Which, If and "neither True nor False"
- To: mathgroup at smc.vnet.net
- Subject: [mg9652] Which, If and "neither True nor False"
- From: "Paul E Howland" <PEHOWLAND at dra.hmg.gb>
- Date: Mon, 17 Nov 1997 19:50:13 -0500
- Organization: Defence Research Agency
- Sender: owner-wri-mathgroup at wolfram.com
I've just stumbled across a bug in some old code of mine, in which I had
a statement of the form:
Which[
a<b, DoSomething[],
a>c, DoSomethingElse[],
True, DoThis[]
]
which I assumed would call DoThis[] if neither a<b nor a>c were True.
However, what I didn't realise is that if a is undefined, then the
first two conditions are neither True nor False, and Mathematica
returns the whole Which statement unevaluated!
The If statement has an optional fourth argument which is executed if
the test returns neither True nor False. Is it possible to do the same
in a Which statement? If not, why not?!
Paul
Paul E Howland PhD MEng CEng MIEE Room BY209 Senior Scientist
DERA (Malvern) Land Systems Sector St Andrews Road
Defence Evaluation & Research Agency Malvern tel.
+44-(0)1684-895767 Worcestershire fax.
+44-(0)1684-896315 UK Email PEHowland at dera.gov.uk
Web Site http://www.dera.gov.uk
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