string 提取字符串的前（或后）n 个字符
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/15897236/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Extract the first (or last) n characters of a string
提问by Lisa Ann
I want to extract the first (or last) ncharacters of a string. This would be the equivalent to Excel's LEFT()and RIGHT(). A small example:
我想提取字符串的第一个（或最后一个）n 个字符。这将等同于 ExcelLEFT()和RIGHT(). 一个小例子：
# create a string
a <- paste('left', 'right', sep = '')
a
# [1] "leftright"
I would like to produce b, a string which is equal to the first 4 letters of a:
我想生成b一个字符串，它等于 的前 4 个字母a：
b
# [1] "left"
What should I do?
我该怎么办？
回答by rcs
See ?substr
看 ?substr
R> substr(a, 1, 4)
[1] "left"
回答by juba
The stringrpackage provides the str_subfunction, which is a bit easier to use than substr, especially if you want to extract right portions of your string :
该stringr包提供了str_sub比 更易于使用的函数，substr特别是如果您想提取字符串的正确部分：
R> str_sub("leftright",1,4)
[1] "left"
R> str_sub("leftright",-5,-1)
[1] "right"
回答by Andrea Cirillo
You can easily obtain Right() and Left() functions starting from the Rbase package:
您可以从 Rbase 包中轻松获取 Right() 和 Left() 函数：
right function
right = function (string, char) { substr(string,nchar(string)-(char-1),nchar(string)) }left function
left = function (string,char) { substr(string,1,char) }
正确的功能
right = function (string, char) { substr(string,nchar(string)-(char-1),nchar(string)) }左函数
left = function (string,char) { substr(string,1,char) }
you can use those two custom-functions exactly as left() and right() in excel. Hope you will find it useful
您可以完全按照 Excel 中的 left() 和 right() 使用这两个自定义函数。希望你会觉得它有用
回答by Marcos RF
Make it simple and use R basic functions:
让它变得简单并使用 R 基本函数：
# To get the LEFT part:
> substr(a, 1, 4)
[1] "left"
>
# To get the MIDDLE part:
> substr(a, 3, 7)
[1] "ftrig"
>
# To get the RIGHT part:
> substr(a, 5, 10)
[1] "right"
The substr()function tells you where start and stop substr(x, start, stop)
该substr()功能告诉您开始和停止的位置substr(x, start, stop)
