Question

How can I add a data frame to a list in R? I am trying the following code.

First, I create a data frame from a csv file (any csv file will do)

> a <- read.csv(csvFile)
> class(a)
[1] "data.frame"

so, I have a data frame called “a”. Now I create a list with one character item as shown below

> b <- list("hello world")

Now, I append my data frame “a” to the list b as shown below

> b[[length(b)+1]] <- a
> class(b)
[1] "list"
> length(b)
[1] 2

so far so good. List b now has 2 items (the character string “hello world” and the data frame a). Now the piece that puzzles me is the following code

> class(b[2])
[1] "list"

why is class(b[2]) showing up as a list instead of a data frame? I want to be able to add my data frame “a” to the list “b” without having a be converted implicitly into a list. Can anyone help me understand how to do this?

Answer 1

This is documented in ?"[" (or ?Extract), in the “Recursive (list-like) objects” section (emphasis added):

Indexing by ‘[’ is similar to atomic vectors and selects a list of
the specified element(s).

Both ‘[[’ and ‘$’ select a single element of the list.

Since b[2] returns a list, you want to use [[, i.e. b[[2]].