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Short-Circuiting with Logical Operators
Message
De
28/10/2014 08:52:03
 
 
À
28/10/2014 08:46:27
Dragan Nedeljkovich (En ligne)
Now officially retired
Zrenjanin, Serbia
Information générale
Forum:
Visual FoxPro
Catégorie:
Codage, syntaxe et commandes
Versions des environnements
Visual FoxPro:
VFP 9 SP1
OS:
Windows Server 2003
Network:
Windows 2008 Server
Database:
MS SQL Server
Application:
Web
Divers
Thread ID:
01609995
Message ID:
01610022
Vues:
46
+1

>>In one of my apps I just found and fixed a subtle edge-case bug caused by short-circuiting of logical operators. With my coding style I'm not usually vulnerable to it so it was interesting to be reminded of this feature of VFP.
>
>Whoever needs the reminder... it's been drilled into me that any compiler will shortcircuit your logicals, period. And I take that into account whenever I'm coding. I've seen a few experienced guys do things like this
>
>
do case
>   case not .FirstProcessGoesWell()
>   case not .SecondProcessGoesWell()
>...
>   case not .LastProcessGoesWell()
>   otherwise
>      oMsg.Message("all is well")
>endcase
>...which is, for me, perhaps the neatest packaging of multiphase batch process, which needs to be abandoned on first critical error. Can't remember whom I first saw doing this, perhaps the Feltmans or someone around the ComCodebook project. This is shortcircuiting by design.
>
>However, I've found a case where I'd expect shortcircuiting to work, but it doesn't. It's the evl() and nvl() functions, where both parameters are evaluated, regardless of what the first one returns. So if you try
>
>xx=evl(call1(), call2())
>
>call2 will be called no matter what call1 does. This is kind of counterintuitive, because the equivalent construct with iif() will not have this side effect.
Thierry Nivelet
FoxinCloud
Give your VFP application a second life, web-based, in YOUR cloud
http://foxincloud.com/
Never explain, never complain (Queen Elizabeth II)
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