Increment and Decrement Integer at Point

While editing you often have to increment or decrement some number (usually an integer) by some step. Obviously this is trivial when the number is something like 10, but not pretty pleasant when the number is 2343566 and you want to increment it by 943. Most of the time, however, you’ll probably be incrementing or decrementing by 1.

A long time ago I found a bit of code by Ryan Thompson to help us deal with such tasks. Here’s a slightly modified version of the original code:

(require 'thingatpt)

(defun thing-at-point-goto-end-of-integer ()
  "Go to end of integer at point."
  (let ((inhibit-changing-match-data t))
    ;; Skip over optional sign
    (when (looking-at "[+-]")
      (forward-char 1))
    ;; Skip over digits
    (skip-chars-forward "[[:digit:]]")
    ;; Check for at least one digit
    (unless (looking-back "[[:digit:]]")
      (error "No integer here"))))
(put 'integer 'beginning-op 'thing-at-point-goto-end-of-integer)

(defun thing-at-point-goto-beginning-of-integer ()
  "Go to end of integer at point."
  (let ((inhibit-changing-match-data t))
    ;; Skip backward over digits
    (skip-chars-backward "[[:digit:]]")
    ;; Check for digits and optional sign
    (unless (looking-at "[+-]?[[:digit:]]")
      (error "No integer here"))
    ;; Skip backward over optional sign
    (when (looking-back "[+-]")
        (backward-char 1))))
(put 'integer 'beginning-op 'thing-at-point-goto-beginning-of-integer)

(defun thing-at-point-bounds-of-integer-at-point ()
  "Get boundaries of integer at point."
    (let (beg end)
      (setq beg (point))
      (setq end (point))
      (cons beg end))))
(put 'integer 'bounds-of-thing-at-point 'thing-at-point-bounds-of-integer-at-point)

(defun thing-at-point-integer-at-point ()
  "Get integer at point."
  (let ((bounds (bounds-of-thing-at-point 'integer)))
    (string-to-number (buffer-substring (car bounds) (cdr bounds)))))
(put 'integer 'thing-at-point 'thing-at-point-integer-at-point)

(defun increment-integer-at-point (&optional inc)
  "Increment integer at point by one.

With numeric prefix arg INC, increment the integer by INC amount."
  (interactive "p")
  (let ((inc (or inc 1))
        (n (thing-at-point 'integer))
        (bounds (bounds-of-thing-at-point 'integer)))
    (delete-region (car bounds) (cdr bounds))
    (insert (int-to-string (+ n inc)))))

(defun decrement-integer-at-point (&optional dec)
  "Decrement integer at point by one.

With numeric prefix arg DEC, decrement the integer by DEC amount."
  (interactive "p")
  (increment-integer-at-point (- (or dec 1))))

The code is based on the popular built-in library thing-at-point and extends it to make it aware of integer numbers. The commands increment-integer-at-point and decrement-integer-at-point operate with a step of 1 by default, but with a prefix argument you can select any step you desire. Unlike other similar commands floating in the Internet, these two handle correctly numbers like -3434 and +343.

I’d suggest binding these commands to C-c + and C-c -:

(global-set-key (kbd "C-c +") #'increment-integer-at-point)
(global-set-key (kbd "C-c -") #'decrement-integer-at-point)

Both commands are available in Prelude (but with a prelude- prefix).