Re: Why does MemberQ[{0,1,2}, expr_] evaluate to True?

• To: mathgroup at smc.vnet.net
• Subject: [mg125930] Re: Why does MemberQ[{0,1,2}, expr_] evaluate to True?
• From: David Reiss <dbreiss at gmail.com>
• Date: Sat, 7 Apr 2012 05:59:25 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <4F7D8716.4010202@fundp.ac.be> <jlmf8n\$mhn\$1@smc.vnet.net>

``` expr_ is a pattern that matches any expression.  Hence it matches at
least one of the members of the list {0,1,2}.

--David

On Apr 6, 6:07 am, Christoph Lhotka <christoph.lho... at fundp.ac.be>
wrote:
> I am really sorry for reposting it but of course the subject of
> discussion should be:
>
> MemberQ[{0, 1, 2}, expr_]
>
> Please replace below and in message ModuleQ by MemberQ.
>
> best,
>
> christoph
>
> On 04/05/2012 01:50 PM, Christoph Lhotka wrote:
>
>
>
>
>
>
>
> > Hello,
>
> > I found and interesting subject of discussion in the post
>
> > "Bug in pattern test, or I did something wrong?"
>
> > I could trace back the problem to an issue with ModuleQ.
>
> > Question: Why does
>
> > In[12]:= ModuleQ[{0,1,2},expr_]
>
> > Out[12]:= True
>
> > evaluate to True?
>
> > My argumentation is as follows:
>
> > On the one hand there could be a chance that expr_ is 0,1 or 2 but on
> > the other
> > hand the probability that expr_ is not 0,1 or 2 is even higher. As a
> > conclusion it should neither
> > evaluate to True nor to False.
>
> > In other words: Is there any reason why the expression of everything
> > (named expr)
> > is contained in the set {0,1,2} ?
>
> > Best,
>
> > Christoph

```

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