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MathGroup Archive 2001

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Re: Appending to Lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27057] Re: [mg27045] Appending to Lists
  • From: BobHanlon at aol.com
  • Date: Sat, 3 Feb 2001 04:58:53 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

b = {0.2, 0.6, 1.2, -0.2, 0.5, 0.3, 0.7, -0.2, -0.6};

c = {1, 2, 3, 1, 2, 1, 2, 1, 1};

a = (Split[Ordering[c], c[[#1]] == c[[#2]]&] /. 
      n_Integer :> b[[n]])

{{0.2, 0.3, -0.2, -0.2, -0.6}, {0.6, 0.5, 0.7}, {1.2}}

PadRight[#, Max[Length /@ a]]& /@ a

{{0.2, 0.3, -0.2, -0.2, -0.6}, {0.6, 0.5, 0.7, 0, 0}, 
  {1.2, 0, 0, 0, 0}}

Bob Hanlon

In a message dated 2001/2/1 3:41:34 AM, j.k.jones at dl.ac.uk writes:

>I have a function that creates a list (a) from another list (b). The list
>elements are re-grouped in the new list according to a third list (c).
>A
>Position command is applied to list (c) for an element, then with this
>output the list (a) is created from list (b) at positions given by the
>element position data, list (c). This is repeated for the large number
>of
>elements in the original lists.
>The Position command is necessary as different elements appear in the list
>a
>different number of times.
>However, with the large number of elements in the lists (approx 50,000
>for a
>simple list), this method is _very_ slow.
>If any one can give me help in speeding this process up I would be very
>grateful.
>
>The data sets would look like this
>
>      b                       c
>
>    0.2                      1
>    0.6                      2
>    1.2                      3
>    -0.2                     1
>    0.5                       2
>    0.3                       1
>    0.7                       2
>   -0.2                      1
>   -0.6                      1
>
>A List would then be created from this data ( the list (a) ) containing
>vectors for 1, 2 and 3. The data in (b) is not important, and the order
>in
>which elements in (c) drop out is not set.
>In this case the (a) list should look like
>
>a = { { 0.2, -0.2, -0.2, -0.6} , {0.6, 0.5, 0.7} , { 1.2 } }
>
>My current function looks like this
>
>Do[AppendTo[xfinal,
>      Flatten[Part[X, #] & /@
>          Position[Global`PARTICLE, i]]], {i, 1,
>      Max[PARTICLE]}];
>
>where xfinal is an (a) list, i.e. to be created.
>          X is the (b) list , i.e. to be addressed, and
>          PARTICLE is the (c) list. It is referenced by number.
>
>and it is very slow!
>
>Also, after producing this list, the different vector elements need to
>be
>made the same length, and so 0.0 are added to the ends of all vector
>elements shorter than the longest. My current function for doing this looks
>like
>
>table = Table[0.0, {Length[First[long]]}]; Print["Table Created!"];
>
>Do[If[Length[Part[xfinal, i]] < Length[First[long]],
>      AppendTo[Part[xfinal, i],
>        Drop[table, (Length[Part[xfinal, i]])] ]], {i, 2,
>      Length[xfinal]}];
>
>where list (long) just sorts the list elements according to length.
>
>This function is also very slow, and I was wondering, again, if anyone
>knew
>a faster way of implementing this. Is the production of a table, once,
>and
>then dropping bits off and appending the fastest method? Of course this
>needs to be done tens of thousands of times per set of data so any small
>speed increase would be very helpful ;->
>


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