How to replace the certain character in certain position in the string?

Question

I have a question that is how to replace a character which is in a certain place. For example:

str <- c("abcdccc","hijklccc","abcuioccc")
#I want to replace character "c" which is in position 3 to "X" how can I do that?
#I know the function gsub and substr, but the only idea I have got so far is 
#use if() to make it. How can I do it quickly?
#ideal result
>str
"abXdccc" "hijklccc" "abXuioccc"

Answer 1

It's a bit awkward, but you can replace a single character dependent on that single character's value like:

ifelse(substr(str,3,3)=="c", `substr<-`(str,3,3,"X"), str)
#[1] "abXdccc"   "hijklccc"  "abXuioccc"

If you are happy to overwrite the value, you could do it a bit cleaner:

substr(str[substr(str,3,3)=="c"],3,3) <- "X"
str
#[1] "abXdccc"   "hijklccc"  "abXuioccc"

Answer 2

I wonder if you can use a regex lookahead here to get what you are after.

str <- c("abcdccc","hijklccc","abcuioccc")
gsub("(^.{2})(?=c)(.*$)", "\\1X\\2", str, perl = T)

Or using a positive lookbehind as suggested by thelatemail

sub("(?<=^.{2})c", "X", str, perl = TRUE)

What this is doing is looking to match the letter c which is after any two characters from the start of the string. The c is replaced with X.

(?<= is the start of positive lookbehind

^.{2} means any two characters from the start of the string

)c is the last part which says it has to be a c after the two characters

---

[1] "abXcdccc"   "hijklccc"   "abXcuioccc"

If you want to read up more about regex being used (link)

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Additionally a generalised function:

switch_letter <- function(x, letter, position, replacement) {
  stopifnot(position > 1)

  pattern <- paste0("(?<=^.{", position - 1, "})", letter)

  sub(pattern, replacement, x, perl = TRUE)

}

switch_letter(str, "c", 3, "X")