How to convert character(0) to NA in a list?

Question

How can character(0) be converted to NA in a list?

Example:

a = list("a", character(0), "b", "c")

to

a = list("a", NA, "b", "c")

Answer 1

Replace character(0) only (assume you meant character(0) instead of "character(0)"):

a = list("a", character(0), "b", "c")

lapply(a, function(x) if(identical(x, character(0))) NA_character_ else x)

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

#[[2]]
#[1] NA

#[[3]]
#[1] "b"

#[[4]]
#[1] "c"

Replace all length-0 elements:

a[lengths(a) == 0] <- NA_character_

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

#[[2]]
#[1] NA

#[[3]]
#[1] "b"

#[[4]]
#[1] "c"

Answer 2

a[a=="character(0)"] <- "NA"

Given your example this provides what you ask for, but is that actually what you want, though?