orngReinforcement: reinforcement learning
This page describes a class for reinforcement learning. Reinforcement learning is machine learning method where an agent (learner and actor) tries to act with a goal of maximising reward.
Class RLSarsa is an implementation of linear, gradient-descendant Sarsa(lambda) with tile coding. The implementation closely follows the boxed algorithm in Figure 8.8 on page 212 in Sutton, 1998. It is a descendant of mountain car example from the book.
This implementation tries to maximise reward, therefore user should devise a way to reward desired behaviour. Actions are integers starting whith zero.
User also has to properly encode state. State is defined as a list of real numbers - state variables. Technique used for discretization/generalization is tile coding.
We can visualise each 2D tiling as mesh of cells. For each tiling the state is transformed to an index of a cell in a mesh: we have achieved discretization. Because location of our meshes (tilings) is not the same, the state is transformed to (possibly) different cell indexes in each tiling. This is the basis for generalization of states. Greater the number of tilings, greater the precision and power of generalization.
When calculating cell indexes in the mesh the width of a cell is always 1. The number of cells in any direction of the mesh isn't explicitly specified anywhere in the state.
Let us try to explain transformation of a given property to state variable on a simple example. Say we want to divide a state variable describing speed of a trolley to 10 subintervals. Let's say we are only interested in speeds between -0.5 and 0.5. Before conversion we have to limit our property to that interval. Then we can divide limited property value with the width of a subinterval. In our case the width is (0.5-(-0.5))/10 = 0.1. Thus, we describe speed with a state variable between -5 and 5.
More subintervals for a variable greatly enhances precision. Keep in mind that doubling the number of subintervals also doubles memory requirements and increases learning time because it makes generalization harder. Doubling the number of subintervals for four variables means 16-times greater memory consumption.
- __init__(numActions, numTilings, memorySize=1000, maxtraces=100, mintracevalue=0.01, rand=random.Random(0))
- Constructor. Before using
- Initializes new episode. Returns first action.
- decide(reward, state)
- Returns state dependant action. Also, modifies "knowledge" with respect to given reward.
- Probability of taking random action (exploring). Default 0.05.
- Step-size parameter for learning, default 0.5
- Trace weakening factor, default 0.9.
- "Devaluation" factor of expected future rewards, because immediate rewards are worth more. Default 0.97.
In the following example an agent tries to learn that taking action 0 in state (0,0) and action 1 in state (1,1) maximises reward.