R apply function to nested list elements using "[["

Question

Given a nested list of numeric vectors like l = list( a = list(1:2, 3:5), b = list(6:10, 11:16)) If I want to apply a function, say length, of the "index 1 / first" numeric vectors I can do it using the subset function [[:

> sapply(lapply(l, "[[", 1), length)
a b 
2 5

I cant figure how to supply arbitrary indeces to [[ in order to get length of (in this example) both vectors in every sub-list (a naive try : sapply(lapply(l, "[[", 1:2), length)).

About the operators `[[` vs `[`: https://stackoverflow.com/q/1169456/5325862 — camille, Dec 11 '19 at 16:08

score 1 · Accepted Answer · answered Dec 11 '19 at 15:49

1

The [[ can only subset a single one. Instead, we need [ for more than 1 and then use lengths

sapply(lapply(l, "[", 1:2), lengths)
#     a b
#[1,] 2 5
#[2,] 3 6

answered Dec 11 '19 at 15:49

akrun

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Keen to realize that I never did hear or use `lengths`. – user3375672 Dec 11 '19 at 15:53

score 1 · Answer 2 · answered Dec 11 '19 at 15:58

1

Not using base, but purrr is a great package for lists.

library(purrr)

map_dfc(l, ~lengths(.[1:2]))

# A tibble: 2 x 2
      a     b
  <int> <int>
1     2     5
2     3     6

answered Dec 11 '19 at 15:58

Adam

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ThomasIsCoding · Answer 3 · 2019-12-11T16:02:06.800

1

Maybe the code below can help...

> sapply(l, function(x) sapply(x, length))
     a b
[1,] 2 5
[2,] 3 6

edited Dec 11 '19 at 16:02

answered Dec 11 '19 at 15:58

ThomasIsCoding

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R apply function to nested list elements using "[["

3 Answers3