# From Q-Learning to Deep Q-Learning [[from-q-to-dqn]]

We learned that **Q-Learning is an algorithm we use to train our Q-Function**, an **action-value function** that determines the value of being at a particular state and taking a specific action at that state.

<figure>
  <img src="https://huggingface.co/datasets/huggingface-deep-rl-course/course-images/resolve/main/en/unit3/Q-function.jpg" alt="Q-function"/>
</figure>

The **Q comes from "the Quality" of that action at that state.**

Internally, our Q-function is encoded by **a Q-table, a table where each cell corresponds to a state-action pair value.** Think of this Q-table as **the memory or cheat sheet of our Q-function.**

The problem is that Q-Learning is a *tabular method*. This becomes a problem if the states and actions spaces **are not small enough to be represented efficiently by arrays and tables**. In other words: it is **not scalable**.
Q-Learning worked well with small state space environments like:

- FrozenLake, we had 16 states.
- Taxi-v3, we had 500 states.

But think of what we're going to do today: we will train an agent to learn to play Space Invaders, a more complex game, using the frames as input.

As **[Nikita Melkozerov mentioned](https://twitter.com/meln1k), Atari environments** have an observation space with a shape of (210, 160, 3)*, containing values ranging from 0 to 255 so that gives us \\(256^{210 \times 160 \times 3} = 256^{100800}\\) possible observations (for comparison, we have approximately \\(10^{80}\\) atoms in the observable universe).

* A single frame in Atari is composed of an image of 210x160 pixels. Given that the images are in color (RGB), there are 3 channels. This is why the shape is (210, 160, 3). For each pixel, the value can go from 0 to 255.

<img src="https://huggingface.co/datasets/huggingface-deep-rl-course/course-images/resolve/main/en/unit4/atari.jpg" alt="Atari State Space"/>

Therefore, the state space is gigantic; due to this, creating and updating a Q-table for that environment would not be efficient. In this case, the best idea is to approximate the Q-values using a parametrized Q-function  \\(Q_{\theta}(s,a)\\)  .

This neural network will approximate, given a state, the different Q-values for each possible action at that state. And that's exactly what Deep Q-Learning does.

<img src="https://huggingface.co/datasets/huggingface-deep-rl-course/course-images/resolve/main/en/unit1/deep.jpg" alt="Deep Q Learning"/>


Now that we understand Deep Q-Learning, let's dive deeper into the Deep Q-Network.
