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

Your data:

d <- list(
  list(
    list(
      iris[sample(1:150,3),],
      iris[sample(1:150,3),]
    ),
    list(
      list(
        iris[sample(1:150,3),],
        list(
          iris[sample(1:150,3),],
          iris[sample(1:150,3),]
        )
      )
    )
  )
)

First, a crude, but effective function:

f <- function(x) {

  if (is.null(x)) return
  n <- length(x)

  if (length(n) == 0) return

  for (i in 1:n) {

    if (is.data.frame(x[[i]])) {
      res <<- append(res, list(x[[i]]))
    } else {
      if (is.list(x[[i]])) {
        f(x[[i]])
      }
    }

  }

}

The crude but effective accompanying global variable:

res <- list()

f(d)

The results:

res
## [[1]]
##    Sepal.Length Sepal.Width Petal.Length Petal.Width Species
## 37          5.5         3.5          1.3         0.2  setosa
## 16          5.7         4.4          1.5         0.4  setosa
## 8           5.0         3.4          1.5         0.2  setosa
## 
## [[2]]
##     Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
## 10           4.9         3.1          1.5         0.1     setosa
## 141          6.7         3.1          5.6         2.4  virginica
## 86           6.0         3.4          4.5         1.6 versicolor
## 
## [[3]]
##     Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
## 134          6.3         2.8          5.1         1.5 virginica
## 40           5.1         3.4          1.5         0.2    setosa
## 3            4.7         3.2          1.3         0.2    setosa
## 
## [[4]]
##     Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
## 33           5.2         4.1          1.5         0.1     setosa
## 132          7.9         3.8          6.4         2.0  virginica
## 76           6.6         3.0          4.4         1.4 versicolor
## 
## [[5]]
##     Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
## 10           4.9         3.1          1.5         0.1    setosa
## 16           5.7         4.4          1.5         0.4    setosa
## 135          6.1         2.6          5.6         1.4 virginica

Less crude (no global variable) solution:

f2 <- function(x) {

  res <- list()

  if (is.null(x)) return
  n <- length(x)

  if (length(n) == 0) return

  for (i in 1:n) {
    if (is.data.frame(x[[i]])) {
      res <- append(res, list(x[[i]]))
    } else {
      if (is.list(x[[i]])) res <- append(res, f2(x[[i]]))
    }
  }

  return(res)

}

f2(d)
## same output