At 11:34 AM 1/28/2002 -0500, you wrote: >On Mon, 28 Jan 2002 11:00:17 -0500, William Carson wrote: > >... Why doesn't R:Base (and some other programming languages) >know that they are not equal? ...
Bill, The following explanation will help you understand the CORRECT evaluation of EQNUL if you are using the latest and greatest version of R:BASE 2000 (ver 6.5+) and higher ... ------------------------------------------------------------------ Correct evaluation of expressions when EQNULL is SET to ON. ------------------------------------------------------------------ R:BASE uses special processing on simple IF commands for maximum speed. However, the way these were processed was not consistent with the way complex IF commands or WHILE commands were processed with regard to comparisons using NULL values. The EQNULL setting at TRUE means that a comparison between two NULL values is a match and that a comparison between a NULL value and a non-NULL value is not. When EQNULL is set to FALSE then a comparison between two NULL values is not a match nor is a comparison between a NULL value and a non-NULL value a mismatch. The NULL value essentially make the whole thing "unknown'. This behavior was followed in WHILE and complex IF commands but was not followed in simple IF commands. A simple IF command has only one comparison and the left side is a simple variable and the right side is either a variable or a constant. A simple IF does not have any expressions. Compare these code samples: SET VAR v1 TEXT = NULL SET VAR v2 TEXT = NULL SET EQNULL OFF IF v1 = .v2 THEN -- will not be a hit ENDIF IF v1 <> .v2 THEN -- will not be a hit ENDIF IF v1 <> 'This' THEN -- will not be a hit (it used to be before this fix) ENDIF SET EQNULL ON IF v1 = .v2 THEN -- will be a hit ENDIF IF v1 <> .v2 THEN -- will not be a hit ENDIF IF v1 <> 'This' THEN -- will be a hit ENDIF Before this fix, the comparison "IF v1 <> 'This' THEN" would be a hit with EQNULL set ON or FALSE when it should only be a hit when EQNULL is ON. This means that now: "IF (.v1) <> 'This' THEN" and "IF v1 <> 'This' THEN" will both process the same way. In the past they would be different because of this problem. In your code if you want the comparison of a NULL variable and a non-NULL constant to be a hit then you should run with EQNULL set ON. Have Fun! Very Best Regards, Razzak. ===================================-============================ R:BASE Developers's Conference: http://www.rbase.com/conference Official R:BASE List Server: mailto:[EMAIL PROTECTED] RBTI Events/Training: http://www.rbase2000.com/events R:DCC Members: http://www.rbase2000.com/rdcc ================================================================ R:BASE, Oterro & R:Tango are registered trademarks of RBTI. ==================================-============================= ================================================ TO SEE MESSAGE POSTING GUIDELINES: Send a plain text email to [EMAIL PROTECTED] In the message body, put just two words: INTRO rbase-l ================================================ TO UNSUBSCRIBE: send a plain text email to [EMAIL PROTECTED] In the message body, put just two words: UNSUBSCRIBE rbase-l ================================================ TO SEARCH ARCHIVES: http://www.mail-archive.com/rbase-l%40sonetmail.com/
