zemir zemir - 4 months ago 17
R Question

Splitting a word into length combination

I'm looking for a function in R that will given a integer allow me to split a word into that length combination but with a rolling effect.

For example

function("stackoverflow", 4)
would render:

c("stac", "tack", "acko", "ckov", "kove", "over", "verf", "rflo", "flow")


Do you guys know if that function exists or must I create it?

Answer
## install.packages("zoo")

x <- unlist(strsplit("stackoverflow",""))
zoo::rollapply(x,width=4,FUN = paste0,collapse="")
# [1] "stac" "tack" "acko" "ckov" "kove" "over" "verf" "erfl" "rflo" "flow"

A function?

foo <- function(input, h) {
  x <- unlist(strsplit(input,""))
  zoo::rollapply(x,width=h,FUN = paste0,collapse="")
  }

foo("stackoverflow", 4)
# [1] "stac" "tack" "acko" "ckov" "kove" "over" "verf" "erfl" "rflo" "flow"

A benchmark

Consider the base R approach with substring():

foo1 <- function(input, h) substring(input, seq_len(nchar(input)-h+1),h:nchar(input))

Let's generate a very long toy character string:

x <- paste0(rep("a",100000), collapse="")

system.time(foo(x,4))
#   user  system elapsed 
#  2.280   0.004   2.288 

system.time(foo1(x,4))
#   user  system elapsed 
# 10.492   0.000  10.509 

So, the seemingly vectorized function substring() is not efficient at all, which is an interesting observation!

Comments