Quickly Adding a Value to a Numeric Property (in Org-Mode)

Question

Suppose we have

*** Sample Task
:PROPERTIES:
:Tally:    42
:END:

Is there a way to quickly add a number to :Tally: without having to compute the sum manually and changing the property by hand?

EDIT: I tried making an interactive region selecting function which replaces the first instance of :Tally: with the added value:

(defun tally-add (start end num)
  (interactive "r\nn Number:")
  (query-replace-regexp ":Tally:\ *\([0-9]+\)" "\,(format ":Tally: %d" (+ num \#1))" start end)) 

However, when running on a region I get "%d Symbol's value as a variable is void".

Answer 1

I would use this instead:

(defun tally-add (n)
  (interactive "nN: ")
  (org-entry-put
   nil "TALLY"
   (format "%s" (+ n (string-to-number
                      (or (org-entry-get nil "TALLY") "0"))))))

Answer 2

You can do a query-replace-regexp to find the numbers and return a function applied to the number. Usual keybinding is M-C-%.

For the query, find lines with the property and match the number parte:

:Tally:\ *\([0-9]+\)

For the replacement, call an elisp form that reproduces the property line (in this case, adding 3 to the number found):

\,(format ":Tally: %d" (+ 3 \#1))

The \#1 tag refers to the first match group interpreted as a number.