R 是否有像 python 一样的函数开始或结束？
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Does R have function startswith or endswith like python?
提问by user4015990
> startsWith('abc', 'a')
[1] TRUE
> startsWith('abc', 'c')
[1] FALSE
> endsWith('abc', 'a')
[1] FALSE
> endsWith('abc', 'c')
[1] TRUE
采纳答案by ijoseph
As added to basein 3.3.0, startsWith(and endsWith) are exactly this.
正如在 3.3.0 中添加的base那样，startsWith(和endsWith) 正是这个。
> startsWith("what", "wha")
[1] TRUE
> startsWith("what", "ha")
[1] FALSE
https://stat.ethz.ch/R-manual/R-devel/library/base/html/startsWith.html
https://stat.ethz.ch/R-manual/R-devel/library/base/html/startsWith.html
回答by mathematical.coffee
Not inbuilt like that.
不是那样内置的。
Options include grepland substr.
选项包括grepl和substr。
x <- 'ABCDE'
grepl('^AB', x) # starts with AB?
grepl('DE$', x) # ends with DE?
substr(x, 1, 2) == 'AB'
substr('ABCDE', nchar(x)-1, nchar(x)) == 'DE'
回答by Maxime Biette
This is relatively simple by using the substring function:
通过使用 substring 函数，这相对简单：
> strings = c("abc", "bcd", "def", "ghi", "xyzzd", "a")
> str_to_find = "de"
> substring(strings, 1, nchar(str_to_find)) == str_to_find
[1] FALSE FALSE TRUE FALSE FALSE FALSE
You cut each string to the desired length with substring. The length being the number of characters you are looking for at the beginning of each string.
您可以使用子字符串将每个字符串剪切到所需的长度。长度是您在每个字符串开头查找的字符数。
回答by JasonAizkalns
Borrowing some code from the dplyrpackage [see this]you could do something like this:
从dplyr包中借用一些代码[see this]你可以做这样的事情：
starts_with <- function(vars, match, ignore.case = TRUE) {
if (ignore.case) match <- tolower(match)
n <- nchar(match)
if (ignore.case) vars <- tolower(vars)
substr(vars, 1, n) == match
}
ends_with <- function(vars, match, ignore.case = TRUE) {
if (ignore.case) match <- tolower(match)
n <- nchar(match)
if (ignore.case) vars <- tolower(vars)
length <- nchar(vars)
substr(vars, pmax(1, length - n + 1), length) == match
}
回答by G. Grothendieck
The dplyr package's selectstatement supports starts_withand ends_with. For example, this selects the columns of the iris data frame that start with Petal
dplyr 包的select声明支持starts_with和ends_with。例如，这将选择 iris 数据框的列以Petal
library(dplyr)
select(iris, starts_with("Petal"))
selectsupports other subcommands too. Try ?select.
select也支持其他子命令。试试?select。
回答by DaniGate
The simplest way I can think of is to use the %like%operator:
我能想到的最简单的方法是使用%like%运算符：
library(data.table)
"foo" %like% "^f"
evaluates as TRUE- Starting with f
评估为TRUE- 以f 开头
"foo" %like% "o$"
evaluates as TRUE- Ending with o
评估为TRUE- 以o结尾
"bar" %like% "a"
evaluates as TRUE- Containing a
作为计算TRUE-包含一个
