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

83

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?

string r

— Lisa Ann
quelle

89

See ?substr

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

— rcs
quelle

I recommend having a look at @juba's answer below. He provides a stringr solution.

— Jochem

57

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"

— juba
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3

Thank for your hint, juba; by the way I think I will accept rcs' answer because it deals with base R :)

— Lisa Ann

4

Knowing base R is good, but if it comes to string functions, your life will be easier if you only use the stringr as mentioned by @juba

— Dieter Menne

1

I agree with Dieter. Learning stringr will save you almost as much aggravation as lubridate.

— Andrew Brēza

25

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

— Andrea Cirillo
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6

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)

— Marcos RF
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2

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"

— gbganalyst
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