如何从列表列表中获取特定列表？

Question

This is such a basic question and for some reason I can't figure out how to get this right. Suppose I have a list of lists

v <- list(
          list(a=1, b=2, c=3), 
          list(a=4, b=5, c=6), 
          list(a=7, b=8, c=9))

How do I pull out a list of all elements that are named "a". ie I would like to get list(1, 4, 7) asking for a .

Answer 1

We can use pluck

library(tidyverse)
map(v, pluck, "a")
#[[1]]
#[1] 1

#[[2]]
# [1] 4

#[[3]]
# [1] 7

---

The corresponding method in base R would be

lapply(v, `[[`, "a")

Answer 2

In base R, we can use

unlist(v)[names(unlist(v))=="a"]

Or, if your prefer not to use unlist twice:

(x <- unlist(v))[names(x)=="a"]