[O] Tables for attendance lists - A problem understanding TBLFM?

[Gunnar Wolf]

Hi,

As mostly everything for my class work is handled through org-mode, I
am trying to use it also for tracking attendance. And it almost works
(given my quite-probably-wrong way to solve it) — Can you help me
pinpoint what am I doing wrong?

In case it's not obvious, I'm a complete Lisp newbie.

My attendance tables look similar to:

#+CAPTION: Attendances for April
|---------+-------------------+---+---+---+---+----+-------|
| Account | Name              | 1 | 3 | 5 | 8 | 10 | Total |
|---------+-------------------+---+---+---+---+----+-------|
|    1234 | Cárdenas, Lázaro  | X |   | X | X |    |     3 |
|    5678 | Madero, Francisco | X | X | X | X |    |     4 |
|    1544 | Villa, Pancho     |   |   |   |   |    |     1 |
|    0113 | Zapata, Emiliano  |   | X | X |   |    |     2 |
|---------+-------------------+---+---+---+---+----+-------|
| Day avg | 2.25              | 2 | 2 | 3 | 2 |  1 |  2.50 |
|---------+-------------------+---+---+---+---+----+-------|
#+tblfm: @address@hidden'(length 
'($3..$7))::@6$2=vmean($3..$7);%.2f::@address@hidden'(length 
'(@address@hidden))::@6$8=vmean(@address@hidden);%.2f

The formula tries to fill all of the numeric values (last row and
column), but has two important bugs. First, the (attempted) logic:

- Last column includes the sum of nonempty cells. I did this by
  counting the vector's length — But, as you can see, Pancho Villa
  lives up to his rebel's name and has consistently failed to come to
  class. Still, his entry shows he attended once.

  Digging a bit, I found (via M-:) that while (vconcat "X" nil "X" "X")
  yields a three element vector [88 88 88], and (vconcat nil nil nil)
  correctly yields an empty vector []. Getting the length of []
  correctly gives 0. So, I must be doing something wrong. Can you spot
  it?

- This same problem happens of course, getting the daily averages —
  We haven't yet reached April 10, but it shows one attendance.

So, is there a way to unb0rk my lists? Org-mode seems to be quite well
equiped for this task, and I'd hate to use other tools if I can stick
to this :)