string 从 R 中的字符串中提取最后 n 个字符
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Extracting the last n characters from a string in R
提问by Brani
How can I get the last n characters from a string in R? Is there a function like SQL's RIGHT?
如何从R中的字符串中获取最后n个字符？有没有像 SQL 的 RIGHT 这样的函数？
回答by Andrie
I'm not aware of anything in base R, but it's straight-forward to make a function to do this using substrand nchar:
我不知道基本 R 中的任何内容，但是使用substrand制作一个函数来执行此操作很简单nchar：
x <- "some text in a string"
substrRight <- function(x, n){
substr(x, nchar(x)-n+1, nchar(x))
}
substrRight(x, 6)
[1] "string"
substrRight(x, 8)
[1] "a string"
This is vectorised, as @mdsumner points out. Consider:
正如@mdsumner 指出的那样，这是矢量化的。考虑：
x <- c("some text in a string", "I really need to learn how to count")
substrRight(x, 6)
[1] "string" " count"
回答by Xu Wang
If you don't mind using the stringrpackage, str_subis handy because you can use negatives to count backward:
如果你不介意使用这个stringr包，这str_sub很方便，因为你可以使用负数来倒数：
x <- "some text in a string"
str_sub(x,-6,-1)
[1] "string"
Or, as Max points out in a comment to this answer,
或者，正如 Max 在对此答案的评论中指出的那样，
str_sub(x, start= -6)
[1] "string"
回答by bartektartanus
Use stri_subfunction from stringipackage.
To get substring from the end, use negative numbers.
Look below for the examples:
使用包中的stri_sub函数stringi。要从末尾获取子字符串，请使用负数。看看下面的例子：
stri_sub("abcde",1,3)
[1] "abc"
stri_sub("abcde",1,1)
[1] "a"
stri_sub("abcde",-3,-1)
[1] "cde"
You can install this package from github: https://github.com/Rexamine/stringi
你可以从 github 安装这个包：https: //github.com/Rexamine/stringi
It is available on CRAN now, simply type
它现在可在 CRAN 上使用，只需键入
install.packages("stringi")
to install this package.
安装这个包。
回答by Andrew
str = 'This is an example'
n = 7
result = substr(str,(nchar(str)+1)-n,nchar(str))
print(result)
> [1] "example"
>
回答by dsb
Another reasonably straightforward way is to use regular expressions and sub:
另一种相当直接的方法是使用正则表达式和sub：
sub('.*(?=.$)', '', string, perl=T)
So, "get rid of everything followed by one character". To grab more characters off the end, add however many dots in the lookahead assertion:
因此，“摆脱所有后跟一个字符的内容”。要从末尾获取更多字符，请在先行断言中添加许多点：
sub('.*(?=.{2}$)', '', string, perl=T)
where .{2}means .., or "any two characters", so meaning "get rid of everything followed by two characters".
where.{2}表示..，或“任意两个字符”，意思是“去掉后面跟着两个字符的所有内容”。
sub('.*(?=.{3}$)', '', string, perl=T)
for three characters, etc. You can set the number of characters to grab with a variable, but you'll have to pastethe variable value into the regular expression string:
三个字符等。您可以设置要使用变量抓取的字符数，但您必须将paste变量值放入正则表达式字符串中：
n = 3
sub(paste('.+(?=.{', n, '})', sep=''), '', string, perl=T)
回答by Laurent
UPDATE: as noted by mdsumner, the original code is already vectorised because substr is. Should have been more careful.
更新：正如mdsumner所指出的，原始代码已经矢量化了，因为 substr 是。应该更加小心。
And if you want a vectorised version (based on Andrie's code)
如果你想要一个矢量化版本（基于Andrie的代码）
substrRight <- function(x, n){
sapply(x, function(xx)
substr(xx, (nchar(xx)-n+1), nchar(xx))
)
}
> substrRight(c("12345","ABCDE"),2)
12345 ABCDE
"45" "DE"
Note that I have changed (nchar(x)-n)to (nchar(x)-n+1)to get ncharacters.
请注意，我已经改变(nchar(x)-n)，以(nchar(x)-n+1)获得n字符。
回答by Andrew Haynes
A simple base R solution using the substring()function (who knew this function even existed?):
使用该substring()函数的简单基本 R 解决方案（谁知道这个函数甚至存在？）：
RIGHT = function(x,n){
substring(x,nchar(x)-n+1)
}
This takes advantage of basically being substr()underneath but has a default end value of 1,000,000.
这利用了基本上substr()低于但具有 1,000,000 的默认结束值。
Examples:
例子：
> RIGHT('Hello World!',2)
[1] "d!"
> RIGHT('Hello World!',8)
[1] "o World!"
回答by mdsumner
An alternative to substris to split the string into a list of single characters and process that:
另一种方法substr是将字符串拆分为单个字符列表并处理：
N <- 2
sapply(strsplit(x, ""), function(x, n) paste(tail(x, n), collapse = ""), N)
回答by Raimi bin Karim
I use substrtoo, but in a different way. I want to extract the last 6 characters of "Give me your food." Here are the steps:
我也用substr，但方式不同。我想提取“给我你的食物”的最后 6 个字符。以下是步骤：
(1) Split the characters
(1) 拆分字符
splits <- strsplit("Give me your food.", split = "")
(2) Extract the last 6 characters
(2) 提取最后6个字符
tail(splits[[1]], n=6)
Output:
输出：
[1] " " "f" "o" "o" "d" "."
Each of the character can be accessed by splits[[1]][x], where x is 1 to 6.
每个字符都可以通过 访问splits[[1]][x]，其中 x 是 1 到 6。
回答by lukasz
Try this:
尝试这个：
x <- "some text in a string"
n <- 5
substr(x, nchar(x)-n, nchar(x))
It shoudl give:
它应该给出：
[1] "string"
