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MathGroup Archive 1999

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Re: Partitioning lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg20509] Re: Partitioning lists
  • From: Hans Staugaard <hans.staugaard at get2net.dk>
  • Date: Wed, 27 Oct 1999 02:05:05 -0400
  • Organization: get2net Internet Kunde
  • References: <7v3dfq$63d@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

DIAMOND Mark wrote:
> 
> I would like a function f to split a list as follows
> 
> a={1,2,3,4,5,6}
> f[a] =
>   {
>     {{1},{2,3,4,5,6}},
>     {{1,2},{3,4,5,6}},
>     {{1,2,3},{4,5,6}},
>     {{1,2,3,4},{5,6}},
>     {{1,2,3,4,5},{6}}
>   }
> 
> I can do this as
> 
>       Table[{Take[a, i], Drop[a, i]}, {i, 1, Length[a] - 1}]
> 
> but it *looks* as though it is very inefficient. Is it?
> It also looks as though there should be a way of using Partition simply to
> chop a list between positions i and i+1, but
> I cannot see how to do it. Any suggestions?
> 
> mark diamond
> no spam email: markd at psy dot uwa dot edu dot au

Here's a suggestion

	In[1]:=
		t = Range[6]

	Out[1]=
		{1, 2, 3, 4, 5, 6}

	In[2]:=
		ReplaceList[t, {a__, b__} -> {{a}, {b}}]

	Out[2]=
		{{{1}, {2, 3, 4, 5, 6}}, {{1, 2}, {3, 4, 5, 6}}, {{1, 2, 3}, {4, 5,
6}},
		{{1, 2, 3, 4}, {5, 6}}, {{1, 2, 3, 4, 5}, {6}}}


I imagine it is not very efficient for large list.


Hans


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