REDEFINES and OCCURS Clauses

The REDEFINES clause lets a second data item share the exact same storage bytes as a previously defined item at the same level, giving a program two different interpretations of one physical location in memory without allocating additional space. Written as 05 WS-DATE-NUMERIC REDEFINES WS-DATE-TEXT PIC 9(8), the redefining item must appear immediately after the item it redefines, at the same level number, and its total size in bytes normally should not exceed the original item's size, though COBOL permits it to be smaller. REDEFINES is fundamentally different from a group item's subordinate breakdown, since a redefined item does not add new bytes to the record; it is purely an alternate lens for viewing storage that already exists.

One of the most common production uses of REDEFINES is splitting a single incoming field into differently typed views for validation or arithmetic, such as receiving an 8-character date as PIC X(8) from an external file, then REDEFINING it with a numeric picture broken into year, month, and day subfields so PROCEDURE DIVISION can validate that the month falls between 01 and 12 using numeric comparisons rather than string parsing. REDEFINES is also used to build tagged unions of record types in a single file, where an initial classification field determines which of several REDEFINED layouts applies to the rest of the record, letting one FD handle multiple record formats, for example a transaction file mixing PAYMENT and REFUND records that share a common header but differ in their trailing fields.

The OCCURS clause turns a data item into an array, called a table in COBOL terminology, by repeating its definition a fixed number of times, as in 05 WS-MONTHLY-SALES OCCURS 12 TIMES PIC 9(7)V99, which creates twelve consecutive numeric fields accessed by subscript like WS-MONTHLY-SALES(3). OCCURS can also apply to a group item, in which case every subordinate field is likewise repeated for each occurrence, letting a single 01-level record represent, for example, a table of twenty employee sub-records each containing name, ID, and salary. For tables whose actual length varies at runtime, OCCURS DEPENDING ON ties the table's active size to a separate counter field, so OCCURS 1 TO 50 TIMES DEPENDING ON WS-ITEM-COUNT lets a program process anywhere from 1 to 50 real entries without wasting space or risking an out-of-bounds reference beyond the counted portion.

A table's entries can be accessed with either a numeric subscript, an ordinary WORKING-STORAGE field like WS-MONTH-IDX PIC 9(2), or with a proper COBOL index declared via INDEXED BY, which is a compiler-managed occurrence displacement rather than an ordinary integer, making index-based access noticeably faster than subscript-based access since the compiler avoids recomputing the byte offset from scratch on every reference. The SEARCH statement performs a linear scan of an indexed table looking for a matching WHEN condition, while SEARCH ALL performs a binary search but requires the table's contents to be sorted in ascending or descending order as declared with the ASCENDING KEY or DESCENDING KEY clause on the OCCURS entry, making SEARCH ALL dramatically faster for large tables at the cost of requiring pre-sorted data.