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Re: ! operator

  • To: mathgroup at smc.vnet.net
  • Subject: [mg55929] Re: [mg55927] ! operator
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Sun, 10 Apr 2005 23:07:05 -0400 (EDT)
  • References: <200504101055.GAA29078@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On 10 Apr 2005, at 19:55, DongGook Park wrote:

>
> Hi,
>
> Could you explain a mysterious behaviour of "! operator" as shown 
> below?
>
>
> In[43]:=
> MemberQ[{1,2,3},3]
>
> Out[43]=
> True
>
> In[47]:=
> !MemberQ[{1,2,3},3]
> (*  Here Mathematica does not give any response!  *)
>
> In[48]:=
> Not@MemberQ[{1,2,3},3]
>
> Out[48]=
> False
>
>
> Considering the description from Mathematica help:
> 	"!expr is the logical NOT function. It gives False if expr is True,
> and True if it is False."
> This behaviour seems quite suspicious?

You stopped reading the help too early:

  If you are using Mathematica with a text$B!>(Bbased front end, then you 
cannot use the notation !expr for Not[expr] if it appears at the very 
beginning of a line. In this case, !expr is interpreted as a shell 
escape.

The only strange thing is this reference to "text$B!>(Bbased front end" 
since this is also true for graphic front ends or at least Mac OS X. 
However, everything works fine if you enclose ! in something, e.g. 
parentheses

(!MemberQ[{1,2,3},3])

False

or


(!2>3)


True


Andrzej Kozlowski
Chiba, Japan
http://www.akikoz.net/andrzej/index.html
http://www.mimuw.edu.pl/~akoz/


  • References:
    • ! operator
      • From: "DongGook Park" <dgpark6@sunchon.ac.kr>
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