Question

I have a string such as:

"aabbccccdd"

I want to break this string into a vector of substrings of length 2 :

"aa" "bb" "cc" "cc" "dd"

Answer 1

Here is one way

substring("aabbccccdd", seq(1, 9, 2), seq(2, 10, 2))
#[1] "aa" "bb" "cc" "cc" "dd"

or more generally

text <- "aabbccccdd"
substring(text, seq(1, nchar(text)-1, 2), seq(2, nchar(text), 2))
#[1] "aa" "bb" "cc" "cc" "dd"

Edit: This is much, much faster

sst <- strsplit(text, "")[[1]]
out <- paste0(sst[c(TRUE, FALSE)], sst[c(FALSE, TRUE)])

It first splits the string into characters. Then, it pastes together the even elements and the odd elements.

Timings

text <- paste(rep(paste0(letters, letters), 1000), collapse="")
g1 <- function(text) {
    substring(text, seq(1, nchar(text)-1, 2), seq(2, nchar(text), 2))
}
g2 <- function(text) {
    sst <- strsplit(text, "")[[1]]
    paste0(sst[c(TRUE, FALSE)], sst[c(FALSE, TRUE)])
}
identical(g1(text), g2(text))
#[1] TRUE
library(rbenchmark)
benchmark(g1=g1(text), g2=g2(text))
#  test replications elapsed relative user.self sys.self user.child sys.child
#1   g1          100  95.451 79.87531    95.438        0          0         0
#2   g2          100   1.195  1.00000     1.196        0          0         0