Internal Tables Part 8: LOOP Patterns — WHERE, Control Levels and Totals

📌 KEY POINTS
✅ LOOP AT … WHERE filters inside the loop statement — cleaner (and on sorted tables faster) than IF inside.
✅ Control levels AT NEW / AT END OF / SUM need a table SORTED by the control fields, from left to right.
✅ Totals and filtered subsets are built with a normal LOOP (WHERE) plus APPEND or addition.
💡 Side note: 7.40+ offers GROUP BY, VALUE FOR and REDUCE for the same tasks — useful to recognise in newer code.

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▶ WHERE WE ARE
Parts 1–7 covered table operations. Loops are where most business logic happens; this part shows the patterns you will meet in classic code and the modern equivalents.

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▶ 1. LOOP WHERE AND RANGES
LOOP AT gt_flight INTO gs_flight WHERE carrid = 'LH' AND seatsocc < seatsmax.
LOOP AT gt_flight INTO gs_flight FROM 11 TO 20. " index range (paging)
On sorted tables, WHERE on leading key fields is optimized.

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▶ 2. CONTROL-LEVEL PROCESSING (CLASSIC)
Classic reports produce subtotals with:
AT FIRST — before the first row (headers)
AT NEW carrid — when carrid (or any field left of it) changes → group header
AT END OF carrid — last row of the group → subtotal with SUM
AT LAST — after the last row → grand total
Rules:

• SORT by the control fields first, and the control field order must match the structure's field order (fields to the left count).
• Inside AT … ENDAT, fields to the right of the control field contain '*' or are initial — copy what you need before AT.
• SUM adds all numeric fields of the group into the work area.

⚠️ WATCH OUT: using LOOP … WHERE together with AT NEW gives unexpected groups — filter first, then loop.

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▶ 3. SIDE NOTE: GROUP BY (7.40+)
LOOP AT gt_flight INTO DATA(ls) GROUP BY ls-carrid INTO DATA(lv_carr).
LOOP AT GROUP lv_carr INTO DATA(ls_member). … ENDLOOP.

ENDLOOP.

No sorting required, explicit group key, members available per group, group keys can be structures (GROUP BY ( carrid = ls-carrid month = ls-fldate(6) )).

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▶ 4. SIDE NOTE: VALUE FOR AND REDUCE (7.40+)

• Build: DATA(lt_lh) = VALUE ty_flights( FOR f IN gt_flight WHERE ( carrid = 'LH' ) ( f ) ).
• Total: DATA(lv_occ) = REDUCE i( INIT s = 0 FOR f IN gt_flight NEXT s = s + f-seatsocc ).
• Transform: CORRESPONDING #( gt_flight MAPPING … ) for structure mapping.

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▶ 5. WHICH PATTERN?

• Existing classic report → keep control levels consistent, but sort correctly.
• Systems below 7.40 or teams with classic standards → control levels and LOOP WHERE; on 7.40+ the newer constructs are an option if the team guideline allows them.
• Complex logic per row → a normal LOOP calling a method.

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▶ STEP BY STEP
Run the code example: occupancy subtotals per airline with AT NEW/AT END OF + SUM, a filtered subset with LOOP WHERE, and a total with a simple loop.

✅ CHECKPOINT: you can read and write classic control-level loops and their modern equivalents.

A classic sales report printed subtotals per customer with AT END OF kunnr, but after a change the table was sorted by date first. Subtotals appeared after every date change, and totals per customer were split into dozens of fragments. Users exported to Excel and summed manually.
The team restored the correct SORT order matching the control fields and, in the next rewrite, replaced control levels with LOOP GROUP BY on the customer, which does not depend on the sort order and made the grouping explicit.

• Control levels without sorting by the control fields.
• Control field not the leftmost relevant field in the structure.
• Reading masked fields inside AT … ENDAT.
• Combining LOOP WHERE with AT NEW.
• IF inside loops instead of WHERE.

• SORT by control fields before AT NEW/AT END OF.
• Copy needed values before AT blocks.
• Use GROUP BY for new grouping logic.
• Use REDUCE for totals and VALUE FOR for filtering/transforming.
• Keep loop bodies small; call methods for complex logic.

Questions: "How do AT NEW and AT END OF work?", "What are the prerequisites for control-level processing?", "What does LOOP GROUP BY offer?"
Strong answer: control levels trigger on changes of the control field (and fields to its left) in a table sorted by those fields; AT NEW for headers, AT END OF with SUM for subtotals, AT FIRST/AT LAST for totals; fields to the right are masked inside AT; GROUP BY groups without sorting, with explicit group keys and member loops; REDUCE and VALUE FOR compute and build without explicit loops.