If your question is about how to build an internal table (in memory) with constructor expressions (ABAP >= 7.40), rather than rendering it on the screen or in a spool file (totals and subtotals are features well integrated in ALV and easy to use), then here's one way to do it (ASSERT is here to show that the final value is as expected) :
TYPES : BEGIN OF ty_line,
category TYPE string,
amount TYPE decfloat16,
END OF ty_line,
ty_lines TYPE STANDARD TABLE OF ty_line WITH DEFAULT KEY.
DATA(gt_main) = VALUE ty_lines( ( category = 'AAA' amount = 10 )
( category = 'AAA' amount = 20 )
( category = 'BBB' amount = 30 )
( category = 'CCC' amount = 40 )
( category = 'CCC' amount = 50 )
( category = 'CCC' amount = 60 ) ).
DATA(lt_display) = VALUE ty_lines(
( LINES OF VALUE #(
FOR GROUPS <g> OF <line> IN gt_main
GROUP BY ( category = <line>-category )
( LINES OF VALUE #( FOR <line2> IN GROUP <g> ( <line2> ) ) )
( category = 'SUBTOTAL'
amount = REDUCE #( INIT subtotal TYPE ty_line-amount
FOR <line2> IN GROUP <g>
NEXT subtotal = subtotal + <line2>-amount ) ) ) )
( category = 'TOTAL'
amount = REDUCE #( INIT total TYPE ty_line-amount
FOR <line> IN gt_main
NEXT total = total + <line>-amount ) ) ).
ASSERT lt_display = VALUE ty_lines( ( category = 'AAA' amount = 10 )
( category = 'AAA' amount = 20 )
( category = 'SUBTOTAL' amount = 30 )
( category = 'BBB' amount = 30 )
( category = 'SUBTOTAL' amount = 30 )
( category = 'CCC' amount = 40 )
( category = 'CCC' amount = 50 )
( category = 'CCC' amount = 60 )
( category = 'SUBTOTAL' amount = 150 )
( category = 'TOTAL' amount = 210 ) ).