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Understanding the agent-environment interaction model, state representations, and environment design in reinforcement learning.
The fundamental loop in RL is the continuous interaction between an agent and its environment.
Agent-Environment Loop
┌─────────────────────────────┐
│ Agent │
│ (Decision Maker) │
└────────────┬────────────────┘
│ action a_t
▼
┌─────────────────────────────┐
│ Environment │
│ (Dynamics, Rewards) │
└────────────┬────────────────┘
│ state s_{t+1}, reward r_t
▼
┌─────────────────────────────┐
│ Agent │
│ (Update Policy) │
└─────────────────────────────┘
State Representation
State: Complete description of environment configuration
Types of States
Fully Observable: Agent sees complete state
Example
state = [position, velocity, angle, angular_velocity]
Partially Observable: Agent sees partial state
Example
# Poker: only own hand visible
state = [own_cards, community_cards_public, betting_history]
Latent: Hidden features must be learned
python example
# Image: raw pixels, must learn features
state = image_array # shape (84, 84, 3)Action Space
Discrete: Finite set of actions
Example
actions = ['forward', 'backward', 'left', 'right']
n_actions = 4
Continuous: Infinite actions in range
Example
# Robotic control
action = [joint_1_angle, joint_2_angle, joint_3_angle]
# Each in [-1.0, 1.0]
Hybrid: Mix of discrete and continuous
Example
action = {
'direction': discrete, # [0, 1, 2] for left/forward/right
'speed': continuous # [0.0, 1.0]
}
Reward Structure
Single Reward
python example
reward = {
+1: goal reached,
-1: collision,
0: otherwise
}Shaped Reward
python example
reward = {
+100: goal,
-50: collision,
-0.1: per step (encourage efficiency),
+5: moved towards goal
}Multi-Objective
python example
rewards = {
'main_goal': 100,
'efficiency': -0.1, # per time step
'safety': -50 # for collision
}
# Combine with weights
total_reward = 1.0*main + 0.5*efficiency + 1.0*safetyAgent Architecture
┌──────────────────────────────┐
│ Observation/State │
│ (from environment) │
└────────────┬─────────────────┘
│
▼
┌──────────────────────────────┐
│ Feature Extraction │
│ (if needed) │
└────────────┬─────────────────┘
│
▼
┌──────────────────────────────┐
│ Policy Network │
│ (or Value Network) │
└────────────┬─────────────────┘
│
▼
┌──────────────────────────────┐
│ Action Selection │
│ (e.g., ε-greedy) │
└────────────┬─────────────────┘
│
▼
┌──────────────────────────────┐
│ Action to Environment │
└──────────────────────────────┘
Environment Types
Deterministic
Same action always produces same outcome:
python example
next_state, reward = env.step(action)
# DeterministicStochastic
Same action can have different outcomes:
python example
# Wind affects ball trajectory
next_state = stochastic_step(state, action)Episodic
Task has natural ending:
python example
# Game: start to win/lose
episode = []
done = False
while not done:
action = agent.get_action(state)
state, reward, done, _ = env.step(action)
episode.append((state, action, reward))Continuing
No natural ending:
python example
# Robot maintaining balance
for t in range(infinite):
action = agent.get_action(state)
state, reward, done, _ = env.step(action)Implementation Example
Example
import gym
import numpy as np
class RLAgent:
def __init__(self, env):
self.env = env
self.state_space = env.observation_space
self.action_space = env.action_space
self.learning_rate = 0.1
self.gamma = 0.99
self.epsilon = 0.1
def get_action(self, state):
"""Choose action (ε-greedy)"""
if np.random.random() < self.epsilon:
return self.action_space.sample()
else:
return self.get_best_action(state)
def run_episode(self):
"""Run one episode"""
state = self.env.reset()
episode_data = []
done = False
while not done:
# Choose action
action = self.get_action(state)
# Step environment
next_state, reward, done, info = self.env.step(action)
# Store transition
episode_data.append({
'state': state,
'action': action,
'reward': reward,
'next_state': next_state,
'done': done
})
# Update
self.update(state, action, reward, next_state, done)
state = next_state
return episode_data
def update(self, state, action, reward, next_state, done):
"""Update policy (Q-Learning)"""
pass # Implement algorithm-specific update
# Usage
env = gym.make('CartPole-v1')
agent = RLAgent(env)
for episode in range(100):
episode_data = agent.run_episode()
total_reward = sum(d['reward'] for d in episode_data)
print(f"Episode {episode}: {total_reward}")
State Space Representations
Tabular
Discrete states indexed in table:
Example
Q_table = {}
for state in all_states:
for action in all_actions:
Q_table[(state, action)] = 0.0
Linear Features
Hand-crafted features:
Example
features = np.array([
position,
velocity,
angle,
angular_velocity
])
Q_value = theta @ features
Neural Network
Learning features:
Example
import tensorflow as tf
model = tf.keras.Sequential([
tf.keras.layers.Dense(128, activation='relu', input_shape=(state_dim,)),
tf.keras.layers.Dense(128, activation='relu'),
tf.keras.layers.Dense(n_actions)
])
Summary
Key concepts:
- Agent observes state
- Takes action
- Receives reward and next state
- Updates policy
- Repeat until convergence
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Agents and Environments in RL.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this Machine Learning topic.
Search Terms
machine-learning, machine learning, machine, learning, reinforcement, agents, and, environments
Related Machine Learning Topics
Continue learning this concept
Reinforcement LearningDeep Q-Networks (DQN)Understanding Deep Q-Networks: combining Q-Learning with neural networks for high-dimensional state spaces.Reinforcement LearningPolicy Gradient Methods — Machine LearningLearning optimal policies directly using gradient ascent on policy parameters.Reinforcement LearningQ-Learning AlgorithmDeep dive into Q-Learning: off-policy learning, Bellman equation, and practical implementation.Reinforcement LearningReinforcement Learning: IntroductionIntroduction to reinforcement learning: agents, environments, rewards, and the fundamentals of learning through interaction.Reinforcement LearningReinforcement Learning ProjectsHands-on RL projects: game playing, robot control, and real-world applications.Machine Learning FundamentalsOverfitting and UnderfittingLearn to identify and prevent overfitting and underfitting in machine learning models with practical techniques, code examples, and regularization strategies.