#### Overview of this book

Reinforcement learning (RL) is a branch of machine learning that has gained popularity in recent times. It allows you to train AI models that learn from their own actions and optimize their behavior. PyTorch has also emerged as the preferred tool for training RL models because of its efficiency and ease of use. With this book, you'll explore the important RL concepts and the implementation of algorithms in PyTorch 1.x. The recipes in the book, along with real-world examples, will help you master various RL techniques, such as dynamic programming, Monte Carlo simulations, temporal difference, and Q-learning. You'll also gain insights into industry-specific applications of these techniques. Later chapters will guide you through solving problems such as the multi-armed bandit problem and the cartpole problem using the multi-armed bandit algorithm and function approximation. You'll also learn how to use Deep Q-Networks to complete Atari games, along with how to effectively implement policy gradients. Finally, you'll discover how RL techniques are applied to Blackjack, Gridworld environments, internet advertising, and the Flappy Bird game. By the end of this book, you'll have developed the skills you need to implement popular RL algorithms and use RL techniques to solve real-world problems.
Preface
Free Chapter
Getting Started with Reinforcement Learning and PyTorch
Markov Decision Processes and Dynamic Programming
Monte Carlo Methods for Making Numerical Estimations
Capstone Project – Playing Flappy Bird with DQN
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# Developing the SARSA algorithm

You will recall that Q-learning is an off-policy TD learning algorithm. In this recipe, we will solve an MDP with an on-policy TD learning algorithm, called State-Action-Reward-State-Action (SARSA).

Similar to Q-learning, SARSA focuses on state-action values. It updates the Q-function based on the following equation:

Here, s' is the resulting state after taking the action, a, in state s; r is the associated reward; α is the learning rate; and γ is the discount factor. You will recall that in Q-learning, a behavior-greedy policy, , is used to update the Q value. In SARSA, we simply pick up the next action, a', by also following an epsilon-greedy policy to update the Q value. And the action a' is taken in the next step. Hence, SARSA is an on-policy algorithm.

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