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MathGroup Archive 2001

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Re: Replacement Rule

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32072] Re: Replacement Rule
  • From: Erk Jensen <Erk.Jensen at cern.ch>
  • Date: Sat, 22 Dec 2001 04:22:44 -0500 (EST)
  • References: <9vuuha$onq$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Brian Higgins wrote:
> 
> I have the following list
> 
> l1={{1,2},{3,4},{5,6}};
> 
> and then I operate on it with the following pattern/replacement rule
> 
> In[3]:= l1 /. {x_, y_} -> x
> 
> Out[3]= {1, 3, 5}
> 
> This is what I would expect. Now consider the following list
> 
> In[4]:= l2 = {{3, 4}, {5, 6}};
> 
> Then I use the same pattern/replacement rule
> 
> In[5]:=l2 /. {x_, y_} -> x
> 
> Out[5]= {3, 4}
> 
> I was hoping to get
> 
> {3,5}
> 
> What am I missing? The FullForm of l2 is basically the same structure
> as l1, as far as I can tell....
> 
> Brian

Replace[l2, rule, {1}]
will give the result you expected. More details in the help on "Replace", along
with Chapter A 3.6 of the Book for the levels. Be aware that "/." does a
"ReplaceAll". It recognizes the pattern given in the rule already at the
outermost level of l2, which is of course a list of two elements itself
(ignoring that these elements again are lists). I guess this sort of explains
it.

Ciao
   -erk-
-- 
 Dr.-Ing. Erk JENSEN                    mailto:Erk.Jensen at cern.ch
 CERN  PS/RF  L19510                    http://cern.ch/Erk.Jensen
 CH-1211 Geneva 23                      Tel.:     +41 22 76 74298
 Switzerland                            Fax.:     +41 22 76 78510


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