Character substitution by position

charsub.R

x2 <- c("apple", "banana", "orange", "pineapple", "kiwi")

# Original function
charsub <- function(x, i, value) {
  out <- lapply(strsplit(x, ""), function(z) {
    z <- `[<-`(z, i, value)
    z[is.na(z)] <- " "
    paste0(z, collapse = "")
  })
  unlist(out)
}

# Rewrite with simpler assignment + vapply
charsub2 <- function(x, i, value) {
  vapply(strsplit(x, ""), function(z) {
    z[i] <- value
    z[is.na(z)] <- " "
    paste0(z, collapse = "")
  }, vector("character", 1), USE.NAMES = FALSE)
}

# Rewrite with stringi
library(stringi)

charsub3 <- function(x, i, value) {
  vapply(x, function(z) {
    z <- stri_pad_right(z, max(i))
    for(j in seq_along(i))
      stri_sub(z, i[j], i[j]) <- value[j]
    z
  }, vector("character", length = 1), USE.NAMES = FALSE)
}

library(microbenchmark)
microbenchmark(charsub(x2, c(1, 7), c("X", "Y")),
               charsub2(x2, c(1, 7), c("X", "Y")),
               charsub3(x2, c(1, 7), c("X", "Y")))
             
# Unit: microseconds
#                               expr    min      lq     mean  median     uq     max neval
#  charsub(x2, c(1, 7), c("X", "Y")) 36.086 37.4100 40.53261 38.4040 41.052  51.647   100
# charsub2(x2, c(1, 7), c("X", "Y")) 38.073 39.7280 43.68439 41.7140 43.701  90.712   100
# charsub3(x2, c(1, 7), c("X", "Y")) 82.767 86.0775 92.52336 87.7325 97.003 135.406   100