Re: Which, If and "neither True nor False"
- To: mathgroup at smc.vnet.net
- Subject: [mg9705] Re: [mg9652] Which, If and "neither True nor False"
- From: David Withoff <withoff>
- Date: Fri, 21 Nov 1997 01:31:53 -0500
- 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 > Official Disclaimer: > The views expressed above are entirely those of the writer and do not > represent the views, policy or understanding of any other person or > official body. Try Which[ TrueQ[a<b], DoSomething[], TrueQ[a>c], DoSomethingElse[], True, DoThis[] ] This will cause indeterminate tests to be treated as False, so that the last test will be used more or less in the same way as the fourth element in If. Dave Withoff Wolfram Research