gotchaR - Bumps in the road for the learneR driver.
Oct 8, 2019
Wes Hinsley
6 minute read

I am about 3 years into the journey of learning R. There is a lot to get used to, when my previous landmarks have been languages based around Java, C, Pascal, Basic and some dabblings with assemblers of various breeds. Mostly, my adjustment has been about doing things the way R likes; turning loop-ish code into vector-ish code, using the right base functions properly and thinking about data in columns for a start.

Occasionally though, R has the capacity to make me doubt either its sanity, or my own, at least for a while. Here are a couple of my lowest points. More may come later, but two is a start, and those PRs for Hacktoberfest aren’t going to happen on their own…

The dodgy dollar?

Here is a piece of code, in which I incrementally ask my friends what their favourite fruit is. I decided to use generic variable names, so that later on, I can maybe ask them what’s their favourite vegetable, or pet, or something. This sort of knowledge, I feel, is essential for being a good considerate colleague. I start writing some code to process my friends’ replies:-

data <- list()
data$thing_type <- "Fruit"

for (f in c("Apple", "Banana", "Cherry")) {
  data$thing <- c(data$thing, f)

So, now I can head to Tesco with a list of my colleague’s favourite fruits.

> data$thing
[1] "Fruit"  "Apple"  "Banana" "Cherry"

Except… what’s with the “Fruit” entry then? How did that get into my list of things? Even though I don’t think I should need it, I instinctively put data$thing <- NULL as the second line. It makes no difference.

I restart RStudio, reboot my computer, update R - the usual sort of random flailing thing. All useless. I start to wonder if R has been tricking me all along, and I do some really basic assignments and concatenations to check it’s not been fatally flawed all these years. It appears fundamentally sound.

Eventually, I ask my more R-experienced colleagues if, in fact, I am going insane. They cannot answer this for sure, but looking at the programming issue, they tell me it is a feature of R in a way.

The first time I read from data$thing, it doesn’t exist in the list, and R auto-completes it into data$thing_type - hence the “Fruit”. It doesn’t auto-complete on the assignment side though, so after the first loop iteration, data$thing exists, and things continue normally.

It’s tempting to think data$thing <- NULL beforehand would help - but with a list, that would only remove the (non-existent) thing from list, because that’s what NULL assignment with lists does.

So what should I instead do? One option is to replace my inner loop with data$thing <- c(data[['thing']], f), because the auto-completion is activated on the use of the dollar sign, but not on a quoted variable name. Moreover, setting these options:

warnPartialMatchAttr = TRUE
warnPartialMatchDollar = TRUE
warnPartialMatchArgs = TRUE

in .Rprofile will throw a warning every time R does a partial-match. This seems like it might save a world of pain, although casting my mind back to various projects where I have frivolously talked about list$thing and list$thing_name and list$thing_id still makes me shudder a little.

Also having set these options, I occasionally see warnings popping up about auto-completion in code for other packages we rely upon…

data.table vs data.frame

So I collected some more data1 for my colleagues, and I put it in a data.frame like this.

data <- data.frame(
  name = c("Alex", "Emma", "James", "Rich", "Rob"),
  food = c("Oysters", "Haggis", "Goose", "Cheese", "Pie Barm"),
  stringsAsFactors = FALSE)

who_likes <- function(food) {
  data[data$food == food, ]

This is very nice. So if I have a haggis and wonder what to do with it2 :-

> who_likes("Haggis")
  name   food
2 Emma Haggis

Great. However, I later want to do something astoundingly complicated with this rich dataset, and I decide that using the popular data.table will be a good idea, since it’s just a higher performance version of data.frame, right? It’s also in very common use. (To be clear, the performance increase is indeed very worthwhile for large datasets).

However… if I edit just my first line from data.frame(...) to data.table(...) then…

> who_likes("Haggis")
  name      food
1 Alex   Oysters
2 Emma    Haggis
3 James    Goose
4 Rich    Cheese
5 Rob   Pie Barm

Now all my friends suddenly seem to like Haggis. And Cheese too. Or do they? A bit more exploring reveals that my friends in the data.table like absolutely everything in the universe. Even mushrooms. And NULL too. This may or may not be true in reality, but it is definitely not what I wanted the subset to do.

What’s happening is: when I write data[data$food == food, ], then data.table interprets the right-hand food as being the column in data, rather than the argument to who_likes. If instead I write:-

who_likes <- function(x) {
  data[data$food == x, ]

then all works as I expected. And if I (oddly) changed x to name, then data.table would perform comparison between columns, returning zero rows.

Looking through my previous code I have quite a number of examples where the most natural thing to write would be data[data$country == country, ] or data[data$id == id, ]. Luckily I did so only with data-frames. I have not yet found any warnings I can switch on, should I forget about this issue in the future. Again.


Well. There are bumps in the road in any learning journey; these couple in R have been my two most notable ones. I think they are quite well known to more experienced R programmers, who would probably acknowledge that they are a bit horrible, but easily avoided once you know about them. Turning on the right warnings is a really good idea.

I am also thinking of reading The R Inferno by the perfectly named Patrick Burns. The preface begins “If you are using R and think you’re in hell, this is a map for you.” I’m not sure I’d go quite that far in describing my experiences, but maybe being vaguely aware of some of the banana-skins in advance might be good preparation to avoid some future diversions…

  1. For the unenlightened, this is a Pie Barm and here is an article in praise of its merits. ↩︎

  2. Emma doesn’t actually like haggis, and if I offered it, she would know exactly what to do with it. ↩︎

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