How to simplify a nested list in R?

Question

I have a nested list like this:

nested_list <- list(a = c(1,2),
          b = list(
            c = c(3,4),
            d = list(
                e = c(5,6,7)
            )))

I want to simplify it so it looks like this (only one level, nested names grouped using colons):

simplified_list <- list(a = c(1,2),
          "b:c" = c(3,4),
          "b:d:e" = c(5,6,7)
         )

What's the best way to do this?

Answer 1

This approach has the advatnage of being quite short. It does not use any packages. It assumes that the input names do not contain trailing digits:

u <- unlist(nested_list)
res <- tapply(u, sub("\\d+$", "", names(u)), unname)

giving:

> res
$a
[1] 1 2

$b.c
[1] 3 4

$b.d.e
[1] 5 6 7

If its important that the names be separated by : instead of . then add this:

names(res) <- chartr(".", ":", names(res))

Answer 2

I make no claims to "best", but this works:

d <- reshape2::melt(nested_list)
> d
  value   L3   L2 L1
1     1 <NA> <NA>  a
2     2 <NA> <NA>  a
3     3 <NA>    c  b
4     4 <NA>    c  b
5     5    e    d  b
6     6    e    d  b
7     7    e    d  b
> d$L <- apply(d[,c('L1','L2','L3')],1,function(x) paste(unique(x[!is.na(x)]),collapse = ":"))
> l <- plyr::dlply(d,"L",function(x) unlist(x$value))
> l
$a
[1] 1 2

$`b:c`
[1] 3 4

$`b:d:e`
[1] 5 6 7