READPREV

This statement is syntactically similar to the READNEXT but it works in reverse order. There are some considerations when the direction is changed from forward search to backward search or vice-versa.

When SELECT statement is first executed forward direction is assumed. Therefore if SELECT is immediately followed by READPREV, then a change of direction is assumed.

During the READNEXT or READPREV sequence a next-key pointer is kept up to date. This is the record or index key to be used, should a READNEXT be executed.

During a change of direction from forward READNEXT to backward READPREV then the next record key or index key read in by the READPREV will be the one preceding the next-key pointer.

When the select list is exhausted it will either point one before the start of the select list (if READPREVs have been executed) or one past the end of the select list (if READNEXTs have been executed). Thus in the event of a change of direction the very first or very last index key or record key will be used.

Behaviour of READNEXT/READPREV depends on emulation. The following example is for jbase emulation:

TAFJ note: READPREV statement isn't supported.

EXAMPLE

Consider the following jBC code 

        list = "DAVE" :@FM: "GREG" :@FM: "JIM"
        SELECT list

The following table shows what happens if you do READNEXTs and READPREVs on the above code and the reasons for it.

Statements executedResult of operationComments
READNEXT key ELSEkey becomes "DAVE"key becomes "DAVE"
READNEXT key ELSEkey becomes "GREG"Second key in list
READPREV key ELSEkey becomes "DAVE"Reversed so take preceding
key
READPREV key ELSETake ELSE clauseThe next key ptr exhausted
at start.
READNEXT key ELSEkey becomes "DAVE"First key in list
READNEXT key ELSEkey becomes "GREG"Second key in list
READNEXT key ELSEkey becomes "JIM"Final key. Next key ptr
exhausted.
READPREV key ELSEkey becomes "JIM"Reversed but list
exhausted.
READPREV key ELSEkey becomes "GREG"Second key in list
READPREV key ELSEkey becomes "DAVE"First key in list

EXAMPLE 2

This code shows the behaviour of READNEXT/READPREV under prime emulation. The difference to the example above starts with the first READPREV: 

       list = "DAVE" :@FM: "GREG" :@FM: "JIM"
       SELECT list
       READNEXT V.ID ELSE CRT 'READNEXT 1 FAILED'
       CRT 'NEXT:' : V.ID                              ;*  NEXT:DAVE
       READNEXT V.ID ELSE CRT 'READNEXT 2 FAILED'
       CRT 'NEXT:' : V.ID                              ;*  NEXT:GREG
       READPREV V.ID ELSE CRT 'READPREV 1 FAILED'
       CRT 'PREVIOUS:' : V.ID                          ;*  PREVIOUS:JIM
       READPREV V.ID ELSE CRT 'READPREV 2 FAILED'      ;*  READPREV 2 FAILED
       CRT 'PREVIOUS:' : V.ID                          ;*  PREVIOUS:
       READNEXT V.ID ELSE CRT 'READNEXT 3 FAILED'      ;*  READNEXT 3 FAILED
       CRT 'NEXT:' : V.ID                              ;*  NEXT:
Last update: Tue, 30 Aug 2022 15:48