Modify IDoc data to update the counter!! How??

Question

Hi All,
I have requirement to trigger custom Outbound IDoc from a custom program.
Basically my IDoc structure has 3 segments
1. TOP - Can coour only once in each IDoc
2. HDR - Can occur only 5 times in each IDoc(under segment TOP)
3. DET - Can occur n times in each IDoc ( under the segment HDR)
say I have one field - COUNT in my TOP segment
once the program is executed If 5 IDocs have triggered, then I have to update 5 in the COUNT field in each 5 IDocs.
(Just basically to know how many IDocs have been triggened in that one specific run)
Say I have field MATNR in my HDR segment.
and
I have a internal table populated with all the final data, with MATNR repeated.
Each HDR is for one MATNR field.
Now the question is what is the best way to find / update COUNT in my TOP segment.
1 . Build a looping logic and find how many HDR segments can occur overall based on final internal table data and
this number divide by 5 will give me total IDoc that will be created .
2. Do I have any option to update the my TOP-COUNT filed after my all IDoc is created. Once all IDoc is created I can get the total number of IDocs. Can I edit and update the IDoc filed within my code, before commit work.

Answer 1

Thanks for the replies.
@RobvS could you please elaborate more. where can I seen the generated conversion program.
@ Vinod . Thanks for your inputs to find the count while populating final internal table.
I use MASTER_IDOC_DISTRIBUTE FM to create IDoc and before COMMIT work, I am able to use FM EDI_DOCUMENT_OPEN_FOR_EDIT passing IDoc number returned by previous FM, this FM is providing me the edidd internal table data. now how can I modify the internal table and update it to IDoc in my program is there any FM available ?
Thanks.
Edited by: Lokesh Naik on Feb 7, 2010 2:13 PM