨迹当做均值# Inner gradient-ascent loop of REINFORCE algorithmT = len(pi.rewards)rets = np.empty(T, dtype=np.float32)  # Initialize returnsfuture_ret = 0.0# compute the returns efficiently in reverse order# R_t(\tau) = \Sigma_{t'=t}^{T} {\gamma^{t'-t} r_{t'}}for t in reversed(range(T)):future_ret = pi.rewards[t] + gamma * future_retrets[t] = future_retbaseline = sum(rets) / Trets = torch.tensor(rets)rets = rets - baseline  # modify the returns by subtracting a baselinelog_probs = torch.stack(pi.log_probs)# - R_t(\tau) * log(\pi_{\theta}(a_t|s_t))# Negative for maximizingloss = - log_probs * rets  #  - \Sigma_{t=0}^{T}  [R_t(\tau) * log(\pi_{\theta}(a_t|s_t))] loss = torch.sum(loss)optimizer.zero_grad()# backpropagate, compute gradients# computes the gradients of the loss with respect to the model's parameters (\theta)loss.backward()   # gradient-ascent, update the weights of the policy network          optimizer.step()            return loss

多回合强化学习训练, 连续多次控制倒立摆成功就结束整个 REINFORCE 算法的训练.

def train_main():env = gym.make('CartPole-v1', render_mode="human")in_dim = env.observation_space.shape[0] # 4out_dim = env.action_space.n # 2pi = Pi(in_dim, out_dim)   # an ibstance of the policy network for REINFORCE algorithmoptimizer = optim.Adam(pi.parameters(), lr=0.01)episode = 0continuous_solved_episode = 0# for epi in range(300): # episode = 300while continuous_solved_episode <= 14:# state = env.reset() # gymstate, _ = env.reset()  # gymnasiumfor t in range(200):  # cartpole max timestep is 200action = pi.act(state)# state, reward, done, _ = env.step(action)  # gymstate, reward, done, _, _ = env.step(action)  # gymnasiumpi.rewards.append(reward)env.render()if done:breakloss = train(pi, optimizer) # train per episodetotal_reward = sum(pi.rewards)   solved = total_reward > 195.0episode += 1if solved:continuous_solved_episode += 1else:continuous_solved_episode = 0print(f'Episode {episode}, loss: {loss}, \total_reward: {total_reward}, solved: {solved}, contnuous_solved: {continuous_solved_episode}')pi.onpolicy_reset()   # onpolicy: clear memory after trainingsave_model(pi)

一个简单的训练录屏

测试需要在神经网络的 evaluation 模式下进行, 测试中可以完成更长时间的倒立摆控制.

def test_process():env = gym.make('CartPole-v1', render_mode="human")# in_dim = env.observation_space.shape[0] # 4# out_dim = env.action_space.n # 2# pi_model = Pi(in_dim, out_dim)pi_model = torch.load(model_path)# set the model to evaluation modepi_model.eval()# 进行前向传播with torch.no_grad():pi_model.onpolicy_reset()   # onpolicy: clear memory after trainingstate, _ = env.reset()  # gymnasiumsteps = 600for t in range(steps):  # cartpole max timestep is 2000action = pi_model.act(state)state, reward, done, _, _ = env.step(action) pi_model.rewards.append(reward)env.render()if done:breaktotal_reward = sum(pi_model.rewards)   solved = total_reward >= stepsprint(f'[Test] total_reward: {total_reward}, solved: {solved}')

一个简单的测试录屏

完整代码:

import gymnasium as gym
# import gymimport numpy as np
import torch
import torch.nn as nn
import torch.optim as optim
import sysgamma = 0.99 # discount factor
model_path = "./reinforce_pi.pt" class Pi(nn.Module):# a policy network to be optimized in reinforcement learning# 待优化的策略网络def __init__(self, in_dim, out_dim): # in_dim = 4, out_dim = 2# super(Pi, self).__init__()super().__init__()# a policy networklayers = [nn.Linear(in_dim, 64), # 4 -> 64nn.ReLU(), # activation functionnn.Linear(64, out_dim), # 64 -> 2]self.model = nn.Sequential(*layers) self.onpolicy_reset()  # initialize memoryself.train()  # Set the model to training modedef onpolicy_reset(self):self.log_probs = []self.rewards = []def forward(self, x): # x -> statepdparam = self.model(x) # forward passreturn pdparam# pdparam -> probability distribution# such as the logits of a categorical distributiondef act(self, state):# Convert the state from a NumPy array to a PyTorch tensor# 由策略网络输出的采样动作和对数概率分布x = torch.from_numpy(state.astype(np.float32)) # print("state: {}".format(state))pdparam = self.forward(x)     # Perform a forward pass through the neural network   # print("pdparam: {}".format(pdparam))# to obtain the probability distribution parameterspd = torch.distributions.Categorical(logits=pdparam) # probability distribution# print("pd.probs: {}\t pd.logits: {}".format(pd.probs, pd.logits))action = pd.sample()            # pi(a|s) in action via pd#calculates the log probability of the sampled action action under the probability distribution pd#$\log(\pi_{\theta}(a_t|s_t))$#where $\pi_{\theta}$ is the policy network,#	$a_t$ is the action at time step $t$,#	$s_t$ is the state at time step $t$log_prob = pd.log_prob(action)  # log_prob of pi(a|s), log_prob = pd.logitsself.log_probs.append(log_prob) # store for trainingreturn action.item()  # extracts the value of a single-element tensor as a scalardef train(pi, optimizer):# 以下利用蒙特卡洛法计算损失函数值，并利用梯度上升法更新策略网络参数# 蒙特卡洛法需要采样多条轨迹来求损失函数的均值，但是为了简化只采样了一条轨迹当做均值# Inner gradient-ascent loop of REINFORCE algorithmT = len(pi.rewards)rets = np.empty(T, dtype=np.float32)  # Initialize returnsfuture_ret = 0.0# compute the returns efficiently in reverse order# R_t(\tau) = \Sigma_{t'=t}^{T} {\gamma^{t'-t} r_{t'}}for t in reversed(range(T)):future_ret = pi.rewards[t] + gamma * future_retrets[t] = future_retbaseline = sum(rets) / Trets = torch.tensor(rets)rets = rets - baseline  # modify the returns by subtracting a baselinelog_probs = torch.stack(pi.log_probs)# - R_t(\tau) * log(\pi_{\theta}(a_t|s_t))# Negative for maximizingloss = - log_probs * rets  #  - \Sigma_{t=0}^{T}  [R_t(\tau) * log(\pi_{\theta}(a_t|s_t))] loss = torch.sum(loss)optimizer.zero_grad()# backpropagate, compute gradients# computes the gradients of the loss with respect to the model's parameters (\theta)loss.backward()   # gradient-ascent, update the weights of the policy network          optimizer.step()            return lossdef save_model(pi):print("pi.state_dict(): {}\n\n".format(pi.state_dict()))for param_tensor in pi.state_dict():print(param_tensor, "\t", pi.state_dict()[param_tensor].size())torch.save(pi, model_path)def train_main():env = gym.make('CartPole-v1', render_mode="human")in_dim = env.observation_space.shape[0] # 4out_dim = env.action_space.n # 2pi = Pi(in_dim, out_dim)   # an ibstance of the policy network for REINFORCE algorithmoptimizer = optim.Adam(pi.parameters(), lr=0.01)episode = 0continuous_solved_episode = 0# for epi in range(300): # episode = 300while continuous_solved_episode <= 14:# state = env.reset() # gymstate, _ = env.reset()  # gymnasiumfor t in range(200):  # cartpole max timestep is 200action = pi.act(state)# state, reward, done, _ = env.step(action)  # gymstate, reward, done, _, _ = env.step(action)  # gymnasiumpi.rewards.append(reward)env.render()if done:breakloss = train(pi, optimizer) # train per episodetotal_reward = sum(pi.rewards)   solved = total_reward > 195.0episode += 1if solved:continuous_solved_episode += 1else:continuous_solved_episode = 0print(f'Episode {episode}, loss: {loss}, \total_reward: {total_reward}, solved: {solved}, contnuous_solved: {continuous_solved_episode}')pi.onpolicy_reset()   # onpolicy: clear memory after trainingsave_model(pi)def usage():if len(sys.argv) != 2:print("Usage: python ./REINFORCE.py --train/--test")sys.exit()mode = sys.argv[1]return mode def test_process():env = gym.make('CartPole-v1', render_mode="human")# in_dim = env.observation_space.shape[0] # 4# out_dim = env.action_space.n # 2# pi_model = Pi(in_dim, out_dim)pi_model = torch.load(model_path)# set the model to evaluation modepi_model.eval()# 进行前向传播with torch.no_grad():pi_model.onpolicy_reset()   # onpolicy: clear memory after trainingstate, _ = env.reset()  # gymnasiumsteps = 600for t in range(steps):  # cartpole max timestep is 2000action = pi_model.act(state)state, reward, done, _, _ = env.step(action) pi_model.rewards.append(reward)env.render()if done:breaktotal_reward = sum(pi_model.rewards)   solved = total_reward >= stepsprint(f'[Test] total_reward: {total_reward}, solved: {solved}')if __name__ == '__main__':mode = usage()if mode == "--train":train_main()elif mode == "--test":test_process()