Emacs Org-mode: Deriving a property value from another property's value

user_15930002 0
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Suppose I have a these properties defined:

:PROPERTIES:
:a: 1
:b: 2
:END:

How do I define a property c such that the value is the sum of the value of property a and the value of property b? I have tried :c: (+ a b) but the value of c is just the string "(+ a b)".

Is it possible to define a property in this way or do I have to have a code-block that is then evaluated? (Using the Properties API for instance)

org-mode emacs
1 Answers

You could define a code block which executes on export, using the Property API:

* Code

#+NAME: PropertyCalculator
#+BEGIN_SRC emacs-lisp
  (let* ( (rootval (string-to-number (org-entry-get nil "root")))
       (squareval (number-to-string (* rootval rootval))) )
    (org-entry-put nil "square" squareval)
    )
#+END_SRC

* Content
** Property Data
:PROPERTIES:
:root:     2
:square:   5
:END:

#+CALL: PropertyCalculator()

This would leave any existing entry in place in the buffer, but overwrite in the export.

You could also correct in-buffer with (org-babel-load-file).

user_26960050
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