Create all combinations of letter substitution in string

Question

I have a string "ECET" and I would like to create all the possible strings where I substitute one or more letters (all but the first) with "X".

So in this case my result would be:

> result
[1] "EXET" "ECXT" "ECEX" "EXXT" "EXEX" "ECXX" "EXXX"

Any ideas as to how to approach the issue?

This is not just create the possible combinations/permutations of "X" but also how to combine them with the existing string.

Answer 1

Using the FUN argument of combn:

a <- "ECET"

fun <- function(n, string) {
  combn(nchar(string), n, function(x) {
    s <- strsplit(string, '')[[1]]
    s[x] <- 'X'
    paste(s, collapse = '')
  } )
}
lapply(seq_len(nchar(a)), fun, string = a)

[[1]]
[1] "XCET" "EXET" "ECXT" "ECEX"

[[2]]
[1] "XXET" "XCXT" "XCEX" "EXXT" "EXEX" "ECXX"

[[3]]
[1] "XXXT" "XXEX" "XCXX" "EXXX"

[[4]]
[1] "XXXX"

unlist to get a single vector. Faster solutions are probably available.

To leave your first character unchanged:

paste0(
  substring(a, 1, 1),
  unlist(lapply(seq_len(nchar(a) - 1), fun, string = substring(a, 2)))
)

[1] "EXET" "ECXT" "ECEX" "EXXT" "EXEX" "ECXX" "EXXX"