ABAP Core Part 2: DATA, TYPES, CONSTANTS, Work Areas and Internal Tables

📌 KEY POINTS
✅ TYPES defines a type (no memory); DATA creates a variable; CONSTANTS creates a value that never changes.
✅ A structure (work area) holds one row; an internal table holds many rows of that structure.
✅ Rows are filled into the work area and added with APPEND; CLEAR empties a variable, REFRESH/CLEAR itab[] empties a table.
💡 Prefer types from the dictionary (S_CARR_ID, SFLIGHT) — they carry length, labels and conversions.

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▶ WHERE WE ARE
Part 1 ran a first program. Now you learn how to declare the data your program works with — the foundation of everything else.

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▶ 1. TYPES — THE BLUEPRINT
TYPES: BEGIN OF ty_flight,
carrid TYPE s_carr_id,
connid TYPE s_conn_id,
fldate TYPE s_date,
seatsmax TYPE s_seatsmax,
seatsocc TYPE s_seatsocc,
END OF ty_flight.
TYPES ty_flights TYPE STANDARD TABLE OF ty_flight.
A type allocates no memory; it describes a shape.

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▶ 2. DATA — VARIABLES, WORK AREAS, TABLES
DATA lv_count TYPE i. " elementary variable
DATA ls_flight TYPE ty_flight. " structure = work area
DATA lt_flights TYPE ty_flights. " internal table
Naming convention used in most projects: lv_ (local variable), ls_/wa_ (structure), lt_/it_ (table), gv_/gs_/gt_ (global), p_ (parameter), s_ (select-option).

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▶ 3. CONSTANTS — FIXED VALUES
CONSTANTS lc_max_rows TYPE i VALUE 100.
Use constants instead of "magic values" scattered in code.

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▶ 4. FILLING A TABLE
Fill the work area field by field, then APPEND it:
ls_flight-carrid = 'LH'. ls_flight-connid = '0400'. APPEND ls_flight TO lt_flights.
CLEAR ls_flight before filling the next row, so no old values remain.

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▶ 5. CLEAR, REFRESH, FREE

• CLEAR lv_x / CLEAR ls_flight — initial value.
• REFRESH lt_flights (or CLEAR lt_flights[]) — delete all rows, keep memory.
• FREE lt_flights — delete rows and release memory (large tables).

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▶ 6. REFERENCE TO DICTIONARY TYPES
TYPE sflight-price, TYPE s_carr_id, TYPE STANDARD TABLE OF sflight — reuse dictionary definitions so lengths and field labels stay consistent.

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▶ 7. MODERN SYNTAX (SIDE NOTE)
From 7.40: inline declarations DATA(lv_x) = … and the VALUE operator build data in one expression. You will see them in newer code; the classic form remains valid everywhere.

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▶ STEP BY STEP

1. Define ty_flight and ty_flights as in the code example.
2. Fill three rows with work area + APPEND, clearing the work area each time.
3. Loop over the table and compute free seats.
4. Output with WRITE.

✅ CHECKPOINT: you can declare types, variables, constants, work areas and internal tables and fill them.

A legacy report declared 60 global variables at the top, many with TYPE c LENGTH 10 for airline and document numbers, and used the literal 'O' for "open" in 14 places. A change of the status code required a risky search-and-replace, and a truncated customer number caused wrong postings.
The refactoring introduced dictionary-based types, a CONSTANTS structure for status values and inline declarations inside small methods. The next status change touched one line, and type mismatches were caught by the syntax check.

• Declaring everything globally at the top.
• Using generic types (c LENGTH 10) instead of dictionary types.
• Magic literals instead of constants.
• Forgetting BASE and overwriting a table with VALUE.
• Assuming inline variables are block-scoped.

• Use dictionary types for business fields.
• Declare close to first use (inline where it helps readability).
• Use CONSTANTS for fixed codes.
• Build data with VALUE instead of many APPEND statements.
• Follow your team's naming convention consistently.

Questions: "Difference between TYPES and DATA?", "What are inline declarations?", "How does the VALUE operator work?"
Strong answer: TYPES defines a type without memory, DATA creates a variable, CONSTANTS a fixed value; inline declarations (DATA(x) = …, INTO TABLE @DATA(…)) derive the type from the right side at first use; VALUE #( ) constructs structures and tables, with BASE to keep existing lines. Recommend dictionary types and constants over literals.