Adding the values of a field using FOR loop - abap

Question

How do I add the values in a field based on the same values in another field using FOR loop?
Types:
Begin of ty_final,
doctype type char5,
posnr type char5,
total type quan5,
End of ty_final.
DATA(lt_final) = VALUE ty_final(
FOR line IN lt_history
         WHERE ( hist_type = 'U' )
         ( doctype = line-hist_type
total  = line-quantity
           posnr   = line-po_item  ) ).
What I have in LT_HISTORY:
HIST_TYPE POSNR QUANTITY
U 10 5
U 10 2
U 20 3
U 20 -3
What I need in LT_FINAL:
DOCTYPE POSNR QUANTITY
U 10 7
U 20 0
I am trying to use GROUP BY to get the sum of the values in TOTAL field based on POSNR and DOCTYPE fields. It's just that I am not sure where exactly I need to add GROUP BY in my FOR loop. REDUCE makes my head spin. So I was trying out as simple as possible.

Answer 1

Below is minimal reproducible example, which uses ABAP Unit (Ctrl+Shift+F10 to run it). I commented your code and replaced it with the solution:
CLASS ltc_test DEFINITION FOR TESTING
DURATION SHORT RISK LEVEL HARMLESS.
PRIVATE SECTION.
METHODS test FOR TESTING.
ENDCLASS.
CLASS ltc_test IMPLEMENTATION.
METHOD test.
TYPES: BEGIN OF ty_line,
doctype TYPE c LENGTH 1,
posnr TYPE i,
quantity TYPE i,
END OF ty_line,
ty_table TYPE STANDARD TABLE OF ty_line WITH DEFAULT KEY.
DATA(lt_history) = VALUE ty_table(
( doctype = 'U' posnr = 10 quantity = 5 )
( doctype = 'U' posnr = 10 quantity = 2 )
( doctype = 'U' posnr = 20 quantity = 3 )
( doctype = 'U' posnr = 20 quantity = -3 ) ).
DATA(lt_final) = VALUE ty_table(
* FOR line IN lt_history
* WHERE ( doctype = 'U' )
* ( doctype = line-doctype
* quantity = line-quantity
* posnr = line-posnr ) ).
FOR GROUPS grp_line OF line IN lt_history
WHERE ( doctype = 'U' )
GROUP BY ( doctype = line-doctype posnr = line-posnr )
( doctype = grp_line-doctype
quantity = REDUCE #( INIT t = 0 FOR <line> IN GROUP grp_line
NEXT t = t + <line>-quantity )
posnr = grp_line-posnr ) ).
cl_abap_unit_assert=>assert_equals( act = lt_final exp = VALUE ty_table(
( doctype = 'U' posnr = 10 quantity = 7 )
( doctype = 'U' posnr = 20 quantity = 0 ) ) ).
ENDMETHOD.
ENDCLASS.