# Extract colnames from a nested list of data.frames

There are already a couple of answers. But let me leave another approach. I used `rapply2()`

in the rawr package.

```
devtools::install_github('raredd/rawr')
library(rawr)
library(purrr)
rapply2(l = l, FUN = colnames) %>%
flatten
$a
[1] "a" "b" "c"
$b
[1] "a" "b" "c"
$c
[1] "a" "b" "c"
```

Here is a base R solution.

You can define a customized function to flatten your nested list (which can deal nested list of ** any depths**, e.g., more than 2 levels), i.e.,

```
flatten <- function(x){
islist <- sapply(x, class) %in% "list"
r <- c(x[!islist], unlist(x[islist],recursive = F))
if(!sum(islist))return(r)
flatten(r)
}
```

and then use the following code to achieve the colnames

```
out <- Map(colnames,flatten(l))
```

such that

```
> out
$a
[1] "a" "b" "c"
$b
[1] "a" "b" "c"
$c
[1] "a" "b" "c"
```

**Example with a deeper nested list**

```
l <- list(a = d, list(b = d, list(c = list(e = list(f= list(g = d))))))
> l
$a
a b c
1 1 1 1
2 2 2 2
3 3 3 3
[[2]]
[[2]]$b
a b c
1 1 1 1
2 2 2 2
3 3 3 3
[[2]][[2]]
[[2]][[2]]$c
[[2]][[2]]$c$e
[[2]][[2]]$c$e$f
[[2]][[2]]$c$e$f$g
a b c
1 1 1 1
2 2 2 2
3 3 3 3
```

and you will get

```
> out
$a
[1] "a" "b" "c"
$b
[1] "a" "b" "c"
$c.e.f.g
[1] "a" "b" "c"
```