Using FOR with a dynamic internal table? - for-loop

Question

I would like to convert the method below to a nested FOR instead of a nested LOOP, but I don't know how to do it since the inner table is dynamic (it can be one of 5 different types).
TYPES: BEGIN OF ty_result,
lgart TYPE string,
betrg TYPE string,
betpe TYPE string,
END OF ty_result,
ty_results TYPE STANDARD TABLE OF ty_result WITH EMPTY KEY.
DATA: known_table TYPE ty_results,
also_known_table TYPE ty_results,
mt_period_results TYPE ty_results.
FIELD-SYMBOLS: <dynamic_table> TYPE STANDARD TABLE,
<betrg>, <betpe>, <lgart>.
LOOP AT known_table REFERENCE INTO DATA(known_line).
READ TABLE <dynamic_table> TRANSPORTING NO FIELDS WITH KEY ('LGART') = known_line->*-lgart.
IF sy-subrc <> 0. CONTINUE. ENDIF.
DATA(lv_tabix) = sy-tabix.
LOOP AT <dynamic_table> ASSIGNING FIELD-SYMBOL(<dynamic_line>) FROM lv_tabix.
UNASSIGN: <betrg>, <betpe>, <lgart>.
ASSIGN COMPONENT: 'BETPE' OF STRUCTURE <dynamic_line> TO <betpe>,
'BETRG' OF STRUCTURE <dynamic_line> TO <betrg>,
'LGART' OF STRUCTURE <dynamic_line> TO <lgart>.
IF <lgart> <> known_line->*-lgart.
EXIT.
ENDIF.
APPEND VALUE ty_result( lgart = <lgart>
betrg = <betrg>
betpe = <betpe> ) TO mt_period_results.
ENDLOOP.
ENDLOOP.
When the inner table is not dynamic, I can do it like this:
append lines of value zwta_t_results(
for known_line in known_table
for also_known_line in also_known_table
where ( lgart = known_line-lgart )
( lgart = known_line-lgart
betrg = also_known_line-betrg
betpe = also_known_line-betpe ) to mt_period_results.
So the question is: is it possible to use FOR iterator (as the second method) with a dynamic table?

Answer 1

My answer was checked for ABAP 7.52. Unfortunately, it's currently only possible to use a subset of the static variant of ASSIGN by using LET <fs> = writable_expression IN inside a construction expression (including "FOR" table iterations), where the "writable expression" is limited to a table expression, NEW and CAST. So it's rather limited, there are no equivalences for the dynamic variants of ASSIGN, so you can use only workarounds.
The syntax after WHERE allows a dynamic expression, so it will be possible to enter WHERE ('LGART = KNOWN_LINE-LGART'). However, it could be very counter-performing if the loop is nested inside another loop (as it is in your case), so an index should be defined so that to accelerate the iteration. If a secondary index is to be used, then the condition should be USING KEY ('KEYNAME') WHERE ('LGART = KNOWN_LINE-LGART').
Now, here is a workaround for your particular case: you define statically the names of the components, so one possibility is to define a static structure with those component names and use the CORRESPONDING construction operator. Note that I didn't test it, but I think for several reasons that the performance of using CORRESPONDING is faster in your case than using ASSIGN.
The following code should work. I assume that the internal table behind <dynamic_table> has a primary key sorted by LGART (TYPE SORTED TABLE OF ... WITH NON-UNIQUE KEY lgart) so that the performance is good:
TYPES: BEGIN OF ty_struc,
lgart TYPE string,
betrg TYPE string,
betpe TYPE string,
END OF ty_struc.
known_table = VALUE #( ( lgart = 'A' ) ( lgart = 'B' ) ).
also_known_table = VALUE #( ( lgart = 'A' ) ( lgart = 'C' ) ( lgart = 'A' ) ).
ASSIGN also_known_table TO <dynamic_table>.
APPEND LINES OF
VALUE ty_results(
FOR known_line IN known_table
FOR <inner_line> IN <dynamic_table>
WHERE ('LGART = KNOWN_LINE-LGART')
LET struc = CORRESPONDING ty_struc( <inner_line> ) IN
( lgart = known_line-lgart
betrg = struc-betrg
betpe = struc-betpe ) )
TO mt_period_results.