R：字符向量的子集

Question

我想从字符向量中获取一个子集。但是我想从特定元素之间的初始向量中获取包含元素的 vector2 。

vector <- c("a", "", "b", "c","","d", "e")
vector

如何抓取元素"b"和"e"之间的所有元素并得到vector2？

#Expected result:
vector2
"c","","d"

Answer 1

这是一个选项

f <- function(x, left, right) {
  idx <- x %in% c(left, right)
  x[as.logical(cumsum(idx) * !idx)]
}

f(vector, "b", "e")
# [1] "c" ""  "d"

第一步是将idx计算为

vector %in% c("b", "e")
# [1] FALSE FALSE  TRUE FALSE FALSE FALSE  TRUE

然后计算累计和

cumsum(vector %in% c("b", "e"))
# [1] 0 0 1 1 1 1 2

乘以!vector %in% c("b", "e")得到

cumsum(vector %in% c("b", "e")) * !vector %in% c("b", "e")
# [1] 0 0 0 1 1 1 0

将其转换为逻辑向量并将其用于子集 x。

对于给定的示例，另一个选项是 charmatch

x <- charmatch(c("b", "e"), vector) + c(1, -1)
vector[seq.int(x[1], x[2])]
# [1] "c" ""  "d"

Answer 2

您也可以这样做：

vector <- c("a", "", "b", "c","","d", "e")
vector[seq(which(vector=="b")+1,which(vector=="e")-1)]
#[1] "c" ""  "d"

Answer 3

带负数下标：

x[-c(1:which(x == 'b'), which(x =='e'):length(x))]
#[1] "c" ""  "d"

如果在 b 之前找到 e 它 returns 空向量：

(y <- rev(x))
#[1] "e" "d" ""  "c" "b" ""  "a"
y[-c(1:which(y == 'b'), which(y =='e'):length(y))]
#character(0)

Answer 4

您也可以试试：

vector[cumsum(vector %in% c("b", "e")) == 1][-1]

[1] "c" ""  "d"

R: subset of character vector