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 User Interface Levels
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     We want to make it easy for you to quickly take advantage of the added
     functionality provided in CA-Clipper 5.2. In order to effectively use
     the RDDs, you should read the following discussions.  They are provided
     as a means of identifying the degree of programming knowledge or
     CA-Clipper experience that will let you effectively use the RDD
     features.

     For this purpose the RDD feature set is arbitrarily divided into levels
     A and B.  Tables listing the commands or functions that comprise these
     access levels are also supplied.  In addition, an RDD Features Summary
     is provided in table form which outlines the features available in each
     driver.  The commands and functions in both of these levels of access
     are described in the Reference chapter of this guide.

 Level A - Command-Level Interface

     Level A.  a simple command-level interface very similar to those
     found in other languages (e.g., dBASE IV, FoxPro).  This is the primary
     access for new CA-Clipper users who may or may not be familiar with
     other languages.

     The following table lists the commands and functions accessible by the
     CA-Clipper programmer with background in languages such as dBASE or
     FoxPro.  The commands and functions in this table provide access to the
     additional features without requiring an advanced knowledge of
     CA-Clipper or other programming concepts.

     Basic Commands and Functions
     ------------------------------------------------------------------------
     Command/Function  Changes
     ------------------------------------------------------------------------
     DELETE TAG        Delete a Tag
     GOTO              Move the pointer to the specified identity
     INDEX             Create an index file
     SEEK              Search an Order for a specified key value
     SET INDEX         Open one or more Order Bags in the current work area
     SET ORDER         Select the controlling Order
     DBAPPEND()        Append a new record to the current Lock List
     DBRLOCK()         Lock the record at the current or specified identity
     DBRLOCKLIST()     Return an array of the current Lock List
     DBRUNLOCK         Release all or specified record locks
     ------------------------------------------------------------------------

 Level B - Function-Level Interface

     Level B.  CA-Clipper also adds a function level interface that not
     only allows access to the enhanced functionality of the drivers, but
     permits the building of higher-level functions using these composing
     behaviors.  This level is meant for more experienced CA-Clipper users
     who need to take advantage of the full power of the driver and Order
     Management System.

     The following table lists the DML and Order Management functions
     recommended to the intermediate to advanced CA-Clipper programmer.
     These functions provide the greatest flexibility in accessing the
     extended features of these drivers

     Advanced Functions (including Order Management)
     ------------------------------------------------------------------------
     Command/Function    Description
     ------------------------------------------------------------------------
     DBAPPEND()          Append a new record to the current Lock List
     DBRLOCK()           Lock the record at the current or specified identity
     DBRLOCKLIST()       Return an array of the current Lock List
     DBRUNLOCK()         Release all or specified record locks
     ORDBAGEXT()         Return the default Order Bag RDD extension
     ORDBAGNAME()        Return the Order Bag name of a specific Order
     ORDCREATE()         Create an Order in an Order Bag
     ORDDESTROY()        Remove a specified Order from an Order Bag
     ORDFOR()            Return the FOR expression of an Order
     ORDKEY()            Return the Key expression of an Order
     ORDLISTADD()        Add Order Bag contents or single Order to the Order
                         List
     ORDLISTCLEAR()      Clear the current Order List
     ORDLISTREBUILD()    Rebuild all Orders in the Order List of the current
                         work area
     ORDNAME()           Return the name of an Order in the work area
     ORDNUMBER()         Return the position of an Order in the current Order
                         List
     ORDSETFOCUS()       Set focus to an Order in an Order List
     RDDLIST()           Return an array of the available Replaceable
                         Database Drivers
     RDDNAME()           Return the name of the RDD active in the current or
                         specified work area
     RDDSETDEFAULT()     Set or return the default RDD for the application
     ------------------------------------------------------------------------

 RDD Features 

     The following decision table summarizes the availability of key features
     across RDDs.  It lists the features available in each RDD so you can use
     it as an aid in correct RDD implementation and data access.

     RDD Features Summary
     ------------------------------------------------------------------------
     Item                                NTX   NDX   MDX   CDX  DBPX
     ------------------------------------------------------------------------
     Implicit record unlocking in        Yes   Yes   Yes   Yes  Yes
     single lock mode
     Multiple Record Locks               Yes   Yes   Yes   Yes  No
     Number of Concurrent Record Locks   *1    *1    *1    *1   1
     Order Management (Tag support)      Yes   Yes   Yes   Yes  No
     Orders (Tags) per Order Bag (File)  1     1     47    50   N/A
     Number of Order Bags (Files)        15    15    15    15   N/A
     per work area
     Conditional Indexes (FOR clause)    Yes   No    Yes   Yes  No
     Temporary (Partial) Indexes         Yes   No    No    Yes  No
     (WHILE, ... )
     Descending via DESCENDING clause    Yes   No    Yes   Yes  No
     Unique via the UNIQUE clause        Yes   Yes   Yes   Yes  No
     EVAL and EVERY clause support       Yes   No    No    Yes  No
     Production/Structural Indexes       No    No    Yes   Yes  No
     Maximum Key Expression length       256   256   220   255  N/A
     (bytes)
     Maximum FOR Condition length        256   N/A   261   255  N/A
     (bytes)
     ------------------------------------------------------------------------

     *1 determined by available memory.