How to subset a vector inside list of list

Question

I have the following list of list:

foo <- list(a = list(x = 1:10, y = 11:25), b = list(x = 1:10, y = 100:110))

It looks like this:

> foo
$a
$a$x
 [1]  1  2  3  4  5  6  7  8  9 10

$a$y
 [1] 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25


$b
$b$x
 [1]  1  2  3  4  5  6  7  8  9 10

$b$y
 [1] 100 101 102 103 104 105 106 107 108 109 110

What I want to do is to only pick first 3 of the list yielding:

$a
$a$x
 [1]  1  2  3  

$a$y
 [1] 11 12 13 


$b
$b$x
 [1]  1  2  3  

$b$y
 [1] 100 101 102

How can I achieve that?

Answer 1

A wild ?rapply appears:

rapply(foo, f=head, n=3, how="list")
#$a
#$a$x
#[1] 1 2 3
#
#$a$y
#[1] 11 12 13
#
#
#$b
#$b$x
#[1] 1 2 3
#
#$b$y
#[1] 100 101 102

This is a recursive apply which will go through each list until a non-list is found and then apply a f=unction. The how="list" argument just ensures that you get the same list structure back again.

Answer 2

Use nested lapply

lapply(foo, lapply, `[`, 1:3)

#$a
#$a$x
#[1] 1 2 3

#$a$y
#[1] 11 12 13


#$b
#$b$x
#[1] 1 2 3

#$b$y
#[1] 100 101 102

Or use head

lapply(foo, lapply, head, 3)

Answer 3

library(purrr)
foo %>% map_depth(2,head,3)
#$a
#$a$x
#[1] 1 2 3
#
#$a$y
#[1] 11 12 13
#
#
#$b
#$b$x
#[1] 1 2 3
#
#$b$y
#[1] 100 101 102