org mode sort problems with (numerical) properties

Question

I use the property "pages" for the number of pages of an article or book.

There are two problems.

First:

If I use C-c ^ to sort the items by the value of pages org-sort seems to interpret the numbers lexicographically: 1<100<20<3. Of course I need the natural order of numbers: 1<3<20<100.

Second:

If I use C-c a m to search for items with pages<20 I get the correct items and the "wrong" items: where the pages property is not defined. How to ignore the "wrong" items?

Answer 1

For the first problem, you have to use a function like this:

#+BEGIN_SRC emacs-lisp
(defun sort-by-pages ()
 (string-to-number (org-entry-get nil "pages")))
#+END_SRC

then type C-c ^ f and select the function sort-by-pages. The problem is properties in org-mode are strings, and you need this to convert them to numbers.

For problem two try: pages<20+pages<>"" the second one should eliminate the nulls.