Thanks for the responses.
In response to, "Is the result of the function being used in other
formulae?" Not always, but often yes, the result is used in further
downstream calculations. Of course, if I haven't acheived the desired
accuracy in this calculation, or it has diverged to some "garbage"
answer, then all downstream calculations aren't as accurate as desired
or are "garbage" as well. I don't know if I'm too worried about the
effect of an error value (whether xlerrorvalue or string message) on
any downstream calculations.
My intent here is to alert the user (namely myself, as I'm the only
intended user) that "this value didn't converge." I generally don't
like to have a computer do too much of my thinking for me, lest the
computer really foul things up. I simply want to be notified that a
specific calculation didn't converge so I can go in and determine
(using the computer in my head) if the calculation simply needs a few
more iterations, or is diverging, or is caught in an oscillation, or
what. Then I can decide how best to respond based on my knowledge of
the numerical algorithm and the specifics of the calculation at hand.
I'm not sure I like the idea of a messagebox in a UDF. If I have 20
incidents of the UDF in a spreadsheet, then I potentially could get 20
successive messageboxes to clear with each calculation event. I would
also have to figure out (not sure that it would be difficult) how to
identify within the message box which incident of the function isn't
converging. A message box would have the advantage of allowing for
more flexible alert options. I could list the oldresult, newresult
values and any other variable values that could help in troubleshooting
just what is going wrong.
One solution I've thought of: have the UDF return two values.
Something like: myudf=array(result,it). Then I could scan down the
second column of return values for (or use conditional formatting to
highlight) cases where it=100. I could also see if the function was
returning a potentially reasonable answer (ie maybe it just needs a few
more iterations) or if it was diverging to garbage.
Thanks for the responses. It helps me think through what I'm trying to
do.
Any other ideas, or comments on the discussion so far, I'm still
thinking this through.
--
MrShorty
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