Re: Partitioning lists

*To*: mathgroup at smc.vnet.net*Subject*: [mg20482] Re: Partitioning lists*From*: "Allan Hayes" <hay at haystack.demon.co.uk>*Date*: Wed, 27 Oct 1999 02:04:48 -0400*References*: <7v3dfq$63d@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

DIAMOND Mark <noname at noname.com> wrote in message news:7v3dfq$63d at smc.vnet.net... > 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 > We could use Table[Split[a, #1 != i &], {i, Length[a] - 1}] // Timing But this is much slower because of all the testing needed. -- Allan --------------------- Allan Hayes Mathematica Training and Consulting Leicester UK www.haystack.demon.co.uk hay at haystack.demon.co.uk Voice: +44 (0)116 271 4198 Fax: +44 (0)870 164 0565