Another R Quirk

For the most part I like programming in R, but it is considerably quirkier than any other language I have used. I'm pretty sure that is what led to the development of what is known now as the "Tidyverse". The Tidyverse in turn introduces other quirks, as I've pointed out in a previous post.

One of the quirks in base R caused me a fair amount of grief recently. The context was an interactive program (written in Shiny, although that is beside the point here). At one point in the program the user would be staring at a table (the display of a data frame) and would select rows and columns for further analysis. The program would reduce the data frame to those rows and columns, and pass the reduced data frame to functions that would do things to it.

The program worked well until I innocently selected a bunch of rows and one column for analysis. That crashed the program with a rather cryptic (to me) error message saying that some function I was unaware of was not designed to work with a vector.

I eventually tracked down the line where the code died. The function I was unaware of apparently was baked into a library function I was using. As for the vector part, that was the result of what I would characterize as a "quirk" (though perhaps "booby trap" might be more accurate). I'll demonstrate using the mtcars data frame that automatically loads with R.

Consider the following code chunk.

rows <- 1:3
cols <- c("mpg", "cyl")
temp <- mtcars[rows, cols]
str(temp)

This extracts a subset of three rows and two columns from mtcars and presents it as a data frame.

'data.frame':    3 obs. of  2 variables:
 $ mpg: num  21 21 22.8
 $ cyl: num  6 6 4

So far, so good. Now suppose we choose only one column and rerun the code.

rows <- 1:3
cols <- c("mpg")
temp <- mtcars[rows, cols]
str(temp)

Here is the result.

num [1:3] 21 21 22.8

Our data frame just became a vector. That was what caused the crash in my program.

Since I was using the dplyr library elsewhere, there was an easy fix once I knew what the culprit was.

rows <- 1:3
cols <- c("mpg")
temp <- mtcars[rows, ] |> select(all_of(cols))
str(temp)

The result, as expected, is a data frame.

 'data.frame':    3 obs. of  1 variable:
 $ mpg: num  21 21 22.8

There will be situations where you grab one column of a data frame and want it to be a vector, and situations (such as mine) where you want it to be a data frame, so the designers of the language have to choose which route to go. I just wish they had opted to retain structure (in this case data frame) until explicitly dropped, rather than drop it without warning.