---
title: Tensorflow for Reinformcement Learning
categories: session
---

**Slides** [rl-3.pdf]()

## Task 1 - Tensorflow/Keras

We will start today with getting used to tensorflow/keras, you can also adapt the exercises to pytorch or similar if prefered (but code-examples will be given in tensorflow).

First, install tensorflow:

```bash
pip install tensorflow
```

We will only be using the Keras API, you can find the documentation [here](https://www.tensorflow.org/guide/keras/sequential_model)

Verify in python with:

```python
import tensorflow as tf
print(tf.__version__)
```

### Part A - Perceptron

We can make a single perceptron with keras like this:

```python
from tensorflow import keras
from tensorflow.python.keras.layers import Dense

model = tf.keras.Sequential([  
  Dense(units=1, input_dim=1)  
])
```

and do a forward propagation with:

```python
import numpy as np

model(np.array([[5]]))  # For x = 5
```

Furthermore, we can get and set the current weights with:

```python
# Get weights
model.layers[0].get_weights()

# Set weights (TODO: replace w1 and b1)
model.layers[0].set_weights([np.array([[w1]]), np.array([b1]))
```

**Tasks/Questions**
- Test out different values for the weight and bias
- How do you forward-propogate multiple values at once? 
- Can you plot the graph for some range of inputs?

## Task 2 - Q-Values from an ANN

We still want to work with Q-values, meaning that we would like some value for all possible actions as output from our neural network. Our FrozenLake environment has 4 possible actions, and we already know the q-values for all possible states, making it easy to fit a neural network.

### Part A - Creating a network

The following code will create a neural network that inputs a state (one value) and outputs 4 values (one for each action), it will also assume 16 possible states (0-15):

```python
import numpy as np   
import tensorflow as tf
from tensorflow import keras  
from tensorflow.python.keras.layers import Dense

x_data = np.linspace(0, 15, 16)
normalizer = keras.layers.Normalization(input_shape=[1,], axis=None)  
normalizer.adapt(np.array(x_data))

model = keras.Sequential([  
	 normalizer,  
	 Dense(64, activation='relu'),  
	 Dense(64, activation='relu'),  
	 Dense(4)  
])  

model.compile(  
    optimizer=tf.optimizers.Adam(learning_rate=0.001),  
	  loss='mse'  
)
```

Some of this code can be safely ignored (normalization and the compile method).

**Tasks/Questions**
- What is the design of this neural network?

### Part B - Training

As we already have Q-Values, let us train the network on the data:


```python
y_data = np.array([  
	[0.54, 0.53, 0.53, 0.52],  
	[0.34, 0.33, 0.32, 0.50],  
	[0.44, 0.43, 0.42, 0.47],  
	[0.31, 0.31, 0.30, 0.46],  
	[0.56, 0.38, 0.37, 0.36],  
	[0., 0., 0., 0.],  
	[0.36, 0.2, 0.36, 0.16],  
	[0., 0., 0., 0.],  
	[0.38, 0.41, 0.40, 0.59],  
	[0.44, 0.64, 0.45, 0.40],  
	[0.62, 0.50, 0.40, 0.33],  
	[0., 0., 0., 0.],  
	[0., 0., 0., 0.],  
	[0.46, 0.53, 0.74, 0.50],  
	[0.73, 0.86, 0.82, 0.78],  
	[1, 1, 1, 1]  
])

model.fit(  
	x_data,  
	y_data,  
	epochs=50000,  
	verbose=0)
```

**Tasks/Questions**
- Test out the forward propagation, are the values similar to what you expect from a Q-table?
- Plot the utility given optimal play.

### Part C - FrozenLake

Given the model trained above and an optimal policy (argmax of output), can you move around the environment/solve the problem?

## Task 3 - DQN

Given exercises from last week, we now only need an implementation of a replay-buffer to implement a DQN (Deep Q-network) agent. The replay-buffer needs two methods, one to store experiences (state, action, reward, next_state), and one to sample from the replay-buffer.

**Implement these two methods**

## Task 4 - MountainCar

Until now we have been working on the FrozenLake environment. Try to solve the [MountainCar](https://www.gymlibrary.ml/environments/classic_control/mountain_car/) environment using techniques we have learned in this course. I will update this page with a DQN-solution later (hopefully before the end of the day).