how to populate internal table................

hii all,

my code is according this..................

the three KEY field which is commom in IT_CDPOS & IT_CDHDR are

1. objectclass = 'VBERCGH'

2. tabname = 'VBAP'

3.changind = 'U'.

loop at it_cdpos into wa_cdpos.

z_var = wa_cdpos-tabkey.

wa_cdhdr-vbeln = z_var+3(10).

wa_cdhdr-posnr = z_var+13(6).

modify it_cdhdr from wa_cdhdr transporting vbeln posnr.

endloop.

now the problem is that when i execute the code DUMP is come due to modify statement.

apart from that if i use in place of MODIFY......

APPEND.

Data will move but not in proper position.like it appends from the last position.

As IT_CDHDR has data before this code.

so according the data that IT_CDHDR contain i want to move vbeln and posnr value in IT_CDHDR at correct position.

Please provide me the suitable code for that......

thanks

Babbal.