Enterprise COBOL for z/OS, Version 4.2, Programming Guide

Identify VSAM RRDS files in a COBOL program by using the ORGANIZATION IS RELATIVE clause.
Use the RELATIVE KEY IS clause to associate each logical record with its relative record number.

The following example shows a relative-record data set (RRDS) that is accessed randomly by the value in the relative key:

   SELECT R-FILE
          ASSIGN TO RELATIVE-FILE
          ORGANIZATION IS RELATIVE
          ACCESS       IS RANDOM
          RELATIVE KEY IS RFILE-RELATIVE-KEY
          FILE STATUS  IS FSTAT-CODE VSAM-CODE.

You can use a randomizing routine to associate a key value in each record with the relative record number for that record.
Although there are many techniques to convert a record key to a relative record number, the most commonly used is the division/remainder technique.
With this technique, you divide the key by a value equal to the number of slots in the data set to produce a quotient and remainder.
When you add one to the remainder, the result is a valid relative record number.

Alternate indexes are not supported for VSAM RRDS.

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