Positional character substitution

charsub.R

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

# SINGLE STRING
(x1 <- "hello world.")
## [1] "hello world."
charsub(x1, nchar(x1), "!")
## [1] "hello world!"
charsub(x1, 5:6, "no")
## [1] "hellnonoworld."

# VECTORIZED
x2 <- c("apple", "banana", "orange", "pineapple", "kiwi")
## [1] "apple"     "banana"    "orange"    "pineapple" "kiwi"
charsub(x2, c(1, 7), c("X", "Y"))
## [1] "Xpple Y"   "XananaY"   "XrangeY"   "XineapYle" "Xiwi  Y"