Programming Languages

I take this opportunity to apologise for my prior belief that all that matters is thinking algorithmically, and language in which the ideas are expressed doesn’t matter.

About a decade ago, I used to make fun of information technology company that hired developers based on the language they coded in. My contention was that writing code is a skill that you either have or you don’t, and what a potential employer needs to look for is the ability to think algorithmically, and then render ideas in code. 

While I’ve never worked as a software engineer I find myself writing more and more code over the years as a part of doing data analysis. The primary tool I use is R, where coding doesn’t really feel like coding, since it is a rather high level language. However, I’m occasionally asked to show code in Python, since some clients are more proficient in that, and the one thing that has done is to teach me the value of domain knowledge of a programming language. 

I take this opportunity to apologise for my prior belief that all that matters is thinking algorithmically, and language in which the ideas are expressed doesn’t matter. 

This is because the language you usually program in subtly nudges you towards thinking in a particular way. Having mostly used R over the last decade, I think in terms of tables and data frames, and after having learnt tidyverse earlier this year, my way of thinking algorithmically has become in a weird way “object oriented” (no, this has nothing to do with classes). I take an “object” (a data frame) and then manipulate it in various ways, changing it, summarising stuff, calculating things on the fly and aggregating, until the point where the result comes out in an elegant manner. 

And while Pandas allows chaining (in fact, it is from Pandas that I suspect the tidyverse guys got the idea for the “%>%” chaining operator), it is by no means as complete in its treatment of chaining as R, and that that makes things tricky. 

Moreover, being proficient in R makes you think in terms of vectorised operations, and when you see that python doesn’t necessarily offer that, and and operations that were once simple in R are now rather complicated in Python, using list comprehension and what not. 

Putting it another way, thinking algorithmically in the framework offered by one programming language makes it rather stressful to express these thoughts in another language where the way of algorithmic thinking is rather different. 

For example, I’ve never got the point of the index in pandas dataframes, and I only find myself “resetting” it constantly so that my way of addressing isn’t mangled. Compared to the intuitive syntax in R, which is first and foremost a data analysis tool, and where the data frame is “native”, the programming language approach of python with its locs and ilocs is again irritating. 

I can go on… 

And I’m guessing this feeling is mutual – someone used to doing things the python way would find R’s syntax and way of doing things rather irritating. R’s machine learning toolkit for example is nowhere as easy as scikit learn is in python (this doesn’t affect me since I seldom need to use machine learning. For example, I use regression less than 5% of the time in my work). 

The next time I see a job opening for a “java developer” I will not laugh like I used to ten years ago. I know that this posting is looking for a developer who can not only think algorithmically, but also algorithmically in the way that is most convenient to express in Java. And unlearning one way of algorithmic thinking and learning another isn’t particularly easy. 

Stirring the pile efficiently

Warning: This is a technical post, and involves some code, etc. 

As I’ve ranted a fair bit on this blog over the last year, a lot of “machine learning” in the industry can be described as “stirring the pile”. Regular readers of this blog will be familiar with this image from XKCD by now:

Source: https://xkcd.com/1838/

Basically people simply take datasets and apply all the machine learning techniques they have heard of (implementation is damn easy – scikit learn allows you to implement just about any model in three similar looking lines of code; See my code here to see how similar the implementation is).

So I thought I’ll help these pile-stirrers by giving some hints of what method to use for different kinds of data. I’ve over-simplified stuff, and so assume that:

  1. There are two predictor variables X and Y. The predicted variable “Z” is binary.
  2. X and Y are each drawn from a standard normal distribution.
  3. The predicted variable Z is “clean” – there is a region in the X-Y plane where Z is always “true” and another region where Z is always “false”
  4. So the idea is to see which machine learning techniques are good at identifying which kind of geometrical figures.
  5. Everything is done “in-sample”. Given the nature of the data, it doesn’t matter if we do it in-sample or out-of-sample.

For those that understand Python (and every pile-stirrer worth his salt is excellent at Python), I’ve put my code in a nice Jupyter Notebook, which can be found here.

So this is what the output looks like. The top row shows the “true values” of Z. Then we have a row for each of the techniques we’ve used, which shows how well these techniques can identify the pattern given in the top row (click on the image for full size).

As you can see, I’ve chosen some common geometrical shapes and seen which methods are good at identifying those. A few pertinent observations:

  1. Logistic regression and linear SVM are broadly similar, and both are shit for this kind of dataset. Being linear models, they fail to deal with non-linear patterns
  2. SVM with RBF kernel is better, but it fails when there are multiple “true regions” in the dataset. At least it’s good at figuring out some non-linear patterns. However, it can’t figure out the triangle or square – it draws curves around them, instead.
  3. Naive Bayesian (I’ve never understood this even though I’m pretty good at Bayesian statistics, but I understand this is a commonly used technique; and I’ve used default parameters so not sure how it is “Bayesian” even) can identify some stuff but does badly when there are disjoint regions where Z is true.
  4. Ensemble methods such as Random Forests and Gradient Boosting do rather well on all the given inputs. They do well for both polygons and curves. Elsewhere, Ada Boost mostly does well but trips up on the hyperbola.
  5. For some reason, Lasso fails to give an output (in the true spirit of pile-stirring, I didn’t explore why). Ridge is again a regression method and so does badly on this non-linear dataset
  6. Neural Networks (Multi Layer Perceptron to be precise) does reasonably well, but can’t figure out the sharp edges of the polygons.
  7. Decision trees again do rather well. I’m pleasantly surprised that they pick up and classify the disjoint sets (multi-circle and hyperbola) correctly. Maybe it’s the way scikit learn implements them?

Of course, the datasets that one comes across in real life are never such simple geometrical figures, but I hope that this set can give you some idea on what techniques to use where.

At least I hope that this makes you think about the suitability of different techniques for the data rather than simply applying all the techniques you know and then picking the one that performs best on your given training and test data.

That would count as nothing different from p-hacking, and there’s an XKCD for that as well!

Source: https://xkcd.com/882/

Python and Hindi

So I’ve recently discovered that using Python to analyse data is, to me, like talking in Hindi. Let me explain.

Back in 2008-9 I lived in Delhi, where the only language spoken was Hindi. Now, while I’ve learnt Hindi formally in school (I got 90 out of 100 in my 10th boards!), and watched plenty of Hindi movies, I’ve never been particularly fluent in the language.

The basic problem is that I don’t know the language well enough to think in it. So when I’m talking Hindi, I usually think in Kannada and then translate my thoughts. This means my speech is slow – even Atal Behari Vajpayee can speak Hindi faster than me.

More importantly, thinking in Kannada and translating means that I can get several idioms wrong (can’t think of particular examples now). And I end up using the language in ways that native speakers don’t (again can’t think of examples here).

I recently realised it’s the same with programming languages. For some 7 years now I’ve mostly used R for data analysis, and have grown super comfortable with it. However, at work nowadays I’m required to use Python for my analysis, to ensure consistency with the rest of the firm.

While I’ve grown reasonably comfortable with using Python over the last few months, I realise that I have the same Hindi problem. I simply can’t think in Python. Any analysis I need to do, I think about it in R terms, and then mentally translate the code before performing it in Python.

This results in several inefficiencies. Firstly, the two languages are constructed differently and optimised for different things. When I think in one language and mentally translate the code to the other, I’m exploiting the efficiencies of the thinking language rather than the efficiencies of the coding language.

Then, the translation process itself can be ugly. What might be one line of code in R can sometimes take 15 lines in Python (and vice versa). So I end up writing insanely verbose code that is hard to read.

Such code also looks ugly – a “native user” of the language finds it rather funnily written, and will find it hard to read.

A decade ago, after a year of struggling in Delhi, I packed my bags and moved back to Bangalore, where I could both think and speak in Kannada. Wonder what this implies in a programming context!