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}}