# Actor-critic

## Definition, Explanations, Examples & Code

Actor-critic is a **temporal difference** algorithm used in **reinforcement learning**. It consists of two networks: the actor, which decides which action to take, and the critic, which evaluates the action produced by the actor by computing the value function and informs the actor how good the action was and how it should adjust. In simple terms, the actor-critic is a temporal difference version of policy gradient. The learning of the actor is based on a policy gradient approach.

{% embed url="https://youtu.be/r2py_Z-bMuY?si=f_aULvOlWo55hC3g" %}

### Actor-critic: Introduction

| Domains          | Learning Methods | Type                |
| ---------------- | ---------------- | ------------------- |
| Machine Learning | Reinforcement    | Temporal Difference |

Actor-critic is a popular algorithm in the field of artificial intelligence and machine learning. It can be defined as a Temporal Difference (TD) version of Policy gradient that utilizes two networks: Actor and Critic. The Actor component is responsible for deciding which action should be taken, while the Critic informs the Actor how good the action was and how it should adjust. The learning method used by the Actor is based on the policy gradient approach. On the other hand, the Critic evaluates the action produced by the Actor by computing the value function. Actor-critic is commonly used in Reinforcement Learning tasks and is a powerful tool for decision-making in various applications.

### Actor-critic: Use Cases & Examples

Actor-critic is a powerful algorithm used in the field of artificial intelligence and machine learning. It is a Temporal Difference(TD) version of Policy gradient, and it has two networks: Actor and Critic. The actor is responsible for deciding which action should be taken, while the critic informs the actor how good the action was and how it should adjust.

One of the most common use cases for actor-critic is in reinforcement learning. The learning of the actor is based on policy gradient approach, where the critic evaluates the action produced by the actor by computing the value function. This allows the algorithm to learn from its own experience and improve over time.

Actor-critic has been used in a variety of applications, including robotics, gaming, and natural language processing. For example, in robotics, actor-critic has been used to train robots to perform complex tasks, such as grasping and manipulation. In gaming, actor-critic has been used to train game agents to play games such as chess and go. In natural language processing, actor-critic has been used to train chatbots to interact with humans in a more natural and intuitive way.

Another example of the use of actor-critic is in the field of finance. It has been used to develop trading algorithms that can learn from market data and adjust their strategies based on changing market conditions. This has the potential to revolutionize the finance industry, as it allows for more accurate and efficient trading.

### Getting Started

To get started with Actor-Critic algorithm, you'll need to have a basic understanding of Reinforcement Learning and Temporal Difference learning. Actor-Critic is a TD version of Policy Gradient, which has two neural networks: Actor and Critic. The Actor network decides which action should be taken, while the Critic network informs the Actor how good the action was and how it should adjust. The Actor's learning is based on the Policy Gradient approach, while the Critic evaluates the action produced by the Actor by computing the value function.

Here's an example of how to implement Actor-Critic using Python and common ML libraries:

```

import numpy as np
import torch
import torch.nn as nn
import torch.optim as optim
import gym

class ActorCritic(nn.Module):
    def __init__(self, input_size, output_size, hidden_size):
        super(ActorCritic, self).__init__()
        self.actor = nn.Sequential(
            nn.Linear(input_size, hidden_size),
            nn.ReLU(),
            nn.Linear(hidden_size, output_size),
            nn.Softmax(dim=-1)
        )
        self.critic = nn.Sequential(
            nn.Linear(input_size, hidden_size),
            nn.ReLU(),
            nn.Linear(hidden_size, 1)
        )

    def forward(self, x):
        return self.actor(x), self.critic(x)

def train(env_name='CartPole-v0', hidden_size=256, lr=1e-3, gamma=0.99, num_episodes=1000):
    env = gym.make(env_name)
    input_size = env.observation_space.shape[0]
    output_size = env.action_space.n

    model = ActorCritic(input_size, output_size, hidden_size)
    optimizer = optim.Adam(model.parameters(), lr=lr)
    criterion = nn.MSELoss()

    for i_episode in range(num_episodes):
        state = env.reset()
        done = False

        while not done:
            action_probs, value = model(torch.FloatTensor(state))
            dist = torch.distributions.Categorical(action_probs)
            action = dist.sample()

            next_state, reward, done, _ = env.step(action.item())
            next_state = torch.FloatTensor(next_state)

            _, next_value = model(next_state)
            td_target = reward + gamma * next_value * (1 - int(done))
            td_error = td_target - value

            actor_loss = -dist.log_prob(action) * td_error.detach()
            critic_loss = criterion(value, td_target.detach())

            loss = actor_loss + critic_loss

            optimizer.zero_grad()
            loss.backward()
            optimizer.step()

            state = next_state

    env.close()

if __name__ == '__main__':
    train()

```

### FAQs

#### What is Actor-critic?

Actor-critic is a Temporal Difference(TD) version of Policy gradient. It has two networks: Actor and Critic. The actor decides which action should be taken and the critic informs the actor how good the action was and how it should be adjusted. In comparison, critics evaluate the action produced by the actor by computing the value function.

#### What is the type of Actor-critic?

Actor-critic is a type of Temporal Difference learning.

#### What are the learning methods used by Actor-critic?

* Reinforcement Learning

#### How does the Actor-critic algorithm work?

The actor-critic algorithm works by having the actor decide which action to take and the critic evaluate that action by computing the value function. The learning of the actor is based on the policy gradient approach.

#### What are the benefits of using Actor-critic?

* It can handle high-dimensional state spaces and continuous action spaces.
* It can provide more stable and efficient learning compared to other reinforcement learning methods.
* It can be used for various applications such as game playing, robotics, and natural language processing.

### Actor-critic: ELI5

Imagine you are trying to learn how to ride a bike for the first time. You know that you need to pedal and steer, but you're not exactly sure how to do those things. This is where Actor-Critic comes in.

Actor-Critic is like having a teacher and a cheerleader when you're learning how to ride a bike. The teacher (the Critic) tells you when you're doing something wrong and gives you tips on how to improve. The cheerleader (the Actor) tells you what you're doing right and encourages you to keep going.

The Critic uses a value function to evaluate your actions and determine how good they are. It's like getting a score for how well you're doing. The Actor uses this feedback to adjust and improve its actions. It's like getting guidance on how to pedal and steer better.

Using this approach, Actor-Critic is able to learn from its mistakes and improve over time. It's like when you fall off the bike and learn what not to do the next time. Eventually, you'll be able to ride the bike without any help from your teacher or cheerleader.

But instead of learning how to ride a bike, Actor-Critic is used in reinforcement learning to train an agent to make the best decisions in a given situation. The Actor decides what action to take, while the Critic evaluates the quality of that action and provides feedback for improvement. Over time, the agent learns from its mistakes and becomes better at making decisions.

[Actor Critic](https://serp.ai/actor-critic/)
