compare values in an table in lua - indexing

Question

I need help. I am having an table like this:
local dict = {}
dict[1] = {achan = '7f', aseq='02'} --ACK
dict[2] = {dchan = '7f', dseq='03'} --DATA
dict[3] = {achan = '7f', aseq='03'} --ACK
dict[4] = {dchan = '7f', dseq='04'} --DATA
dict[5] = {achan = '7f', aseq='04'} --ACK
dict[6] = {dchan = '7f', dseq='02'} --DATA
Basically I am using this in an Dissector so I don't know the Index except the one I am actually "standing" at the moment.
So what I want to have is:
if the "achan" and the "dchan" is the same and the "aseq" i am standing at the moment is the same as an "dseq" value on positions from the past which are already saved into the table then it should give me back the index from the same "dseq" value from the past.
if (dict[position at the moment].achan == dict[?].dchan) and (dict[position at the moment].aseq == dict[?].dseq) then
return index
end
for example: dchan from position 6 is the same es achan from position 1 and dseq from position 6 is the same as aseq from position 1. So I want to get the position 1 back

Answer 1

You can use a numeric for loop with a negative step size to go back in your table, starting from the previous element. Check wether the achan and aseq fields exist, then compare them vs the dchan and dseq fields of your current entry.
function getPreviousIndex(dict, currentIndex)
for i = currentIndex - 1, 1, -1 do
if dict[i].achan and dict[currentIndex].dchan
and dict[i].achan == dict[currentIndex].dchan
and dict[i].aseq and dict[currentIndex].dseq
and dict[i].aseq == dict[currentIndex].dseq then
return i
end
end
end
This code assumes you have no gaps in your table. You should also add some error handling that makes sure you actually are at a dchan entry and that your index is in range and so on...