Combining lists, possibly with mapply - r

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

c(1, 2, 3, 6, "A") is not what you think, it will be converted to c("1", "2", "3", "6", "A"). If you want mixed class, you cannot unlist, it must stay a list.
Some thoughts:
my_list[my_indices]
# [[1]]
# [1] 1 2 3
# [[2]]
# [1] 1 2 6
# [[3]]
# [1] "A"
unlist(my_list[my_indices])
# [1] "1" "2" "3" "1" "2" "6" "A"
unique(unlist(my_list[my_indices]))
# [1] "1" "2" "3" "6" "A"
To preserve class and ensure uniqueness, you can do
func <- function(a, b) {
a_chrs <- as.character(a)
b_chrs <- as.character(b)
b[ match(setdiff(b_chrs, a_chrs), b_chrs) ]
}
Reduce(func, my_list[my_indices], accumulate = TRUE)
# [[1]]
# [1] 1 2 3
# [[2]]
# [1] 6
# [[3]]
# [1] "A"
The _chrs fancy footwork is because setdiff by itself will not reduce correctly:
out <- Reduce(setdiff, my_list[my_indices], accumulate = TRUE)
out
# [[1]]
# [1] 1 2 3
# [[2]]
# [1] 3
# [[3]]
# [1] 3
If you need that with individually-indexable values, then
unlist(lapply(out, as.list), recursive = FALSE)
# [[1]]
# [1] 1
# [[2]]
# [1] 2
# [[3]]
# [1] 3
# [[4]]
# [1] 6
# [[5]]
# [1] "A"

Answer 2

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)