Re: Partitioning lists
- To: mathgroup at smc.vnet.net
- Subject: [mg20535] Re: [mg20439] Partitioning lists
- From: "David Park" <djmp at earthlink.net>
- Date: Sat, 30 Oct 1999 00:13:53 -0400
- Sender: owner-wri-mathgroup at wolfram.com
Mark,
Your method looks pretty good: Here are some alternative methods and timings.
a={1,2,3,4,5,6};
Your method:
TwoPartSplit0[a_] := Table[{Take[a, i], Drop[a, i]}, {i, 1, Length[a] - 1}]
Do[TwoPartSplit0[a], {1000}] // Timing
{0.6 Second, Null}
This is a method using the Partition function;
TwoPartSplit1[a_List] :=
With[{len = Length[a]},
Table[If[i <= len/2,
Partition[a, len - i, len - i, {len + 1 - 2i, len - i}, {}],
Partition[a, i, i, {1, len - i}, {}]], {i, 1, len - 1}]]
Do[TwoPartSplit1[a], {1000}] // Timing
{2.86 Second, Null}
This is a method taking the parts:
TwoPartSplit2[a_List] :=
With[{len = Length[a]},
Table[{a[[Range[1, i]]], a[[Range[1 + i, len]]]}, {i, 1, len - 1}]]
Do[TwoPartSplit2[a], {1000}] // Timing
{1.37 Second, Null}
So Take and Drop seem to be relatively efficient.
David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/
>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
>