# Meeting Don Knuth

Everyone has heroes, but not everyone gets to meet them. This past Tuesday, I was fortunate to meet one of mine — Don Knuth. If you are in computer science and don’t know who Don Knuth is, I highly recommend you take a break from this post and do a bit of reading about him. He is most well-known for authoring his (still in progress) Art of Computer Programming, a compendium of all kinds of information on algorithms, but his contributions to computer science and software development reach far beyond that. He invented TeX (precursor to LaTeX), wrote Concrete Mathematics (a great mathematical foundations of computer science book), and received a Turing award.

# Raspberry Pi Default Groups

In setting up my Raspberry Pi for a home fileshare, I noticed the pi user is a part of several default groups. These are:

(I’m using the 2017-09-07 image of Raspbian Stretch Lite.)

This looked like a lot of groups to me! To make sure my new user only has the minimum permissions needed, let’s look at the what each group is and why it’s there.

# How to Request a Regrade

One of the highlights of my time at UVA was working as a teaching assistant for the computer science department. In this capacity, I proctored labs and exams, held office hours, created exam questions, and even graded homework and exams. Due, in part, to the large class sizes of our introductory courses and the necessity of multiple graders for each assignment, most professors have a “regrade” policy – if the grader has made an error in grading a student’s work, there is a formal process for the student to request a second look at his or her work.

For CS 2150 (the course I TA’d), I was one of two or three TAs who processed most – if not all – of the regrades for exams in the past two semesters. Although grades are ideally determined solely by the answer’s merit, there are a few simple ways you can make your grader’s life easier. (And that’s always a good thing, right?)

# A Brief Exploration of a Möbius Transformation

As part of a recent homework set in my complex analysis course, I was tasked with a problem that required a slight generalization on part of Schwarz’s Lemma:

Lemma (Schwarz): Let $f$ be analytic on the unit disk with $|f(z)| \leq 1$ for all $z$ on the disk and $f(0) = 0$. Then $|f(z)| < |z|$ and $f’(0)\leq 1$.
If either $|f(z)|=|z|$ for some $z\neq0$ or if $|f’(0)|=1$, then $f$ is a rotation, i.e., $f(z)=az$ for some complex constant $a$ with $|a|=1$.

The homework assignment asked us (within the context of a larger problem) to consider the case when $f(\zeta) = 0$ for some $\zeta \neq 0$ on the interior of the unit disk. The secret to this problem was to find some analytic function $\varphi$ that maps the unit disk to itself, but swaps $0$ and $\zeta$. Then, we may consider $\varphi^{-1}\circ f\circ \varphi$ and apply Schwarz’s Lemma.

# How I wrote a GroupMe Chatbot in 24 hours

For the past couple years, I have worked as a teaching assistant for UVa’s CS 2150 (Program and Data Representation) course. We recently started a GroupMe chat for the course staff, and I thought it would be fun to create a chatbot to help remind all the TAs to submit timesheets, keep track of when people are holding office hours, and remember when/where TA meetings are being held. Setting up a basic chatbot is a lot simpler than it sounds and is really fun–I wrote my bot from scratch using Python in just one day.

## GroupMe Bot Overview

GroupMe has a very brief tutorial explaining how their API may be used for bots. The easiest way to create a bot is through their web form, which prompts you for the bot’s name, callback URL (technically optional, but you want it), avatar URL (optional), and the name of the group where the bot will live. Once you’ve done this, you will be given a unique bot ID token. Anyone with this token can pretend to be your bot, so keep it safe. (Security is somewhat laughable here: your bot asserts its ID and the server believes it with no “login” procedure.) We’ll talk more about the callback URL in a moment; for now, just leave it blank.

Once you’ve done these steps, you have created a bot–as far as GroupMe is concerned. If you send a specifically formatted JSON mssage, the newly created bot will post in your group. However, if left at this point, your “bot” is little more than a proxy for human-written messages submitted with curl. Your bot needs some way of reading messages sent to the group, formulating a response, and only then sending messages to the GroupMe servers.

# TensorFlow with the Surface Book

While interning at Microsoft over the summer, I received a first-generation Surface Book with an i5-6300U CPU (2.4 GHz dual core with up to 3.0 GHz), 8GB RAM, and a “GeForce GPU” (officially unnamed, but believed to be equivalent to a GT 940). This is a huge step up from my older laptop, so I wanted to set it up for my ML work. In this post, I’ll outline how I set it up with TensorFlow and GPU acceleration.

## CUDA + cuDNN

If you want to use GPU acceleration, the typical way to do so is with NVIDIA’s CUDA API. CUDA 8.0 is compatible with the Surface Book and is (as of this writing) the most up-to-date version of CUDA. Download it from the NVIDIA website and run their installer.

For work with deep learning, you’ll also want to install cuDNN. To install, just download the library from NVIDIA’s website and unzip it in a convenient place (I chose C:\cudnn). The only “installation” you need to do is to add C:\cudnn\bin to your PATH environment variable.

# Visualizing Multidimensional Data in Python

Nearly everyone is familiar with two-dimensional plots, and most college students in the hard sciences are familiar with three dimensional plots. However, modern datasets are rarely two- or three-dimensional. In machine learning, it is commonplace to have dozens if not hundreds of dimensions, and even human-generated datasets can have a dozen or so dimensions. At the same time, visualization is an important first step in working with data. In this blog entry, I’ll explore how we can use Python to work with n-dimensional data, where $n\geq 4$.

## Packages

I’m going to assume we have the numpy, pandas, matplotlib, and sklearn packages installed for Python. In particular, the components I will use are as below:

## Plotting 2D Data

Before dealing with multidimensional data, let’s see how a scatter plot works with two-dimensional data in Python.

First, we’ll generate some random 2D data using sklearn.samples_generator.make_blobs. We’ll create three classes of points and plot each class in a different color. After running the following code, we have datapoints in X, while classifications are in y.

To create a 2D scatter plot, we simply use the scatter function from matplotlib. Since we want each class to be a separate color, we use the c parameter to set the datapoint color according to the y (class) vector.

# Election 2016: Moving Forward

Like many of my fellow Americans, I stayed up late tonight to watch the polling results for the 2016 General Election. As of my writing this, it appears that Donald Trump will win by a slight margin. The New York Times is predicting that the popular vote will go to Hillary Clinton, while Politico and the Wall Street Journal are showing the current popular vote is Trump’s by about 1 million.

# New Feature: Commenting!

Thanks to a helpful blog post by CodeBlocQ, I’ve now enabled Disqus-powered comments on the blog! Let me know what you think about my posts, and I’ll keep an eye on discussions to respond to questions/comments/concerns!

The second part of the Microsoft series should be out soon; I wanted to get comments working before I did so, but it took me a while to find the time to actually get it up and running.

# A Microsoft Summer, Part 1: Seattle Fun

As suggested by this post’s title, I spent this past summer as an intern with Microsoft in Redmond, Washington. The experience was highly educational for me–as my first (and last!) “real” internship, I learned a lot about software development and the importance of corporate culture, as well as discovering a lot about myself. Overall, the experience was a positive one, though, and I had an enormous amount of fun!

This is the first of a three-part series on my time at Microsoft. This post focuses on fun recreational activities for interns in the Seattle area.

## Outdoors

The Pacific Northwest is home to some of the most amazing views I’ve ever seen. Seattle is conveniently located close to the beach, the mountains, Puget Sound, rainforests, and many hiking trails and campsites. Exploring the outdoors also has the advantage of being very inexpensive, which is great if you’re saving your internship money for college expenses. If you visit National Parks, consider the National Park Passport Program–if you’re going to once-in-a-lifetime parks, it’s a good idea to get your passbook stamped!