At 10:27 PM 7/22/2002 -0400, Thomas J Cimicato wrote:
>What is the prevailing thought on the use of the EQNULL setting??
>Should you code for it ON or OFF as a standard? (Off seems to be
>the default in the rbase.cfg file. )
>When would you not want NULL to equal NULL??
Thom,
Using The Glorious R:BASE 2000 (ver 6.5++) and higher, here's how the
expressions are evaluated CORRECTLY 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 the fix in 6.5++, 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.
Hope that helps you understand the CORRECT use of EQNULL.
Enjoy,
Very Best Regards,
Razzak.
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