Extrahieren Sie die ersten (oder letzten) n Zeichen einer Zeichenfolge

Ich möchte die ersten (oder letzten) n Zeichen einer Zeichenfolge extrahieren . Dies wäre das Äquivalent zu Excel LEFT()und RIGHT(). Ein kleines Beispiel:

# 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:

b
# [1] "left"

What should I do?

See ?substr

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

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"

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

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)

If you are coming from Microsoft Excel, the following functions will be similar to LEFT(), RIGHT(), and MID() functions.

# This counts from the left and then extract n characters

str_left <- function(string, n) {
  substr(string, 1, n)
}



# This counts from the right and then extract n characters

str_right <- function(string, n) {
  substr(string, nchar(string) - (n - 1), nchar(string))
}


# This extract characters from the middle

str_mid <- function(string, from = 2, to = 5){
  
  substr(string, from, to)
  }

Examples:

x <- "some text in a string"
str_left(x, 4)
[1] "some"

str_right(x, 6)
[1] "string"

str_mid(x, 6, 9)
[1] "text"