r - Combining lists, possibly with mapply

Question

I have a list of lists - a simple example is given below:

my_list <- vector(mode = "list", length = 4)
my_list[[1]] <- c(1, 2, 3)
my_list[[2]] <- c(1, 2, 6)
my_list[[3]] <- c("A")
my_list[[4]] <- c("A", "B")

I would like to combine a subset of these lists based on their indices in a vector. For example if my_indices <- c(1,2,3), I would like to combine the first three lists and eliminates duplicates to get

c(1, 2, 3, 6, "A")

I can do this manually as follows:

c(my_list[[1]], my_list[[2]], my_list[[3]]) %>% 
      unique()
[1] "1" "2" "3" "6" "A"

but when i try and simplify / generalize this to

my_indices <- c(1, 2, 3)
c(my_list[[my_indices ]]) %>% 
  unique()

I get an error message:

error in my_list[[my_indices]] : recursive indexing failed at level 2

How can i combine lists in this setting. I do want a general solution, as my list of lists is large, and I want to be able to extract any subset of it. I have seen posts that use mapply in a related setting, but have not successfully got it to work.

Many thanks in advance for your help

Thomas Philips

Answer 1

Here's a tidyverse solution using reduce.

library(tidyverse)

my_list <- vector(mode = "list", length = 4)
my_list[[1]] <- c(1, 2, 3)
my_list[[2]] <- c(1, 2, 6)
my_list[[3]] <- c("A")
my_list[[4]] <- c("A", "B")

to_merge <- c(1,2,3)
unique(reduce(my_list[to_merge], c))
#> [1] "1" "2" "3" "6" "A"

^{Created on 2021-01-08 by the reprex package (v0.3.0)}