# Tensorflow

## On this page:

1. [Tensorflow](/cee-5735/machine-learning-libraries/tensorflow-and-keras.md#tensorflow)
   1. [Version 1.x](/cee-5735/machine-learning-libraries/tensorflow-and-keras.md#tensorflow-1-x)
   2. [Version 2.x](/cee-5735/machine-learning-libraries/tensorflow-and-keras.md#tensorflow-2-x-eager-execution)
   3. [Tensorboard](/cee-5735/machine-learning-libraries/tensorflow-and-keras.md#tensorboard)
2. [Keras](/cee-5735/machine-learning-libraries/tensorflow-and-keras.md#keras)
3. [Tensorflow Probability](/cee-5735/machine-learning-libraries/tensorflow-and-keras.md#tensorflow-probability)

## Tensorflow

Tensorflow (TF) is a powerful ML framework that lets us develop and train models that are computationally optimized and compatible with different processing units, including CPUs, GPUs, and the more recent TPUs. 

### Tensorflow 1.x

{% embed url="<https://www.tensorflow.org/versions/r1.15/api_docs>" %}

In TF 1.x, the ***computational*** ***graph***, which maps out operation order and dependencies, is specified first, before any computations occur. This type of graph is commonly called a ***static graph*** since it doesn't change during a ***session***, which is when the graph is executed and values are computed.&#x20;

![Tensorflow graph visualized using Tensorboard](/files/-MN9jGxVzVgvNTPKHh78)

A TF computational graph is specified using a combination of these objects and operations:&#x20;

* ***TF Placeholders*** are objects that are given information at the time of computation. Ex: the input layer of a neural network.
* ***TF Variables*** are the parameters that are evaluated during a session. Ex: weights and biases.
* ***TF operations,*** which are performed on placeholders and variables, closely follow NumPy operations. TF also includes more ML specific functions, such as one-hot encoding (*i.e.* turning classifications into integer representation) and nonlinear activation functions.&#x20;

In addition to the graph, we also need to specify the training method and any functions to execute during training:

* ***Optimizer*** minimizes the specified loss function to iteratively update the TF variables.
* ***Callback functions*** are functions that are executed during training after a certain number of iterations. Popular callbacks include early stopping (*i.e.* determining when to stop training), using [TensorBoard](/cee-5735/machine-learning-libraries/tensorflow-and-keras.md#tensorboard-visualization-tool), etc.&#x20;

Finally, to perform the computation, we start a TF session and feed the placeholders with our dataset.&#x20;

### Tensorflow 2.x (Eager Execution)

{% embed url="<https://www.tensorflow.org/api_docs>" %}

With TF 2.x, there are no longer TF "sessions" and everything is executed eagerly.

### TensorBoard (Visualization Tool)

{% embed url="<https://www.tensorflow.org/tensorboard/get_started>" %}

Tensorboard is Tensorflow's visualization tool for viewing computational graphs, tracking variables & performance metrics, etc. Installation of Tensorflow includes TensorBoard. To launch the dashboard, run this on terminal and point the --logdir to the appropriate directory.

```
tensorboard --logdir=summaries
```

Open the link shown on the terminal (default: localhost:8888) on a web browser.

<div align="center"><img src="/files/-MN9qm3tonz-gUgkujph" alt="TensorBoard dashboard"></div>

## Keras

{% embed url="<https://keras.io>" %}

Keras is a popular high-level API for building deep neural networks using prebuilt layers (*i.e.* dense layers, convolutional layers, batch normalization layers, etc.). Keras is great for quickly building regular models, but can't accommodate custom operations like in Tensorflow/Pytorch.

Another benefit of Keras is the abundance of [pre-trained model architectures](https://keras.io/api/applications/) that are available for transfer learning.

## Tensorflow Probability

{% embed url="<https://www.tensorflow.org/probability>" %}

Tensorflow Probability is a library for probabilistic deep learning models and includes useful tools for Variational Inference, Bayesian Neural Networks, MCMC, etc.


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