“Embedded” data.frame in R. What is it, what is it called, why does it behave the way it does?

Question

I have the following data structure in R:

df <- structure(
  list(
    ID = c(1L, 2L, 3L, 4L, 5L),
    var1 = c(\'a\', \'b\', \'c\', \'d\', \'e\'),
    var2 =          


        

Answer 1

I think the strucutre makes it pretty clear

str(df)
# 'data.frame':   5 obs. of  4 variables:
#  $ ID  : int  1 2 3 4 5
#  $ var1: chr  "a" "b" "c" "d" ...
#  $ var2:'data.frame':   5 obs. of  2 variables:
#   ..$ var2a: chr  "v" "w" "x" "y" ...
#   ..$ var2b: chr  "vv" "ww" "xx" "yy" ...
#  $ var3: chr  "aa" "bb" "cc" "dd" ...

It's a data.frame with a column (var2) that contains a data.frame. This isn't super easy to create so i'm not quite sure how you did it but it isn't technically "illegal" in R.

data.frames can contain matrices and other data.frames. So it doesn't just look at the length() of the elements, it looks at the dim() of the elements to see if it has the right number of "rows".

I often "fix" or expand these data.frames using

fixed <- do.call("data.frame", df)