The idea that we learn by interacting with our environment is probably the first to occur to us when we think about the nature of learning. When an infant plays, waves its arms, or looks about, it has no explicit teacher, but it does have a direct sensorimotor connection to its environment. Reinforcement learning is a computational approach to understanding and automating goal-directed learning and decision-making. It is distinguished from other computational approaches by its emphasis on learning by an agent from direct interaction with its environment, without relying on exemplary supervision or complete models of the environment.
Reinforcement learning is learning what to do--how to map situations to actions--so as to maximize a numerical reward signal. The learner is not told which actions to take, as in most forms of machine learning, but instead must discover which actions yield the most reward by trying them. In the most interesting and challenging cases, actions may affect not only the immediate reward but also the next situation and, through that, all subsequent rewards. These two characteristics--trial-and-error search and delayed reward--are the two most important distinguishing features of reinforcement learning. Reinforcement Learning is a type of Machine Learning, thereby also a branch of Artificial Intelligence. It allows machines and software agents to automatically determine the ideal behavior within a specific context, in order to maximize its performance. Simple reward feedback is required for the agent to learn its behavior; this is known as the reinforcement signal.
Reinforcement learning is a learning paradigm concerned with learning to control a system so as to maximize a numerical performance measure that expresses a long-term objective. What distinguishes reinforcement learning from supervised learning is that only partial feedback is given to the learner about the learner’s predictions. Further, the predictions may have long-term effects by influencing the future state of the controlled system. Thus, time plays a special role. The goal of reinforcement learning is to develop efficient learning algorithms.
Figure 1: The Basic Reinforcement Learning Scenario
Reinforcement learning is defined not by characterizing learning methods, but by characterizing a learning problem. Any method that is well suited to solving that problem, we consider to be a reinforcement learning method. Reinforcement learning is different from supervised learning, the kind of learning studied in most current research in machine learning, statistical pattern recognition, and artificial neural networks. Supervised learning is learning from examples provided by a knowledgeable external supervisor. This is an important kind of learning, but alone it is not adequate for learning from interaction. In interactive problems, it is often impractical to obtain examples of desired behavior that are both correct and representative of all the situations in which the agent has to act. In uncharted territory-- where one would expect learning to be most beneficial--an agent must be able to learn from its own experience.
Elements of Reinforcement Learning
Beyond the agent and the environment, one can identify four main sub-elements of a reinforcement learning system: a policy, a reward signal, a value function, and, optionally, a model of the environment.
A policy defines the learning agent's way of behaving at a given time. Roughly speaking, a policy is a mapping from perceived states of the environment to actions to be taken when in those states. It corresponds to what in psychology would be called a set of stimulus-response rules or associations. In some cases, the policy may be a simple function or lookup table, whereas in others it may involve extensive computation such as a search process. The policy is the core of a reinforcement learning agent in the sense that it alone is sufficient to determine behavior. In general, policies may be stochastic.
A reward signal defines the goal in a reinforcement learning problem. On each time step, the environment sends to the reinforcement learning agent a single number, a reward. The agent’s sole objective is to maximize the total reward it receives over the long run. The reward signal thus defines what the good and bad events are for the agent. In a biological system, we might think of rewards as analogous to the experiences of pleasure or pain. They are the immediate and defining features of the problem faced by the agent. As such, the process that generates the reward signal must be unalterable by the agent. The agent can alter the signal that the process produces directly by its actions and indirectly by changing its environment’s state— since the reward signal depends on these—but it cannot change the function that generates the signal.
Whereas the reward signal indicates what is good in an immediate sense, a value function specifies what is good in the long run. Roughly speaking, the value of a state is the total amount of reward an agent can expect to accumulate over the future, starting from that state. Whereas rewards determine the immediate, intrinsic desirability of environmental states, values indicate the long-term desirability of states after taking into account the states that are likely to follow, and the rewards available in those states. For example, a state might always yield a low immediate reward but still have a high value because it is regularly followed by other states that yield high rewards. Or the reverse could be true. To make a human analogy, rewards are somewhat like pleasure (if high) and pain (if low), whereas values correspond to a more refined and farsighted judgment of how pleased or displeased we are that our environment is in a particular state. Expressed this way, we hope it is clear that value functions formalize a basic and familiar idea.
The fourth and final element of some reinforcement learning systems is a model of the environment. This is something that mimics the behavior of the environment, or more generally, that allows inferences to be made about how the environment will behave. For example, given a state and action, the model might predict the resultant next state and next reward. Models are used for planning, by which we mean any way of deciding on a course of action by considering possible future situations before they are actually experienced.
One reason that reinforcement learning is popular is that it serves as a theoretical tool for studying the principles of agents learning to act. But it is unsurprising that it has also been used by a number of researchers as a practical computational tool for constructing autonomous systems that improve themselves with experience. These applications have ranged from robotics to industrial manufacturing, to combinatorial search problems such as computer game playing. Some of the practical applications of reinforcement learning are:
Manufacturing: In Fanuc, a robot uses deep reinforcement learning to pick a device from one box and put it in a container. Whether it succeeds or fails, it memorizes the object and gains knowledge and train’s it to do this job with great speed and precision.
Inventory Management: A major issue in supply chain inventory management is the coordination of inventory policies adopted by different supply chain actors, such as suppliers, manufacturers, and distributors, so as to smooth material flow and minimize costs while responsively meeting customer demand.
Delivery Management: Reinforcement learning is used to solve the problem of Split Delivery Vehicle Routing. Q-learning is used to serve appropriate customers with just one vehicle.
Power Systems: Reinforcement Learning and optimization techniques are utilized to assess the security of the electric power systems and to enhance Microgrid performance. Adaptive learning methods are employed to develop control and protection schemes. Transmission technologies with High-Voltage Direct Current (HVDC) and Flexible Alternating Current Transmission System devices (FACTS) based on adaptive learning techniques can effectively help to reduce transmission losses and CO2 emissions.
Finance Sector: AI is at the forefront of leveraging reinforcement learning for evaluating trading strategies. It is turning out to be a robust tool for training systems to optimize financial objectives. It has immense applications in stock market trading where the Q-Learning algorithm is able to learn an optimal trading strategy with one simple instruction.
 Szepesvári, Csaba, "Algorithms for reinforcement learning", Synthesis lectures on artificial intelligence and machine learning 4, no. 1 (2010): 1-103.
 Sutton, Richard S., and Andrew G. Barto. Reinforcement learning: An introduction. Vol. 1, no. 1, Cambridge: MIT Press, 1998.
 Maruti Techlabs, “Reinforcement Learning and Its Practical Applications”, available online at: https://chatbotsmagazine.com/reinforcement-learning-and-its-practical-applications-8499e60cf751.Read More