Question

Suppose I have a list nested within a list and I have some function which only works on vectors (like str_replace from the stringr package). The function should do its work on every element which actually entails information, …

Question 1: Is there a specific solution to my problem?
Question 2: Is there a general solution?

There should be a solution using loops, but that is all but elegant and probably very slow – efficiency does play a role here.

Let’s have an example:

# let's start easy:
test1 <- list(c("a","d"),c("b","d"),c("c","d"))

# does not work:
str_replace(test1,"d","changed")

# but this does:
lapply(test1,str_replace,"d","changed")

# but what now ?
test2 <- list(c(list("a"),"d"),c("b","d"),c("c","d"))

# does not work! :-(
lapply(test2,str_replace,"d","changed")

Answer 1

You can use unlist/relist:

library(stringr)
test <- list(c(list("a"),"d"),c("b","d"),c("c","d"))
test2 <- unlist(test)
test2 <- str_replace(test2,"d","changed")
relist(test2,test)

[[1]]
[[1]][[1]]
[1] "a"

[[1]][[2]]
[1] "changed"


[[2]]
[1] "b"       "changed"

[[3]]
[1] "c"       "changed"

I believe this is pretty efficient, but have not tested.