org table apply formula from specific row onwards

Question

| BookmarkLevel | BookmarkPageNumber | ActualPageNumber |
|---------------+--------------------+------------------|
|             1 |                 11 |                  |
|             2 |                 11 |                  |
|             2 |                 34 |                  |
|             1 |                 59 |                  |
|             2 |                 59 |                  |
|             3 |                    |                3 |
|             3 |                    |               10 |
|             2 |                    |               17 |
|             3 |                    |               17 |
|             3 |                    |               29 |
|             3 |                    |               35 |
|             3 |                    |               39 |
|             3 |                    |               43 |
|             4 |                    |               43 |
|             4 |                    |               45 |

#+tblfm: $2=$3+58

Where BookmarkPageNumber is empty, I would like to use ActualPageNumber (from the TOC) to calculate its values (by adding a constant to it). How should tblfm be written for the effect?

Answer 1

In this particular situation, you could use a range formula to fill in rows 7 through the last row:

#+TBLFM: @7$2..@>$2 = $3 + 58

Counting rows is probably not what you want to do though and is error-prone, so you could say

#+TBLFM: $2 = if ($2 > 0, $2, $3+58)

leaving the non-empty entries alone, and changing the empty entries (which in a numerical context evaluate to 0).

See Formula Syntax for Calc but be forewarned that the conversions that Org mode makes behind the scenes sometimes make it frustrating to figure out a formula that works. The formula debugger is your friend but is not a panacea.

Another possibility is using Lisp for the formula and handling the conversions yourself. E.g. here is a more robust check for empty vs 0 cells:

 #+TBLFM: $2 = '(if (not (equal $2 "")) $2 (+ (string-to-number $3) 58))

See Formula syntax for Lisp.