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Re: How to do a negative pattern?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90585] Re: How to do a negative pattern?
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Mon, 14 Jul 2008 05:23:18 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <g5dm6i$ht4$1@smc.vnet.net>

Steven Siew wrote:

> I have used the function DeleteCases which has the format
> 
> DeleteCases[expr,pattern]
> 
> However what I wanted is a negative pattern like
> 
> DeleteCases[expr,negpattern]
> 
> But I have no idea how to craft a negative pattern. Alternatively I
> suppose I could do with a DeleteCasesNot function. Something like
> this:
> 
> $ cat file012.out
> Mathematica 5.2 for Students: Microsoft Windows Version
> Copyright 1988-2005 Wolfram Research, Inc.
> 
> In[1]:=
> Out[1]= {stdout}
> 
> In[2]:= (* Write your mathematica code below *)
> 
> In[3]:= mylist={ {1,1},{1,2},{1,3},{2,1},{2,2},{2,3} }
> 
> Out[3]= {{1, 1}, {1, 2}, {1, 3}, {2, 1}, {2, 2}, {2, 3}}
> 
> In[4]:= myselectionA = DeleteCases[mylist,{_,1}]
> 
> Out[4]= {{1, 2}, {1, 3}, {2, 2}, {2, 3}}
> 
> In[5]:= myselectionB = DeleteCasesNot[mylist,{_,3}]
> 
> Out[5]= {{1, 3}, {2, 3}}
> 
> In[6]:= (* End of mathematica code *)
> 
> In[7]:= Quit[];
> 
> 
> Does anyone has any idea how I could code up a DeleteCasesNot
> function?

I do not have the slightest idea about what a "negative" pattern
can possibly  be (at least in the context of Mathematica programming). 
It seems that you are looking for *Cases[]*, however.

     lst = {{1, 1}, {1, 2}, {1, 3}, {2, 1}, {2, 2}, {2, 3}};

     Cases[lst, {_, 3}]

     DeleteCases[lst, {_, 3}]


     {{1, 3}, {2, 3}}

     {{1, 1}, {1, 2}, {2, 1}, {2, 2}}


Regards,
-- Jean-Marc



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