[next] [previous] [contents]

  14.6.10 Accessing Records by Record File Address
  A Record File Address (RFA) uniquely specifies a record in a
  file. Accessing records by RFA is therefore more efficient and
  faster than other forms of random record access

                                                                        1

  Because an RFA requires six bytes of storage, BASIC has
  a special data type, RFA, that denotes variables that con-
  tain RFA information. Variables of data type RFA can be
  used only with the I/O statements and functions that use
  RFA information, and in comparison and assignment state-
  ments. You cannot print these variables or use them in any
  arithmetic operation. However, you can compare RFA vari-
  ables using the equal to ( = ) and not equal to (<>) relational
  operators.

  You cannot create named constants of the RFA data type.
  However, you can assign values from one RFA variable to
  another, and you can use RFA variables as parameters.

  Accessing a record by RFA requires the following steps:

  1. Explicitly declare the variable or array of data type RFA
        to hold the address.
  2. Assign the address to the variable or array element.
        You can do this either with the GETRFA function, or by
        reading a file of RFAs generated by previous GETRFA
        functions or by the VMS Sort Utility.
  3. Specify the variable in the RFA clause of a GET or FIND
        statement.

  The GETRFA function returns the RFA of the last record
  accessed on a channel. Therefore, you must access a record
  in the file with a GET, FIND, or PUT statement before using
  the GETRFA function. Otherwise, GETRFA returns a zero,
  which is an invalid RFA.

  The following example declares an array of type RFA con-
  taining 100 elements. After each PUT operation, the RFA of
  the record is assigned to an element of the array. Once the
  RFA information is assigned to a program variable or ar-
  ray element, you can use the RFA clause on a GET or FIND
  statement to retrieve the record.
  DECLARE RFA R_array(1 TO 100)
  DECLARE LONG I
  MAP (XYZ) STRING A = 80
  OPEN "TEST.DAT" FOR OUTPUT AS FILE #1, &
            SEQUENTIAL, MAP XYZ
  FOR I = 1% TO 100%
      .
      .
      .
        PUT #1
        R_array(I) = GETRFA(1%)
  NEXT I

  You can use the RFA clause on GET or FIND statements for
  any file organization; the only restriction is that the file must
  reside on a disk that is accessible to the node that is executing
  the program. An RFA value is only valid for the life of a spe-
  cific version of a file. If a new version of a file is created, the
  RFA values may change. If you attempt to access a record
  with an invalid RFA value, BASIC signals a run-time error.

  The following example continues the previous one. It ran-
  domly retrieves the records in a sequential file by using RFAs
  stored in the array.
  DECLARE RFA R_array(1% TO 100%)
  DECLARE LONG I
  MAP (XYZ) STRING A = 80
  OPEN "TEST.DAT" FOR OUTPUT AS FILE #1, &
            SEQUENTIAL, MAP XYZ
  FOR I = 1% TO 100%
      .
      .
      .
        PUT #1
        R_array(I) = GETRFA(1%)
  NEXT I
  WHILE -1%
        PRINT "Which record would you like to see";
        INPUT "(type a carriage return to exit)";Rec_num%
        EXIT PROGRAM IF Rec_num% = 0%
        GET #1, RFA R_array(Rec_num%)
        PRINT A
  NEXT