How to change string at specific byte index nicely?

Question

I want to change 3rd byte of a string to x, i'm currently using this code:

let s = s[:1] . 'x' . s[3:]

It's a bit awkward, i wish i can do let s[2] = 'x', but that's not allowed.

Are there any other built-in ways to do it nicely?

Answer 1

In vimscript, strings are invariant: it would be very hard to poke odd bytes into a series of Unicode characters without trashing the entire thing.

Concatenation of parts as you have done is not so hard!

An alternative approach uses the substitute() function:

let s = substitute(s, "^\\(..\\).", "\\1x", "")

but that is not much more readable or flexible. It is possible to wrap up both attacks in a more useable fashion:

function! s:replacechar(src, pos, chr)
  if len(a:src) < a:pos
    return a:src
  else
        return (a:pos ? a:src[0:(a:pos-1)] : "") . a:chr . a:src[(a:pos+1):]
  endif
endfunction

function! s:rereplacechar(src, pos, chr)
  let l:pat = "^\\(" . repeat(".", a:pos) . "\\).\\(.*\\)"
  let l:rep = "\\1" . a:chr . "\\2"
  return substitute(a:src, l:pat, l:rep, "")
endfunction

(Edit to correct a:p == 0 bug spotted by Mass)

Hope that helps

Tim