提取字符串的第一个（或最后一个）n个字符

I want to extract the first (or last) n characters 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。这相当于Excel的LEFT（）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，一个等于a的前4个字母的字符串：

b
# [1] "left"

What should I do?

我该怎么办？

4 个解决方案

#1

See ?substr

见？substr

R> substr(a, 1, 4)
[1] "left"

#2

The stringr package provides the str_sub function, which is a bit easier to use than substr, especially if you want to extract right portions of your string :

stringr包提供了str_sub函数，它比substr更容易使用，特别是如果你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"

#3

You can easily obtain Right() and Left() functions starting from the Rbase package:

您可以从Rbase包开始轻松获取Right（）和Left（）函数：

right function

正确的功能

right = function (string, char){ substr(string,nchar(string)-(char-1),nchar(string)) }

right = function（string，char）{substr（string，nchar（string） - （char-1），nchar（string））}
left function

左功能

left = function (string,char){ substr(string,1,char) }

left = function（string，char）{substr（string，1，char）}

you can use those two custom-functios exactly as left() and right() in excel. Hope you will find it useful

您可以在excel中将这两个自定义函数完全用作left（）和right（）。希望你会发现它很有用

#4

Make it simple and use R basic functions:

简单使用R基本功能：

# 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)

substr（）函数告诉你在哪里开始和停止substr（x，start，stop）

#1