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MathGroup Archive 2006

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Re: How to tell Mathematica to stop conditional testing in an If

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69542] Re: How to tell Mathematica to stop conditional testing in an If
  • From: dh <dh at metrohm.ch>
  • Date: Fri, 15 Sep 2006 06:45:02 -0400 (EDT)

  statment if one condition is niether True or False? McCarthy evaluation rules?
 
  'and then' test?
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Hi Nasser,

SameQ(or "===" will return "False" if the condition is false or 

undecideable.



Daniel



Nasser Abbasi wrote:

> I can better describe this with simple example:

> 

> ------------- code ------------

> 

> Remove["Global`*"];

> xc = Table[i, {i, 1, 3}]

> x = 5;

> If[x == b && xc[[10]] == 4, Print["True"], Print["False"], Print["Can't 

> decide"]]

> 

> ----- end code -------------

> In the above, 'x==b' is neither True nor False, since 'b' has no numerical 

> value.

> 

> But what I want is when this happens, for Mathematica to NOT continue with 

> the  testing if xc[[10]]==4  is True (because even if it is True, it will 

> not change the outcome, which is can't decide.

> 

> I am looking for something like 'and then'  which says to test the next 

> condition only if the one just tested was true.

> 

> The interesting thing is that if 'b' had a value, say 7, which makes the 

> first test (the x==b) to be False, then Mathematica does the right thing, 

> and will not try to check the xc[[10]]==4 condition.  I need it to do the 

> same thing when also the result of the check is 'undecided', not just 

> 'False' or 'True'.

> 

> Is there a way to do this?  Notice in the example above, I get the error 

> that xc[[10]] is out of bound, but still get the can't decide message.

> 

> It is clear to me that the way Mathematica does it now is not the right way. 

> I do not see why it tries to check for xc[[10]]==4 when it will not make a 

> difference to the final result.

> 

> any thoughts?

> 

> thanks,

> Nasser

> 

> 

> 



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