How to remove an unnamed element from a single item list?

问题

This may sound a very beginner's question and very well it could also be a very basic and stupid question, but somehow I am having headache in doing it.

Let's suppose I have a single item list

v <- as.list("1, 2, 3,")

v
[[1]]
[1] "1, 2, 3,"

Now I want to split all of its items as separate items

v2 <- lapply(str_split(v, pattern = ","), trimws)
v2
[[1]]
[1] "1" "2" "3" ""

Now I want to remove this "" from the first and only item of this list without using []?

回答1:

Using nzchar.

lapply(v2, function(x) x[nzchar(x)])
# [[1]]
# [1] "1" "2" "3"

Or use base::strsplit in the first place which appears to be more sophisticated.

lapply(strsplit(v[[1]], ","), trimws)
# [[1]]
# [1] "1" "2" "3"

回答2:

You can use Filter with nchar

v2 <- lapply(str_split(v, pattern = ",\\s?"), Filter, f = nchar)

which gives

> v2
[[1]]
[1] "1" "2" "3"

回答3:

An option is also setdiff to remove the ""

lapply(str_split(v, pattern = ",\\s*"), setdiff, "")
#[[1]]
#[1] "1" "2" "3"

回答4:

Whether you have a list with just one item or more, I'd probably do something like:

str_split(gsub("^,|,$|\\s+", "", v), ",")

Or better yet:

strsplit(gsub("^,|,$|\\s+", "", v), ",", TRUE)
# [[1]]
# [1] "1" "2" "3"

(Or, maybe even strsplit(gsub("^,|,$|\\s", "", gsub(", ,", ",", v, fixed = TRUE)), ",", TRUE) depending on your actual data.)

Here's an example with a list with multiple elements as opposed to a list with just one element.

v <- rep(v, 2500)

I've put the other answers into functions, modifying as appropriate to make them work on multiple list elements. Here are the functions I've tested:

fun_a5a <- function() str_split(gsub("^,|,$|\\s+", "", v), ",")
fun_a5b <- function() strsplit(gsub("^,|,$|\\s+", "", v), ",", TRUE)

fun_ak <- function() lapply(str_split(v, pattern = ",\\s*"), setdiff, "")

fun_des <- function() {
  v2 <- lapply(str_split(v, pattern = ","), trimws)
  lapply(v2, function(x) x[x != ""])
}

fun_hfa <- function() Map(function(x){trimws(unlist(strsplit(x, ",")))}, v)
fun_hfb <- function() sapply(v, strsplit, ",\\s*")

fun_jay <- function() lapply(unlist(lapply(v, strsplit, ","), recursive = FALSE), trimws)

fun_tica <- function() lapply(str_split(v, pattern = ",\\s?"), Filter, f = nchar)
fun_ticb <- function() lapply(str_split(v, pattern = ",\\s?"), Filter, f = nzchar)

Here are the results:

bench::mark(fun_a5a(), fun_a5b(), 
            fun_ak(), 
            fun_des(), 
            fun_hfa(), fun_hfb(), 
            fun_jay(),
            fun_tica(), fun_ticb())
# # A tibble: 9 x 13
#   expression     min  median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time result
#   <bch:expr> <bch:t> <bch:t>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm> <list>
# 1 fun_a5a()   2.47ms  2.63ms     372.     58.7KB     2.04   183     1    491.4ms <list…
# 2 fun_a5b()   1.85ms   1.9ms     517.     58.7KB     2.03   255     1    493.4ms <list…
# 3 fun_ak()   14.17ms 14.85ms      66.8    58.7KB    44.5     15    10    224.5ms <list…
# 4 fun_des()  62.86ms 62.86ms      15.9    78.3KB   111.       1     7     62.9ms <list…
# 5 fun_hfa()  82.17ms 82.17ms      12.2    19.6KB    73.0      1     6     82.2ms <list…
# 6 fun_hfb()  13.36ms 13.59ms      72.9    90.8KB     9.11    32     4    438.9ms <list…
# 7 fun_jay()   71.3ms  71.3ms      14.0    58.7KB    84.2      1     6     71.3ms <list…
# 8 fun_tica() 21.97ms  22.2ms      44.5    58.7KB    66.8      8    12    179.7ms <list…
# 9 fun_ticb() 13.12ms 13.59ms      73.5    58.7KB    44.1     20    12    272.2ms <list…
# # … with 3 more variables: memory <list>, time <list>, gc <list>

ggplot::autoplot(.Last.value)

回答5:

A more ugly version (just as an additional idea, but the solution from jay.sf seems to be preferable):

Based on your v2 input:

delete <- which(v2[[1]] == "")

v2[[1]] <- v2[[1]][-delete]

# [[1]]
# [1] "1" "2" "3"

回答6:

Using Map():

Map(function(x){trimws(unlist(strsplit(x, ",")))}, v)

Using sapply():

sapply(v, strsplit, ",\\s*")

来源：https://stackoverflow.com/questions/65408309/how-to-remove-an-unnamed-element-from-a-single-item-list

标签

string

list

remove