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RE: Building List

• To: mathgroup at smc.vnet.net
• Subject: [mg48035] RE: [mg48015] Building List
• From: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>
• Date: Sat, 8 May 2004 01:23:39 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

>-----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: [mg48035] [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
>
>

If all your sublists are of even length, it's easy:

In[1]:= l = {{1, 2}, {3, 4, 5, 6}, {7, 8}};

In[2]:= Partition[Flatten[l, 1], 2]
Out[2]= {{1, 2}, {3, 4}, {5, 6}, {7, 8}}


...else, if you then drop the last elements:

In[3]:= l2 = {{1, 2}, {3, 4, 5, 6, 6.5}, {7, 8}};

In[4]:= Flatten[#, 1] &[Partition[#, 2] & /@ l2]
Out[4]= {{1, 2}, {3, 4}, {5, 6}, {7, 8}}


...or if you want to retain it (as singleton):

In[5]:= Flatten[#, 1] &[Partition[#, 2, 2, {1, 1}, {}] & /@ l2]
Out[5]= {{1, 2}, {3, 4}, {5, 6}, {6.5}, {7, 8}}

...or retain the first element as singleton:

In[21]:=
Flatten[#, 1] &[
    Partition[#, 2, 2, {1 + Mod[Length[#], 2], 1}, {}] & /@ l2]
Out[21]= {{1, 2}, {3}, {4, 5}, {6, 6.5}, {7, 8}}

...??

--
Hartmut Wolf