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COBOLintermediate

Tables and OCCURS Clause

Understand how COBOL implements arrays via OCCURS and how INDEXED BY drives efficient table access.

A COBOL table is an array. It is declared with OCCURS.

A COBOL table is a wall of labeled pigeonholes fixed in place at compile time — INDEXED BY gives you a fast pointer finger that jumps straight to a slot instead of counting from the first pigeonhole every time.

Key Concepts

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    05  RATE-TABLE  OCCURS 50 TIMES INDEXED BY RT-IDX.
        10  RT-CODE   PIC X(4).
        10  RT-AMOUNT PIC S9(7)V99 COMP-3.
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Fifty copies of that group, laid out contiguously, reached as RT-CODE(5).
RT-CODE(5)
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Subscript or index, and the difference is real.
    subscript   an ordinary numeric field. The compiler multiplies it
                by the entry length on every reference.
    index       declared with INDEXED BY. Holds a byte displacement
                already, so no multiplication is needed.
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Indexes are faster, and inside a loop over a large table that difference shows. They also cannot be used in arithmetic directly — SET RT-IDX UP BY 1, not ADD 1 TO RT-IDX.
SET RT-IDX UP BY 1ADD 1 TO RT-IDX
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OCCURS DEPENDING ON makes the length variable.
    05  LINE-COUNT  PIC 9(3).
    05  ORDER-LINE  OCCURS 1 TO 200 TIMES
                    DEPENDING ON LINE-COUNT.
OCCURS DEPENDING ON
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An order with three lines occupies three entries, not two hundred. The record's length varies with the data, which is how variable-length records work.
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The trap is the order of operations. LINE-COUNT must hold the right value before the table is referenced or the record is written.
LINE-COUNT
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    write with LINE-COUNT = 0   -> a record with no lines, silently
    read  with LINE-COUNT stale -> reading past what was populated
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Nothing checks this. The field is just a number, and getting it wrong produces a malformed record rather than an error.
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SEARCH and SEARCH ALL are the two lookups, and choosing wrongly is the classic question.
    SEARCH RATE-TABLE
        AT END      PERFORM NOT-FOUND
        WHEN RT-CODE(RT-IDX) = WS-WANTED
                    PERFORM FOUND
    END-SEARCH
SEARCHSEARCH ALL
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That is SEARCH: linear, from the current index, works on any table in any order.
SEARCH
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    SEARCH ALL RATE-TABLE
        AT END      PERFORM NOT-FOUND
        WHEN RT-CODE(RT-IDX) = WS-WANTED
                    PERFORM FOUND
    END-SEARCH
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That is SEARCH ALL: a binary search. On a 1,000-entry table it takes about 10 comparisons instead of 500.
SEARCH ALL
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SEARCH ALL has two requirements, and violating them does not raise an error:
SEARCH ALL
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  - the table must be declared with `ASCENDING KEY IS` (or descending)
  - the data must actually be in that order
ASCENDING KEY IS
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An unsorted table searched with SEARCH ALL returns "not found" for entries that are present. The program runs, and the results are wrong.
SEARCH ALL
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Multi-dimensional tables nest OCCURS inside a group and are walked with nested PERFORM VARYING ... AFTER. Reference them with both subscripts: RATE(WS-I, WS-J).
OCCURSPERFORM VARYING ... AFTERRATE(WS-I, WS-J)
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What the interviewer is probing.1. "What is the difference between an index and a subscript?" Probing: the distinction. Stalls: "Nothing." Moves up: a subscript is an ordinary numeric field holding an occurrence number; an index is a displacement maintained by the compiler and is faster.
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2. "What does OCCURS DEPENDING ON give you?" Probing: the capability. Stalls: "A bigger table." Moves up: a variable-length table whose length comes from another field, which is how variable record formats are handled.
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3. "What happens on a subscript out of range?" Probing: the hazard. Stalls: "An error message." Moves up: without bounds checking compiled in, it reads or writes adjacent storage silently — which is why SSRANGE matters in test builds.
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4. "What is SEARCH ALL and what does it require?" Probing: the precondition. Stalls: "A sorted search." Moves up: a binary search, which requires the table to be sorted on the key and the ASCENDING KEY clause declared — an unsorted table gives wrong answers, not errors.
Use cases:tablesoccursarrayssearch-all