Elisp / How to insert a char at a fixed position in a string

Let say you have (setq myId "00223344556677") and now you want to format it as "002233-44556677" I can't find a good way to insert a character. Thanks

I'd use a temp buffer `(with-temp-buffer)`. Insert the value of myId `(goto-char) ` wherever you want to put the hyphen Insert it Get the text from the buffer, for example with `(buffer-substring-no-properties)`

(concat (substring myId 0 6) "-" (substring myId 6))

Thanks, exactly what I was looking for.

Are you doing this enough to worry about efficiency? If so, you might care that string literals are effectively immutable in elisp (IIRC), and so taking a string apart and adding pieces will create multiple strings in memory. Garbage Collection means that you don’t have to worry about memory management, until you’re doing something often/quickly enough that gc becomes a concern. (The alternative is *probably* to use **and re-use** an emacs buffer, which is expressly designed to hold mutable sequences of characters.)

One timer, just to fix the correct id to send to the compiler. So no. But thanks for the heads up!