如何选择空格后的第一个字符

Question

我想在空格后选择第一个字符。我的MWE是由

Input

a <- c("United States", "South America")
a

Output desired

"S" "A"

Answer 1

base-R的例子：

gsub(".* (\\C).*", "\\1", a, perl = TRUE)
[1] "S" "A"

Answer 2

inds = regexpr(" ", a) + 1
substring(a, inds, inds)
#[1] "S" "A"

或者使用str_extract的stringr

library(stringr)
str_extract(string = a, pattern = "(?<=\\s).")
#[1] "S" "A"