string 如何从 R 向量中的每个元素中删除最后 n 个字符
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How to remove last n characters from every element in the R vector
提问by LucasSeveryn
I am very new to R, and I could not find a simple example online of how to remove the last n characters from every element of a vector (array?)
我对 R 很陌生，我在网上找不到一个简单的例子，说明如何从向量（数组？）的每个元素中删除最后 n 个字符
I come from a Java background, so what I would like to do is to iterate over every element of a$dataand remove the last 3 characters from every element.
我来自 Java 背景，所以我想做的是迭代每个元素a$data并从每个元素中删除最后 3 个字符。
How would you go about it?
你会怎么做？
回答by nfmcclure
Here is an example of what I would do. I hope it's what you're looking for.
这是我会做的一个例子。我希望这就是你要找的。
char_array = c("foo_bar","bar_foo","apple","beer")
a = data.frame("data"=char_array,"data2"=1:4)
a$data = substr(a$data,1,nchar(a$data)-3)
a should now contain:
a 现在应该包含：
data data2
1 foo_ 1
2 bar_ 2
3 ap 3
4 b 4
回答by Matthew Plourde
Here's a way with gsub:
这是一种方法gsub：
cs <- c("foo_bar","bar_foo","apple","beer")
gsub('.{3}$', '', cs)
# [1] "foo_" "bar_" "ap" "b"
回答by Blaszard
Although this is mostly the same with the answer by @nfmcclure, I prefer using stringrpackage as it provdies a set of functions whose names are most consistent and descriptive than those in base R (in fact I always google for "how to get the number of characters in R"as I can't remember the name nchar()).
尽管这与@nfmcclure 的答案大致相同，但我更喜欢使用stringrpackage，因为它提供了一组函数，其名称与基础 R 中的函数名称最为一致和具有描述性（事实上，我总是在谷歌上搜索“如何获得R 中的字符”，因为我不记得名字了nchar()）。
library(stringr)
str_sub(iris$Species, end=-4)
#or
str_sub(iris$Species, 1, str_length(iris$Species)-3)
This removes the last 3 characters from each value at Speciescolumn.
这将从Species列的每个值中删除最后 3 个字符。
回答by gagolews
回答by krads
Similar to @Matthew_Plourde using gsub
类似于@Matthew_Plourde 使用 gsub
However, using a pattern that will trim to zero characters i.e. return "" if the original string is shorterthan the number of characters to cut:
但是，如果原始字符串短于要剪切的字符数，则使用将修剪为零字符的模式，即返回“” ：
cs <- c("foo_bar","bar_foo","apple","beer","so","a")
gsub('.{0,3}$', '', cs)
# [1] "foo_" "bar_" "ap" "b" "" ""
Difference is, {0,3}quantifier indicates 0 to 3 matches, whereas {3}requires exactly 3 matches otherwise no match is found in which case gsubreturns the original, unmodified string.
区别在于，{0,3}量词表示 0 到 3 个匹配，而{3}需要正好 3 个匹配，否则找不到匹配，在这种情况下gsub返回原始的、未修改的字符串。
N.B. using {,3}would be equivalent to {0,3}, I simply prefer the latter notation.
NB using{,3}等价于{0,3}，我更喜欢后一种表示法。
See here for more information on regex quantifiers: https://www.regular-expressions.info/refrepeat.html
有关正则表达式量词的更多信息，请参见此处：https: //www.regular-expressions.info/refrepeat.html
