Handling zero length character vectors as empty strings - r

Question

By way of example, see the extraction of Twitter handles below. The target is to have a character string that resembles tweets but has only handles separated by commas. str_replace_all yields empty vectors when no matches are found and that threw some unexpected errors further down the track.
library(purrr)
library(stringr)
tweets <- c(
"",
"This tweet has no handles",
"This is a tweet for #you",
"This is another tweet for #you and #me",
"This, #bla, is another tweet for #me and #you"
)
mention_rx <- "#\\w+"
This was my first attempt:
map_chr(tweets, ~str_c(str_extract_all(.x, mention_rx)[[1]], collapse = ", "))
#> Error: Result 1 must be a single string, not a character vector of length 0
Then I played around with things:
mentions <- map(tweets, ~str_c(str_extract_all(.x, mention_rx)[[1]], collapse = ", "))
mentions
#> [[1]]
#> character(0)
#>
#> [[2]]
#> character(0)
#>
#> [[3]]
#> [1] "#you"
#>
#> [[4]]
#> [1] "#you, #me"
#>
#> [[5]]
#> [1] "#bla, #me, #you"
as.character(mentions)
#> [1] "character(0)" "character(0)" "#you" "#you, #me"
#> [5] "#bla, #me, #you"
Until it dawned on me that paste could also be used here:
map_chr(tweets, ~paste(str_extract_all(.x, mention_rx)[[1]], collapse = ", "))
#> "" "" "#you" "#you, #me" "#bla, #me, #you"
My questions are:
Is there a more elegant way of getting there?
Why doesn't str_c behave the same as paste with an identical collapse argument?
Why don't as.character and map_chr recognise a character vector
of length zero as equivalent to an empty string but paste does?
I found some good references on str(i)_c, paste, and the difference between them; but none of these addressed the situation with empty strings.

Answer 1

You don't need to map over tweets, str_extract_all can handle vectors
library(stringr)
str_extract_all(tweets, mention_rx)
#[[1]]
#character(0)
#[[2]]
#character(0)
#[[3]]
#[1] "#you"
#[[4]]
#[1] "#you" "#me"
#[[5]]
#[1] "#bla" "#me" "#you"
Now if you need one comma-separated string then you can use map
purrr::map_chr(str_extract_all(tweets, mention_rx), toString)
#[1] "" "" "#you" "#you, #me" "#bla, #me, #you"
To answer the "why" questions, we can look at the documentation of paste and str_c functions.
From ?paste
Vector arguments are recycled as needed, with zero-length arguments being recycled to "".
From ?str_c
Zero length arguments are removed.
Hence, by default str_c removes zero-length arguments which makes the output a 0-length string which fails for map_chr but it works with map as map returns a list
map(tweets, ~str_c(str_extract_all(.x, mention_rx)[[1]], collapse = ", "))
#[[1]]
#character(0)
#[[2]]
#character(0)
#[[3]]
#[1] "#you"
#[[4]]
#[1] "#you, #me"
#[[5]]
#[1] "#bla, #me, #you"