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MathGroup Archive 2004

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RE: Building List

  • To: mathgroup at smc.vnet.net
  • Subject: [mg48069] RE: [mg48015] Building List
  • From: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>
  • Date: Tue, 11 May 2004 05:19:52 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com
  • Thread-topic: [mg48015] Building List

>-----Original Message-----
>From: Bruce W. Colletti [mailto:bcolletti at compuserve.com]
To: mathgroup at smc.vnet.net
>Sent: Friday, May 07, 2004 10:30 AM
>To: mathgroup at smc.vnet.net
>Subject: [mg48069] [mg48015] Building List
>
>
>Re Mathematica 5.0.1.0.
>
>I have a list of lists -- e.g., L = { {1,2},{3,4,5,6},{7,8} } -- and 
>want to replace element-lists (whose length exceeds 2) with 
>another list 
>built from that element.  All else is untouched.
>
>For instance, using L above, replace {3,4,5,6} with {3,4}, 
>{3,5}, {3,6}. 
> This transforms L to the desired form { {1,2}, {3,4}, {3,5}, {3,6}, 
>{7,8} }.
>
>Although I can do this using Sow and Reap, am hoping there's an easier 
>way using rules (/.).  In general, I want to replace those 
>L-elements x 
>(that meet a criterion) with foo[x]. 
>
>Thanks.
>
>Bruce
>
>


I have to apologize for my nonsensious first reply (to a question you didn't pose). So here something better:

In[2]:=
Thread[{First[#], Rest[#]}] & /@ {{1, 2}, {3, 4, 5, 6}, {7, 8}}
Out[2]= {{{1, 2}}, {{3, 4}, {3, 5}, {3, 6}}, {{7, 8}}}

In[3]:=
Replace[{{1, 2}, {3, 4, 5, 6}, {7, 8}}, {a_, b__} :> Thread[{a, {b}}], {1}]
Out[3]= {{{1, 2}}, {{3, 4}, {3, 5}, {3, 6}}, {{7, 8}}}


--
Hartmut Wolf


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