R Mateys!

For the first language of the month article, the choice was clear: I wanted to learn more about statistics, so I enrolled myself into a course on Coursera to learn about it. It just so happens that the course is taught using R, so I get to kill two birds with one stone, as it were. I won't bore you with an exhaustive overview of the syntax, so let's just touch upon some of the features R provides.

Vectors

One of the core concepts in R is that everything is a vector. Even the literal 1 is a vector; it just happens to be a vector that's one element in length. Because everything is a vector, most operations are vectorized; that is, code like this:

1:10 + 1

doesn't append 1 to the vector consisting of numbers between 1 and 10; rather, it adds 1 to each element. This can seem a bit odd when you're not used to it, but once you get used to it, it makes so much sense. I feel like how I felt when I figured out how to properly use map in Perl. In fact, because vectorized code is so concise and (in the case of R) so optimized, you hardly see for loops in R at all.

Vectors may also be multi-dimensional. and R supports things like matrix multiplication out of the box. R's matrix multiplication operator %*% looks a little weird, especially since the only other language I've used with matrix multiplication as a built-in operator has been Octave, but as someone who appreciates Perl and Lua, I also appreciate it when a language makes me indicate what I mean by my choice of operator.

Lazy Evaluation

One thing that I discovered that really interested me is the fact that in R, function arguments are lazily evaluated. 1) That is, if I call a function with an expression as an argument, like so:

my_function(print("hi"))

that print expression is not run unless my_function refers to it, and then it is run only once. I made use of this when playing around with R to make a little function that just times how long an expression takes to run:

time_it <- function(expr) {
  start <- Sys.time()
  expr
  end <- Sys.time()
  print(paste(start, end)) # paste concatenates two values
}

time_it(Sys.sleep(5))
# [1] "2016-02-07 09:50:53 2016-02-07 09:50:58"

Metaprogramming

Like some languages, R has support for metaprogramming, but it, like Lisp and other languages, treats code as data. That is, you can assemble, manipulate, and evaluate code objects at runtime. I don't know if I'd go so far as to call R homoiconic, but from my limited experience with the concept, it sure seems like it. 2)

Let's say that we're not always big fans of R's lazy evaluation when it comes to function arguments. So let's make a function that we'll call eager to force a function to eagerly evaluate its arguments before running its body. Here's one way of implementing it:

eager <- function(f) {
    names <- names(formals(f)) # formals returns a pairlist of the arguments to
                               # a function
                               # names extracts the keys from a pairlist

    body_list <- as.list(body(f)) # body returns the code for a function
    prefix <- body_list[1] # multi-statement functions start with the '{'
                           # operator; extending this function to work with
                           # single-statement function is left as an exercise
                           # for the reader =)
    remainder <- body_list[2:length(body_list)]


    eager_names <- lapply(names, as.name) # lapply is like map in other
                                          # languages, it applies a function
                                          # to each element of a list,
                                          # returning the results as a new list

    body(f) <- as.call(c(prefix, eager_names, remainder)) # c concatenates lists
    return(f)
}

Now, if apply eager to time_it, we'll find that time_it's functionality of timing a lazily evaluated expression has disappeared:

time_it <- eager(time_it)
time_it(Sys.sleep())
# [1] "2016-02-07 11:16:50 2016-02-07 11:16:50"

CRAN

Since I'm just starting out with statistics and numerical analysis, I often feel like this when working on a problem:

i-have-no-idea-what-im-doing.jpg

You'll see more details about this in a future post, but for my R project, I wanted to try using some advanced numerical analysis algorithms; I just had no clue how to implement them properly. Fortunately, R has CRAN, which is a place for people much smarter than I to put their implementations of these advanced algorithms for others to use.

R puts the R in REPL

I found R's REPL to be easy to use; it supports what most REPLs out there do, in addition to tab completion of existing functions and variables. One nice feature of R is that of images/workspaces; R allows you to save your current working environment to disk. One trick I used when working on my project was I would snapshot the environment every so often; if a failure occurred, I could simply load the image in the REPL and run the code that had given me an error so I could fix it. Since some parts of my program took fifteen or more minutes to run, you can imagine what a timesaver that was!

No destructuring bind =(

This is a feature I miss in any language that doesn't have it. Even if you're not familiar with the term, you may be familiar with the concept; in Perl, I can do this:

  my ( $x, $y ) = function_that_returns_two_values();

and the returned list is automatically destructured into $x and $y. A lot of languages support this, including Python, Ruby, and Octave. R doesn't, unfortunately, and I really wish it did, since it has a number of functions that return more than one value.

Other Statistical Environments

R provides a nice introduction to statistical concepts; if you want to apply them in other languages, there are things like PDL for Perl and numpy for Python. There are also other statisitical languages like Octave, Matlab, and Julia, if those are more your thing.

I said in my introductory Language of the Month post that I would talk about implementing a program using the language I was trying out, and I've been referring to a mysterious program that I wrote in R. For this post, I wanted to give the language the attention it deserves without overwhelming you, dear reader; next time I'll write about the program I wrote!

,
  1. 1) I assume that this is because R copies data a lot, so it's a way of speeding things up.
  2. 2) What gives me pause is that expressions like 2 + 2 aren't stored as such once parsed; however, that's just sugar for `+`(2, 2), so it's a little murky.