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Re: How to do a negative pattern?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90579] Re: How to do a negative pattern?
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Mon, 14 Jul 2008 05:22:09 -0400 (EDT)

On 7/13/08 at 3:47 PM, stevensiew2 at gmail.com (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:

>In[3]:= mylist={ {1,1},{1,2},{1,3},{2,1},{2,2},{2,3} }

>In[5]:= myselectionB = DeleteCasesNot[mylist,{_,3}]

>Out[5]= {{1, 3}, {2, 3}}

>Does anyone has any idea how I could code up a DeleteCasesNot
>function?

You can achieve what you want using DeleteCases with the
appropriate pattern, i.e.,

In[2]:= DeleteCases[mylist, {_, _?(# != 3 &)}]

Out[2]= {{1, 3}, {2, 3}}


But note deleting all cases that do not match a given pattern is
the same as keeping all cases that do match the pattern. So, the
same result can be achieved with

In[3]:= Cases[mylist, {_, 3}]

Out[3]= {{1, 3}, {2, 3}}


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