Actor Critic Method

Introduction

An implementation of Actor Critic method on CartPole-V0 environment.

Actor Critic Method

As an agent takes actions and moves through an environment, it learns to map the observed state of the environment to two possible outputs:

Recommended action: A probabiltiy value for each action in the action space. The part of the agent responsible for this output is called the actor.
Estimated rewards in the future: Sum of all rewards it expects to receive in the future. The part of the agent responsible for this output is the critic.

Agent and Critic learn to perform their tasks, such that the recommended actions from the actor maximize the rewards.

CartPole-V0

A pole is attached to a cart placed on a frictionless track. The agent has to apply force to move the cart. It is rewarded for for every time step the pole remains upright. The agent, therefore, must learn to keep the pole from falling over.

import gym
import numpy as np
import tensorflow as tf
from tensorflow import keras
from tensorflow.keras import layers

# Configuration paramaters for the whole setup
seed = 42
gamma = 0.99  # Discount factor for past rewards
max_steps_per_episode = 10000
env = gym.make("CartPole-v0")  # Create the environment
env.seed(seed)
eps = np.finfo(np.float32).eps.item()  # Smallest number such that 1.0 + eps != 1.0

Implement Actor Critic network

This network learns two functions:

Actor: This takes as input the state of our environment and returns a probability value for each action in its action space.
Critic: This takes as input the state of our environment and returns an estimate of total rewards in the future.

In our implementation, they share the initial layer.

num_inputs = 4
num_actions = 2
num_hidden = 128

inputs = layers.Input(shape=(num_inputs,))
common = layers.Dense(num_hidden, activation="relu")(inputs)
action = layers.Dense(num_actions, activation="softmax")(common)
critic = layers.Dense(1)(common)

model = keras.Model(inputs=inputs, outputs=[action, critic])

Train

optimizer = keras.optimizers.Adam(learning_rate=0.01)
huber_loss = keras.losses.Huber()
action_probs_history = []
critic_value_history = []
rewards_history = []
running_reward = 0
episode_count = 0

while True:  # Run until solved
    state = env.reset()
    episode_reward = 0
    with tf.GradientTape() as tape:
        for timestep in range(1, max_steps_per_episode):
            # env.render(); Adding this line would show the attempts
            # of the agent in a pop up window.

            state = tf.convert_to_tensor(state)
            state = tf.expand_dims(state, 0)

            # Predict action probabilities and estimated future rewards
            # from environment state
            action_probs, critic_value = model(state)
            critic_value_history.append(critic_value[0, 0])

            # Sample action from action probability distribution
            action = np.random.choice(num_actions, p=np.squeeze(action_probs))
            action_probs_history.append(tf.math.log(action_probs[0, action]))

            # Apply the sampled action in our environment
            state, reward, done, _ = env.step(action)
            rewards_history.append(reward)
            episode_reward += reward

            if done:
                break

        # Update running reward to check condition for solving
        running_reward = 0.05 * episode_reward + (1 - 0.05) * running_reward

        # Calculate expected value from rewards
        # - At each timestep what was the total reward received after that timestep
        # - Rewards in the past are discounted by multiplying them with gamma
        # - These are the labels for our critic
        returns = []
        discounted_sum = 0
        for r in rewards_history[::-1]:
            discounted_sum = r + gamma * discounted_sum
            returns.insert(0, discounted_sum)

        # Normalize
        returns = np.array(returns)
        returns = (returns - np.mean(returns)) / (np.std(returns) + eps)
        returns = returns.tolist()

        # Calculating loss values to update our network
        history = zip(action_probs_history, critic_value_history, returns)
        actor_losses = []
        critic_losses = []
        for log_prob, value, ret in history:
            # At this point in history, the critic estimated that we would get a
            # total reward = `value` in the future. We took an action with log probability
            # of `log_prob` and ended up recieving a total reward = `ret`.
            # The actor must be updated so that it predicts an action that leads to
            # high rewards (compared to critic's estimate) with high probability.
            diff = ret - value
            actor_losses.append(-log_prob * diff)  # actor loss

            # The critic must be updated so that it predicts a better estimate of
            # the future rewards.
            critic_losses.append(
                huber_loss(tf.expand_dims(value, 0), tf.expand_dims(ret, 0))
            )

        # Backpropagation
        loss_value = sum(actor_losses) + sum(critic_losses)
        grads = tape.gradient(loss_value, model.trainable_variables)
        optimizer.apply_gradients(zip(grads, model.trainable_variables))

        # Clear the loss and reward history
        action_probs_history.clear()
        critic_value_history.clear()
        rewards_history.clear()

    # Log details
    episode_count += 1
    if episode_count % 10 == 0:
        template = "running reward: {:.2f} at episode {}"
        print(template.format(running_reward, episode_count))

    if running_reward > 195:  # Condition to consider the task solved
        print("Solved at episode {}!".format(episode_count))
        break

Visualizations

In early stages of training:
Imgur

In later stages of training:
Imgur

Introduction