interface using idoc..creating function module??

Iam trying to do an interface with idocs.just tryin to put some default values before uploading to sap.

iam creating a function module with input and output parameters as same as that of IDOC_INPUT_ACC_BILLING.

Iam declaring sements like this..

E1BPACHE01 like E1BPACHE01,

E1BPACAR01 like E1BPACAR01,

E1BPACGL01 like E1BPACGL01,

E1BPACCR01 LIKE E1BPACCR01,

wa_control like line of IDOC_CONTRL,

wa_data like line of IDOC_data,

I_EDIDD TYPE ZEDIDD.

REFRESH I_EDIDD.

adding data to internal table.

LOOP AT IDOC_CONTRL INTO WA_CONTROL.

LOOP AT IDOC_DATA into wa_data WHERE DOCNUM = WA_CONTROL-DOCNUM.

APPEND wa_DATA TO I_EDIDD.

ENDLOOP.

LOOP AT I_EDIDD INTO wa_DATA.

CASE wa_DATA-SEGNAM.

WHEN 'E1BPACHE01'.

E1BPACHE01 = wa_DATA-SDATA.

*********

IF E1BPACHE01-COMP_CODE

IS INITIAL.

move '0000' to E1BPACHE01-COMP_CODE.

endif.

*********

like this am checking other segments and changing values of field wat i require.the

then..

endcase.

modify I_EDIDD FROM WA_DATA.

ENDLoop.

ENDLOOP.

then am calling one standard function module IDOC_INPUT_ACC_BILLING for posting purpose.

please help me

the whole scenario described above is not working ..just tell me where i went wrong