Flatten recursive list

Question

There are quite a few questions apparently on this topic, but I can\'t see any general solution proposed: I have a deeply recursive list and want to flatten it into a single lis

Answer 1

This is a general flatten function using only base R:

flatten <- function(x) {
  if (!inherits(x, "list")) return(list(x))
  else return(unlist(c(lapply(x, flatten)), recursive = FALSE))
}

Result:

flatten(d)
#[[1]]
#    Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
#44           5.0         3.5          1.6         0.6     setosa
#138          6.4         3.1          5.5         1.8  virginica
#87           6.7         3.1          4.7         1.5 versicolor
#
#[[2]]
#   Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
#19          5.7         3.8          1.7         0.3     setosa
#1           5.1         3.5          1.4         0.2     setosa
#71          5.9         3.2          4.8         1.8 versicolor
#
#[[3]]
#    Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
#31           4.8         3.1          1.6         0.2     setosa
#98           6.2         2.9          4.3         1.3 versicolor
#134          6.3         2.8          5.1         1.5  virginica
#
#[[4]]
#    Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
#140          6.9         3.1          5.4         2.1  virginica
#119          7.7         2.6          6.9         2.3  virginica
#57           6.3         3.3          4.7         1.6 versicolor
#
#[[5]]
#    Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
#73           6.3         2.5          4.9         1.5 versicolor
#54           5.5         2.3          4.0         1.3 versicolor
#146          6.7         3.0          5.2         2.3  virginica

Similarly:

x <- list(list("A"), list(list("A"), list("A")))
flatten(x)
#[[1]]
#[1] "A"
#
#[[2]]
#[1] "A"
#
#[[3]]
#[1] "A"

x <- list(list(1), list(list(2), list(3)))
flatten(x)
#[[1]]
#[1] 1
#
#[[2]]
#[1] 2
#
#[[3]]
#[1] 3

It seems a bit round-about to add more lists when the goal is to remove them, but the list/unlist route is the only reliable way to concatenate lists with different numbers of elements.