Train Your First Deep Q Learning based RL Agent: A Step-by-Step Guide


Reinforcement Learning (RL) is a fascinating field of Artificial Intelligence (AI) that enables machines to learn and make decisions through interaction with their environment. Training an RL agent involves a trial-and-error process where the agent learns from its actions and the subsequent rewards or penalties it receives. In this blog, we will explore the steps involved in training your first RL agent, along with code snippets to illustrate the process.

Step 1: Define the Environment

The first step in training an RL agent is to define the environment in which it will operate. The environment can be a simulation or a real-world scenario. It provides the agent with observations and rewards, allowing it to learn and make decisions. OpenAI Gym is a popular Python library that provides a wide range of pre-built environments. Let’s consider the classic “CartPole” environment for this example.

import gym

env = gym.make(‘CartPole-v1’)

Step 2: Understand the Agent-Environment Interaction

In RL, the agent interacts with the environment by taking actions based on its observations. It receives feedback in the form of rewards or penalties, which are used to guide its learning process. The agent’s objective is to maximize the cumulative rewards over time. To do this, the agent learns a policy — a mapping from observations to actions — that helps it make the best decisions.

Step 3: Choose an RL Algorithm

Various RL algorithms are available, each with its own strengths and weaknesses. One popular algorithm is Q-Learning, which is suitable for discrete action spaces. Another commonly used algorithm is Deep Q-Networks (DQN), which utilizes deep neural networks to handle complex environments. For this example, let’s use the DQN algorithm.

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Step 4: Build the RL Agent

To build an RL agent using the DQN algorithm, we need to define a neural network as the function approximator. The network takes observations as input and outputs Q-values for each possible action. We also need to implement a replay memory to store and sample experiences for training.

import torch
import torch.nn as nn
import torch.optim as optim

class DQN(nn.Module):
def __init__(self, input_dim, output_dim):
super(DQN, self).__init__()
self.fc1 = nn.Linear(input_dim, 64)
self.fc2 = nn.Linear(64, 64)
self.fc3 = nn.Linear(64, output_dim)

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

# Create an instance of the DQN agent
input_dim = env.observation_space.shape[0]
output_dim = env.action_space.n
agent = DQN(input_dim, output_dim)

Step 5: Train the RL Agent

Now, we can train the RL agent using the DQN algorithm. The agent interacts with the environment, observes the current state, selects an action based on its policy, receives a reward, and updates its Q-values accordingly. This process is repeated for a specified number of episodes or until the agent achieves a satisfactory level of performance.

optimizer = optim.Adam(agent.parameters(), lr=0.001)

def train_agent(agent, env, episodes):
for episode in range(episodes):
state = env.reset()
done = False
episode_reward = 0

while not done:
action = agent.select_action(state)
next_state, reward, done, _ = env.step(action)
agent.store_experience(state, action, reward, next_state, done)


In this blog, we explored the process of training your first RL agent. We started by defining the environment using OpenAI Gym, which provides a range of pre-built environments for RL tasks. We then discussed the agent-environment interaction and the objective of the agent to maximize cumulative rewards.

Next, we chose the DQN algorithm as our RL algorithm of choice, which combines deep neural networks with Q-learning to handle complex environments. We built an RL agent using a neural network as the function approximator and implemented a replay memory to store and sample experiences for training.

Finally, we trained the RL agent by having it interact with the environment, observe states, select actions based on its policy, receive rewards, and update its Q-values. This process was repeated for a specified number of episodes, allowing the agent to learn and improve its decision-making capabilities.

Reinforcement Learning opens up a world of possibilities for training intelligent agents that can autonomously learn and make decisions in dynamic environments. By following the steps outlined in this blog, you can embark on your journey of training RL agents and exploring various algorithms, environments, and applications.

Remember, RL training requires experimentation, fine-tuning, and patience. As you delve deeper into RL, you can explore advanced techniques such as deep RL, policy gradients, and multi-agent systems. So, keep learning, iterating, and pushing the boundaries of what your RL agents can achieve.

Happy training!

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Train Your First Deep Q Learning based RL Agent: A Step-by-Step Guide was originally published in Becoming Human: Artificial Intelligence Magazine on Medium, where people are continuing the conversation by highlighting and responding to this story.

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