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Row descriptor -X leads outside table

Emacs Asked on December 26, 2020

I’m trying to calculate a new column on an org table, I’m using the following table formula:

#+TBLFM: $9=if (@# > 15, ([email protected]$6)*100000/10724599, 0)

It’s taking the difference on column 6 between the current row and 14 rows previous.

To prevent it from overrunning the start of the table the current row @# is checked to see there are 14 previous rows to look back over. If not, a zero if filled in.

However when using this formula I get the message:

Row descriptor -14 leads outside table

But it shouldn’t do that as it should only evaluate the true branch when there are >14 previous rows – at least that’s what I thought?

In case Calc formulas don’t respect short-circuiting with if, I tried a lisp construct too – but get the same result using if or when:

#+TBLFM: $9='(if (> @# 15) (/ (* (- $6 @-14$6) 100000) 10724599) 0);N

If I replace the -14 for -1 the logic doesn’t fail – I presume, because of the header.

How can I achieve a lookback in a formula like the example above?

Note – if I comment out the condition that it testing for out of bounds indexes in org-table.el it works fine – which suggests org-table--row-type is being called eagerly on all parameters of if even if they are not used? So perhaps I need to write my own lazy version of if or similar?

    (cond ;;((or (< i 0) (>= i l))
       ;;(user-error "Row descriptor %s leads outside table" desc))
      ;; The last hline doesn't exist.  Instead, point to last row
      ;; in table.
      ((= i (1- l)) (1- i))
      (t i))

If I then turn on debugging formula I can see what is happening for the first row – it does look to me like expansion of the out of bound array happens even though I never use it?

With out commenting out the code the debugging does nothing – I assume because the user error is hit before the debugger is started.

Substitution history of formula
Orig:   if (@# > 15, ([email protected]$6)*100000/10724599, 0)
$xyz->  if (@# > 15, ([email protected]$6)*100000/10724599, 0)
@r$c->  if (2 > 15, ($6-(Cumulative_cases))*100000/10724599, 0)
$1->    if (2 > 15, ((1)-(Cumulative_cases))*100000/10724599, 0)
Result: 0
Format: NONE
Final:  0

Update – it’s not necessary to understand the problem but for reference the full code can be found here:
https://github.com/falloutphil/Misc/blob/master/covid.el

One Answer

You may well be right about the eager evaluation (which might be a bug). EDIT: Actually, there is no evaluation at all: the user error is precipitated just by pattern matching - while in row 2, it finds the @-14$6 in the formula and proceeds to check it immediately; it does not matter at all that the formula would never hit that. Fixing that is probably difficult (to say the least).

The only thing I can suggest as a workaround is to fill the first 14 rows of column 9 with zeros, and apply the formula unconditionally to the rest of the rows, using range formulas:

#+TBLFM: #+TBLFM: @[email protected]$9 = 0 :: @[email protected]>$9=([email protected]$6)*100000/1072459

Correct answer by NickD on December 26, 2020

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