Using a number in register as prefix argument

Question

How can I use a number in a register after C-u ?

For example my register looks like 1: 24, and I want to do C-u 24 x.

Answer 1

This should do the trick.

(defun reg-as-pref-arg (register)
  "Use numeric register as prefix arg for next command
You are prompted for the register, showing preview of registers."
  (interactive (list (register-read-with-preview "Increment register: ")))
  (let ((reg-val (get-register register)))
    (when (numberp reg-val)
      (setq prefix-arg  reg-val))))

E.g.:

C-u 42 C-x r n a         ; Put 42 in register `a'

M-x reg-as-pref-arg a X  ; Use 42 as prefix arg for command to insert `X': 42 X's

Bind it to a key.

(global-set-key (kbd "C-o") #'reg-as-pref-arg)

Then use it.

C-o a X                  ; Insert 42 X's.
C-o a C-M-f              ; Move forward 42 words.