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 >