Build a Gridworld

Key Insight

A reinforcement-learning problem is fully described by five pieces — states, actions, transition probabilities, a reward function, and a discount factor — bundled together as an MDP. A gridworld is the smallest environment where you can write all five down by hand and read them straight off a picture, so it turns abstract symbols into something you can point at. Building one yourself forces you to decide concretely what counts as a state, what the transition function does when you walk into a wall, and where the reward actually lives — the three spots where a beginner's mental model is usually fuzziest.