如何按字符拆分字符串而不忽略尾随拆分字符？

Question

我有一个类似于以下内容的字符串

my_string <- "apple,banana,orange,"

我想分割,以产生输出：

list(c('apple', 'banana', 'orange', ""))

我认为 strsplit 可以实现这一点，但它对待尾随的 ',' 就像它不存在一样

my_string <- "apple,banana,orange,"

^{由reprex 包(v2.0.1)于 2023 年 11 月 15 日创建}

实现所需输出的最简单方法是什么？

更多带有示例字符串和所需输出的测试用例

string1 = "apple,banana,orange,"
output1 = list(c('apple', 'banana', 'orange', ''))

string2 =  "apple,banana,orange,pear"
output2 = list(c('apple', 'banana', 'orange', 'pear'))

string3 =  ",apple,banana,orange"
output3 = list(c('', 'apple', 'banana', 'orange'))

## Examples of non-comma separated strings
# '|' separator
string4 =  "|apple|banana|orange|"
output4 = list(c('', 'apple', 'banana', 'orange', ''))

# 'x' separator
string5 =  "xapplexbananaxorangex"
output5 = list(c('', 'apple', 'banana', 'orange', ''))

编辑：

理想的解决方案应该推广到任何分裂字符

还更喜欢 base-R 解决方案（尽管仍然链接提供此功能的任何包，因为它们的源代码可能有助于查看！）

Answer 1

为什么strsplit没有给出期望的输出？

当您输入时?strsplit，您将看到以下语句

请注意，这意味着如果在（非空）字符串的开头有匹配，则输出的第一个元素是“”，但如果在字符串的末尾有匹配，则输出是相同的与删除匹配项一样。

""这就是您在使用 时看不到尾随的原因strsplit。

下面是一些演示

> strsplit("apple,banana,orange,", ",")
[[1]]
[1] "apple"  "banana" "orange"


> strsplit(",apple,banana,orange,", ",")
[[1]]
[1] ""       "apple"  "banana" "orange"


> strsplit(",apple,banana,orange", ",")
[[1]]
[1] ""       "apple"  "banana" "orange"


> strsplit("apple,banana,orange", ",")
[[1]]
[1] "apple"  "banana" "orange"

Base R 解决方法

如果您想进行编码练习，一个基本 R 选项可以定义一个自定义函数（递归），如下所示

f <- function(x, sep = ",") {
  pat <- sprintf("^(.*?)%s.*", sep)
  s1 <- sub(pat, "\\1", x)
  s2 <- sub(paste0("^.*?", sep), "", x)
  if (s2 == x) {
    return(x)
  }
  c(s1, Recall(s2, sep))
}

substr或带有+的变体regexpr

f <- function(x, sep = ",") {
  idx <- regexpr(sep, x)
  s1 <- substr(x, 1, idx - 1)
  s2 <- substr(x, idx + 1, nchar(x))
  if (s2 == x) {
    return(x)
  }
  c(s1, Recall(s2, sep))
}

这样

> f("apple,banana,orange,")
[1] "apple"  "banana" "orange" ""

> f(",apple,banana,orange,")
[1] ""       "apple"  "banana" "orange" ""      

> f(",apple,banana,orange")
[1] ""       "apple"  "banana" "orange"

> f("apple,banana,orange")
[1] "apple"  "banana" "orange"

Answer 2

使用纵梁

library(stringr)

str_split(my_string, ",")

[[1]]
[1] "apple"  "banana" "orange" ""  

Answer 3

在末尾粘贴另一个分隔符应该可以按strsplit预期运行。
否则，您可以回退到使用该scan函数，该read.csv/table函数支撑着这些函数：

strsplit(paste0(string1, ","), ",")
##[[1]]
##[1] "apple"  "banana" "orange" ""

一般考虑正则表达式替换：

L <- list(string1, string2, string3, string4, string5)
mapply(
    function(x,s) strsplit(paste0(x, gsub("\\\\", "", s)), split=s),
    L,
    c(",", ",", ",", "\\|", "x")
)

##[[1]]
##[1] "apple"  "banana" "orange" ""      
##
##[[2]]
##[1] "apple"  "banana" "orange" "pear"  
##
##[[3]]
##[1] ""       "apple"  "banana" "orange"
##
##[[4]]
##[1] ""       "apple"  "banana" "orange" ""      
##
##[[5]]
##[1] ""       "apple"  "banana" "orange" "" 

scan选项：

scan(text=string1, sep=",", what="")
##Read 4 items
##[1] "apple"  "banana" "orange" ""

概括：

mapply(
    function(x,s) scan(text=x, sep=s, what=""),
    L,
    c(",", ",", ",", "|", "x")
)