Tag: sequence # Sorting Algorithms 101: Visualized

Sorting is one of the central topic in most Computer Science degrees. In general, sorting refers to the process of rearranging data according to a defined pattern with the end goal of transforming the original unsorted sequence into a sorted sequence. It lies at the heart of successful businesses ventures — such as Google and Amazon — but is also present in many applications we use daily — such as Excel or Facebook.

Many different algorithms have been developed to sort data. Wikipedia lists as many as 45 and there are probably many more. Some work by exchanging data points in a sequence, others insert and/or merge parts of the sequence. More importantly, some algorithms are quite effective in terms of the time they take to sort data — taking only $n$ time to sort $n$ datapoints — whereas others are very slow — taking as much as $n^2$. Moreover, some algorithms are stable — in the sense that they always take the same amount of time to process $n$ datapoints — whereas others may fluctuate in terms of processing time based on the original order of the data.

I really enjoyed this video by TED-Ed on how to best sort your book collection. It provides a very intuitive introduction into sorting strategies (i.e., algorithms). Moreover, Algorithms to Live By (Christian & Griffiths, 2016) provided the amazing suggestion to get friends and pizza in whenever you need to sort something, next to the great explanation of various algorithms and their computational demand.

The main reason for this blog is that I stumbled across some nice video’s and GIFs of sorting algorithms in action. These visualizations are not only wonderfully intriguing to look at, but also help so much in understanding how the sorting algorithms process the data under the hood. You might want to start with the 4-minute YouTube video below, demonstrating how nine different sorting algorithms (Selection Sort, Shell Sort, Insertion Sort, Merge Sort, Quick Sort, Heap Sort, Bubble Sort, Comb Sort, & Cocktail Sort) process a variety of datasets.

This interactive website toptal.com allows you to play around with the most well-known sorting algorithms, putting them to work on different datasets. For the grande finale, I found these GIFs and short video’s of several sorting algorithms on imgur. In the visualizations below, each row of the image represents an independent list being sorted. You can see that Bubble Sort is quite slow: Cocktail Shaker Sort already seems somewhat faster, but still takes quite a while. For some algorithms, the visualization clearly shows that the settings you pick matter. For instance, Heap Sort is much quicker if you choose to shift down instead of up.

In contrast, for Merge Sort it doesn’t matter whether you sort by breadth first or depth first.

The imgur overview includes many more visualized sorting algorithms but I don’t want to overload WordPress or your computer, so I’ll leave you with two types of Radix Sort, the rest you can look up yourself! # Machine Learning & Deep Learning book

The Deep Learning textbook helps students and practitioners enter the field of machine learning in general and deep learning in particular. Its online version is available online for free whereas a hardcover copy can be ordered here on Amazon. You can click on the topics below to be redirected to the book chapter:

# Keras: Deep Learning in R or Python within 30 seconds

Keras is a high-level neural networks API that was developed to enabling fast experimentation with Deep Learning in both Python and R. According to its author Taylor Arnold: Being able to go from idea to result with the least possible delay is key to doing good research. The ideas behind deep learning are simple, so why should their implementation be painful?

Keras comes with the following key features:

• Allows the same code to run on CPU or on GPU, seamlessly.
• User-friendly API which makes it easy to quickly prototype deep learning models.
• Built-in support for convolutional networks (for computer vision), recurrent networks (for sequence processing), and any combination of both.
• Supports arbitrary network architectures: multi-input or multi-output models, layer sharing, model sharing, etc. This means that Keras is appropriate for building essentially any deep learning model, from a memory network to a neural Turing machine
• Fast implementation of dense neural networks, convolution neural networks (CNN) and recurrent neural networks (RNN) in R or Python, on top of  TensorFlow or Theano.

# R

## R: Installation

The R interface to Keras uses TensorFlow™ as it’s underlying computation engine. First, you have to install the keras R package from GitHub:

``devtools::install_github("rstudio/keras")``

Using the `install_tensorflow()` function you can then install TensorFlow:

``````library(keras)
install_tensorflow()``````

This will provide you with a default installation of TensorFlow suitable for use with the keras R package. See the article on TensorFlow installation to learn about more advanced options, including installing a version of TensorFlow that takes advantage of Nvidia GPUs if you have the correct CUDA libraries installed.

## R: Getting started in 30 seconds

Keras uses models to organize layers. Sequential models are the simplest structure, simply stacking layers. More complex architectures require the Keras functional API, which allows to build arbitrary graphs of layers.

Here is an example of a sequential model (hosted on this website):

``````library(keras)

model keras_model_sequential()

model %>%
layer_dense(units = 64, input_shape = 100) %>%
layer_activation(activation = 'relu') %>%
layer_dense(units = 10) %>%
layer_activation(activation = 'softmax')

model %>% compile(
loss = 'categorical_crossentropy',
optimizer = optimizer_sgd(lr = 0.02),
metrics = c('accuracy')
)
``````

The above demonstrates the little effort needed to define your model. Now, you can iteratively train your model on batches of training data:

``model %>% fit(x_train, y_train, epochs = 5, batch_size = 32)``

Next, performance evaluation can be prompted in a single line of code:

``loss_and_metrics %>% evaluate(x_test, y_test, batch_size = 128)``

Similarly, generating predictions on new data is easily done:

``classes %>% predict(x_test, batch_size = 128)``

Building more complex models, for example, to answer questions or classify images, is just as fast.

# Python

A step-by-step implementation of several Neural Network architectures with Keras in Python can be found on DataCamp. Similarly, one may use this quick cheatsheet to deploy the most basic models.