Python函数仅返回索引

Question

因此，我现在正在观看有关马尔可夫链的代码，事情是，我不知道该代码如何工作。

transition_probabilities = [ 
     [[0.7, 0.3, 0.0], [1.0, 0.0, 0.0], [0.8, 0.2, 0.0]], # in s0, if action a0 then proba 0.7 to state s0 and 0.3 to state s1, etc. 
     [[0.0, 1.0, 0.0], None, [0.0, 0.0, 1.0]], 
     [None, [0.8, 0.1, 0.1], None], 
    ] 

rewards = [ 
     [[+10, 0, 0], [0, 0, 0], [0, 0, 0]], 
     [[0, 0, 0], [0, 0, 0], [0, 0, -50]], 
     [[0, 0, 0], [+40, 0, 0], [0, 0, 0]], 
    ] 

possible_actions = [[0, 1, 2], [0, 2], [1]] 

def policy_fire(state): 
    return [0, 2, 1][state] 

def policy_random(state): 
    return rnd.choice(possible_actions[state]) 

def policy_safe(state): 
    return [0, 0, 1][state] 

class MDPEnvironment(object): 
    def __init__(self, start_state=0): 
     self.start_state=start_state 
     self.reset() 
    def reset(self): 
     self.total_rewards = 0 
     self.state = self.start_state 
    def step(self, action): 
     next_state = rnd.choice(range(3), p=transition_probabilities[self.state][action]) 
     reward = rewards[self.state][action][next_state] 
     self.state = next_state 
     self.total_rewards += reward 
     return self.state, reward 

def run_episode(policy, n_steps, start_state=0, display=True): 
    env = MDPEnvironment() 
    if display: 
     print("States (+rewards):", end=" ") 
    for step in range(n_steps): 
     if display: 
      if step == 10: 
       print("...", end=" ") 
      elif step < 10: 
       print(env.state, end=" ") 
     action = policy(env.state) 
     state, reward = env.step(action) 
     if display and step < 10: 
      if reward: 
       print("({})".format(reward), end=" ") 
    if display: 
     print("Total rewards =", env.total_rewards) 
    return env.total_rewards 

for policy in (policy_fire, policy_random, policy_safe): 
    all_totals = [] 
    print(policy.__name__) 
    for episode in range(1000): 
     all_totals.append(run_episode(policy, n_steps=100, display=(episode<5))) 
    print("Summary: mean={:.1f}, std={:1f}, min={}, max={}".format(np.mean(all_totals), np.std(all_totals), np.min(all_totals), np.max(all_totals))) 
    print() 

在第16行中，有一个函数只返回索引。我从来没有听说过这样的事情。我所知道的索引是它必须有一个相应的l_value，以便索引可以订阅数组内的元素或任何内容。那么你们会告诉我发生了什么事？

Answer 1

线15-16：

def policy_fire(state): 
    return [0, 2, 1][state] 

此函数假定state将0和2之间的整数，并且使用该值来索引列表[0, 2, 1]并返回结果值。所以例如policy_fire(1)将返回2。

此功能在功能run_episode中调用，以env.state作为参数，其中env = MDPEnvironment()。