提取字符串的第一个(或最后一个) n 个字符

我想提取字符串的第一个(或最后一个) N字符。这相当于 Excel 的 LEFT()和 RIGHT()。举个小例子:

# create a string
a <- paste('left', 'right', sep = '')
a
# [1] "leftright"

我想生成一个字符串 b，它等于 a的前4个字母:

b
# [1] "left"

我该怎么办？

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最佳答案

See ?substr

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

R> str_sub("leftright",1,4)
[1] "left"
R> str_sub("leftright",-5,-1)
[1] "right"
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You can easily obtain Right() and Left() functions starting from the Rbase package:

  • right function

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

  • left function

    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

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Make it simple and use R basic functions:

# 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)

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For those coming from Microsoft Excel or Google Sheets, you would have seen functions like LEFT(), RIGHT(), and MID(). I have created a package known as forstringr and its development version is currently on Github.

if(!require("devtools")){
install.packages("devtools")
}


devtools::install_github("gbganalyst/forstringr")


library(forstringr)

  • the str_left(): This counts from the left and then extract n characters

  • the str_right()- This counts from the right and then extract n characters

  • the str_mid()- This extract characters from the middle

Examples:


x <- "some text in a string"


str_left(x, 4)


[1] "some"


str_right(x, 6)


[1] "string"


str_mid(x, 6, 4)


[1] "text"

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