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MathGroup Archive 2006

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Re: How to tell Mathematica to stop conditional testing in an If statment if one condition is niether True or False? McCarthy evaluation rules? 'and then' test?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69566] Re: [mg69525] How to tell Mathematica to stop conditional testing in an If statment if one condition is niether True or False? McCarthy evaluation rules? 'and then' test?
  • From: "Chris Chiasson" <chris at chiasson.name>
  • Date: Fri, 15 Sep 2006 06:46:45 -0400 (EDT)
  • References: <200609141057.GAA21660@smc.vnet.net>

use ===

On 9/14/06, Nasser Abbasi <nma at 12000.org> 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
>
>
>
>


-- 
http://chris.chiasson.name/


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