Maximum PC

Hey everyone. I've done a fair bit of work during past year using R. Recently, I wrote a small script that analyzes some data from a poker league which I think demonstrates a few intersting language features in R which some of you might find interesting. I used the term metaprogramming in the subject line somewhat loosely and probably have used pseudo-metaprogramming because the code that I've written doesn't actually create new code. Rather, I've used a combination of the builtin language functions and runtime environment to save me from having to write code that I would have to write under most languages (eg Java). Also, I won't be demonstrating any of the builtin statistical features like linear models.

Before we get to the R features, I should start by telling you about how I scraped the player data and demonstrate an interesting data structure in R called a data frame. The player data was scraped from the league website using a small Java program which then writes the data in R. The following is an example of the Java output.

To save space, I've selected two players that only played one session. As you can probably make out, the <- operator assigns a data.frame to a players name (eg Travis_r). As with most statistical analysis, columns indicate the factors or dimensions of the data. Rows represent the instances, in this case, information related to how a player performed on a given date, time, location, etc. In total, fifity-five players played during the previous quarter with each represented by a data frame with their name. The data from the previous years contains has over two hundred players.

Here is what the data frame looks like in R.

People with database experience, will probably notice a fair bit of similarity between a data frame and database table. Also, the data frame supports features that are similar to SQL queries. I'll show a couple of brief examples, if anyone is interested in discussing this aspect in further detail, we can create another thread.

Notice that most of the builtin R functions are able to work on vectors or matrices of data (similar to Matlab and other math software). This is a great aid in doing mathematical analysis. In many cases, the output is surprisingly complete saving you the trouble of having to write "reporting" code.

Getting to the pseudo-metaprogramming (finally), as I mentioned previously, fifty-five players were involved during this quarter. I really don't want to write some silly code like players<-list(player1, player2, player3, ..., player55). Further, all of us have probably dealt with or can imagine data sets that are much larger (eg publically traded stocks, DNA records for individuals, star databases, etc). Obviously, this type of code can be very tedious. Thankfully, I can use some of the builtin R functions to eliminate most of this work.

The first line in the code above, "sources" the data file, which brings the fifty-five player data frames into the runtime environment. The ls() function in the next line, "lists" the variables currently available.

You'll probably notice that the result from ls() are strings, not the actual data frames. We'll handle this issue in a moment. You might also be thinking to yourself, what if other variables have already been defined? Well, I left off one line of code above [ie rm(list=ls(all=TRUE))], which removes all the existing variables. Now that I have a vector of players (ie 1dim array), how do I use this in my code? Let's take a simple example, I want to know how many players (in total, not unique players) have played at CDs versus Plaza Bowl.


As you can see, the get() function is used to perform reflection on the variable names in the players vector. Next I update the variables CDs and PB to count the number of times people have played at each location. A pretty nifty feature that helps eliminate some coding tedium. Has anyone else here used R?

I've used R before. It's great for statistics. I never knew about the get function, it looks really cool.

An alternative way to do this would be to create a list of the player data frames. This way you'd maintain all the functionality but wouldn't mess the environment up (remove declared variables).

For example:

Metaprogramming / Reflection can come in really handy. At Boeing, I once replaced a 1400 line if statement with 6 or 7 lines of code using reflection. Somewhat ironically, I also wrote a data structure in Java that was basically data frame. I really wish that I would have known about R at the time!

Did you use R at school or at a job? What type of statistical work were you doing?


When people are working in an interactive environment like R or Matlab, they don't mix up their environments. You always start clean. For example, I would never start an instance of R for analyzing a poker data set then switch to another data set like the stock market.

For a small data set, probably less than 500 records, I wouldn't use a list because having to type something like you suggested or...
... kind of defeats the point of working in an environment like R. I want to be able to work the data frames directly (especially if I'm analyzing the data with non-programmers who are going to be WTF are you doing!).

On those occasions when I'm working with a large data set, I like to break the data up into multiple lists based on some type of categorization then use attach() and detach() to bring the data frames in and out of scope.