Keep last n characters of cells in a function in R - r

Question

Consider the following data.frame:
df <- setNames(data.frame(rep("text_2010"),rep(1,5)), c("id", "value"))
I only want to keep the 4 last characters of the cells in the column "id". Therefore, I can use the following code:
df$id <- substr(df$id,nchar(df$id)-3,nchar(df$id))
However, I want to create a function that does the same. Therefore, I create the following function and apply it:
testfunction <- function(x) {
x$id <- substr(x$id,nchar(x$id)-3,nchar(x$id))
}
df <- testfunction(df)
But I do not get the same result. Why is that?

Answer 1

Add return(x) in your function to return the changed object.
testfunction <- function(x) {
x$id <- substr(x$id,nchar(x$id)-3,nchar(x$id))
return(x)
}
df <- testfunction(df)
However, you don't need an explicit return statement always (although it is better to have one). R by default returns the last line in your function so here you can also do
testfunction <- function(x) {
transform(x, id = substring(id, nchar(id)-3))
}
df <- testfunction(df)
which should work the same.

Answer 2

We can also create a function that takes an argument n (otherwise, the function would be static for the n and only useful as a dynamic function for different data) and constructs a regex pattern to be used with sub
testfunction <- function(x, n) {
pat <- sprintf(".*(%s)$", strrep(".", n))
x$id <- sub(pat, "\\1", x$id)
return(x)
}
-testing
testfunction(df, n = 4)
# id value
#1 2010 1
#2 2010 1
#3 2010 1
#4 2010 1
#5 2010 1

Answer 3

Base R solution attempting to mirror Excel's RIGHT() function:
# Function to extract the right n characters from each element of a provided vector:
right <- function(char_vec, n = 1){
# Check if vector provided isn't of type character:
if(!is.character(char_vec)){
# Coerce it, if not: char_vec => character vector
char_vec <- vapply(char_vec, as.character, "character")
}
# Store the number of characters in each element of the provided vector:
# num_chars => integer vector
num_chars <- nchar(char_vec)
# Return the right hand n characters of the string: character vector => Global Env()
return(substr(char_vec, (num_chars + 1) - n, num_chars))
}
# Application:
right(df$id, 4)
Data:
df <- setNames(data.frame(rep("text_2010"),rep(1,5)), c("id", "value"))