所有算法实现都是在main()实现算法与环境交换与训练。因此，我们对该函数进行抽象，获得范式。

def main():
    '''
    使用'gym'创建游戏环境
    创建buffer
    创建并初始化网络
    初始化更新interval
    为网络建立优化器
    主循环（episode次数）
        env重置
        当done为false：
            采样动作，以及对动作做必要处理
            env.step(动作)【agent采取动作与env交互】
            存储样本于buffer

buffer容量超过阈值：
            多轮训练网络，用batch_size的样本对网络做反传，更新参数
    关闭环境env
    '''  

本节对所有算法的伪代码对照，并进行代码学习。

2.1 DQN

NIPS 13版本的DQN：

对于CartPole问题，我们进一步简化： * 无需预处理Preprocessing。也就是直接获取观察Observation作为状态state输入。 * CartPole问题observation的四个元素分别表示：小车位置、小车速度、杆子夹角及角变化率 * 只使用最基本的MLP神经网络，而不使用卷积神经网络。

算法训练对应代码：

def train(q, q_target, memory, optimizer):
    for i in range(10):
        s,a,r,s_prime,done_mask = memory.sample(batch_size) # tensor形式，每个变量装batch_size的样本 

q_out = q(s)
        q_a = q_out.gather(1,a) # 求当前状态s，执行各个动作的q估计
        max_q_prime = q_target(s_prime).max(1)[0].unsqueeze(1) # 取q估计最大值
        target = r + gamma * max_q_prime * done_mask # 取q估计最大值，通过done_mask考虑了non terminal & terminal state
        loss = F.smooth_l1_loss(q_a, target)

optimizer.zero_grad() # 把梯度置零，也就是把loss关于weight的导数变成0.
        loss.backward()
        optimizer.step() # Performs a single optimization step

class Qnet(nn.Module):
    def __init__(self):
        super(Qnet, self).__init__()
        self.fc1 = nn.Linear(4, 128)
        self.fc2 = nn.Linear(128, 128)
        self.fc3 = nn.Linear(128, 2)

def forward(self, x):
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = self.fc3(x)
        return x

def sample_action(self, obs, epsilon):
        out = self.forward(obs)
        coin = random.random()
        if coin < epsilon:
            return random.randint(0,1)
        else : 
            return out.argmax().item()

epsilon = max(0.01, 0.08 - 0.01*(n_epi/200)) 
#Linear annealing from 8% to 1%：一开始需要更多的探索，所以动作偏随机；渐渐需要动作能够有效，因此减少随机

done_mask = 0.0 if done else 1.0 
memory.put((s,a,r/100.0,s_prime, done_mask)) # 设置 done——mask = 1 or 0, 从buffer取样本时，能处理y target在最终state时的不同情况

2.2 Actor critic

2.3 Advantage actor critic [a2c]

算法对应代码：

envs = ParallelEnv(n_train_processes)

model = ActorCritic()
optimizer = optim.Adam(model.parameters(), lr=learning_rate)

step_idx = 0
s = envs.reset()
print('s',s)
while step_idx < max_train_steps:
    s_lst, a_lst, r_lst, mask_lst = list(), list(), list(), list()
    for _ in range(update_interval): 
        prob = model.pi(torch.from_numpy(s).float()) 
        a = Categorical(prob).sample().numpy() 
        s_prime, r, done, info = envs.step(a) 
        s_lst.append(s)
        a_lst.append(a)
        r_lst.append(r/100.0)
        mask_lst.append(1 - done)

s = s_prime
        step_idx += 1

# 计算 update_interval 内 td target值 【tensor】ie update_interval = 5，torch.Size([5, 3])
    s_final = torch.from_numpy(s_prime).float()
    v_final = model.v(s_final).detach().clone().numpy()
    td_target = compute_target(v_final, r_lst, mask_lst) 

# 计算 update_interval 内 advantage
    td_target_vec = td_target.reshape(-1) 
    s_vec = torch.tensor(s_lst).float().reshape(-1, 4)  # 4 == Dimension of state 
    a_vec = torch.tensor(a_lst).reshape(-1).unsqueeze(1) 
    advantage = td_target_vec - model.v(s_vec).reshape(-1) 

# actor critic loss一并计算
    pi = model.pi(s_vec, softmax_dim=1)
    pi_a = pi.gather(1, a_vec).reshape(-1)
    loss = -(torch.log(pi_a) * advantage.detach()).mean() +\
        F.smooth_l1_loss(model.v(s_vec).reshape(-1), td_target_vec)

# 梯度更新
    optimizer.zero_grad()
    loss.backward()
    optimizer.step()

# 训练后在其他环境测试
    if step_idx % PRINT_INTERVAL == 0:
        test(step_idx, model)

envs.close()

class ActorCritic(nn.Module):
    def __init__(self):
        super(ActorCritic, self).__init__()
        self.fc1 = nn.Linear(4, 256)
        self.fc_pi = nn.Linear(256, 2)
        self.fc_v = nn.Linear(256, 1)

def pi(self, x, softmax_dim=1):
        x = F.relu(self.fc1(x))
        x = self.fc_pi(x)
        prob = F.softmax(x, dim=softmax_dim)
        return prob

def v(self, x):
        x = F.relu(self.fc1(x))
        v = self.fc_v(x)
        return v

2.4 a3c

2.5 DDPG

算法训练对应代码：

# 训练包括了2部分：actor 和 critic 梯度更新；target network更新参数
train(mu, mu_target, q, q_target, memory, q_optimizer, mu_optimizer)
soft_update(mu, mu_target)
soft_update(q,  q_target)

def train(mu, mu_target, q, q_target, memory, q_optimizer, mu_optimizer):
    s,a,r,s_prime,done_mask  = memory.sample(batch_size)

target = r + gamma * q_target(s_prime, mu_target(s_prime)) * done_mask # target policy network决定了target value network
    q_loss = F.smooth_l1_loss(q(s,a), target.detach()) # 从而target policy network和target value network决定了q梯度
    q_optimizer.zero_grad()
    q_loss.backward()
    q_optimizer.step()

mu_loss = -q(s,mu(s)).mean() # That's all for the policy loss. 目标函数是q值，q越大越好
    mu_optimizer.zero_grad()
    mu_loss.backward()
    mu_optimizer.step()

def soft_update(net, net_target):
    for param_target, param in zip(net_target.parameters(), net.parameters()):
        param_target.data.copy_(param_target.data * (1.0 - tau) + param.data * tau)

class MuNet(nn.Module):
    def __init__(self):
        super(MuNet, self).__init__()
        self.fc1 = nn.Linear(3, 128)
        self.fc2 = nn.Linear(128, 64)
        self.fc_mu = nn.Linear(64, 1)

def forward(self, x):
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        mu = torch.tanh(self.fc_mu(x))*2 # Multipled by 2 because the action space of the Pendulum-v0 is [-2,2]
        return mu

class QNet(nn.Module):
    def __init__(self):
        super(QNet, self).__init__()
        self.fc_s = nn.Linear(3, 64)
        self.fc_a = nn.Linear(1,64)
        self.fc_q = nn.Linear(128, 32)
        self.fc_out = nn.Linear(32,1)

def forward(self, x, a):
        h1 = F.relu(self.fc_s(x))
        h2 = F.relu(self.fc_a(a))
        cat = torch.cat([h1,h2], dim=1)
        q = F.relu(self.fc_q(cat))
        q = self.fc_out(q)
        return q

class OrnsteinUhlenbeckNoise:
    def __init__(self, mu):
        self.theta, self.dt, self.sigma = 0.1, 0.01, 0.1
        self.mu = mu
        self.x_prev = np.zeros_like(self.mu)

def __call__(self):
        x = self.x_prev + self.theta * (self.mu - self.x_prev) * self.dt + \
                self.sigma * np.sqrt(self.dt) * np.random.normal(size=self.mu.shape)
        self.x_prev = x
        return x

ou_noise = OrnsteinUhlenbeckNoise(mu=np.zeros(1))

while not done:
            a = mu(torch.from_numpy(s).float())  
            a = a.item() + ou_noise()[0] # 根据当前policy和exploration noise选取动作

2.6 PPO

理解：critic更新方式与ac一样，loss是估计值与实际值对比，做mean squared error 反向传播梯度。

actor更新用advantage，即 每一步多少优势。对loss理解：新policy与老policy比。若优势大，新老policy比大，更新的幅度也大些，后项是减去kl penalty，新老policy差的数，差的太多，当成对loss的惩罚，限制了新策略更新幅度

load_state_dict

q_target.load_state_dict(q.state_dict()) 
# 将q网络中state_dict的parameters和buffers复制到q_target网络中
# 网络继承于module