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MathGroup Archive 2001

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RE: Appending to Lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27061] RE: [mg27045] Appending to Lists
  • From: "David Park" <djmp at earthlink.net>
  • Date: Sat, 3 Feb 2001 04:58:56 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

James,

generateAList[blist_, clist_] :=
 Module[{temp, maxlength},
   temp = Sort[Transpose[{clist, blist}]];
    temp = Split[temp, #1[[1]] == #2[[1]] & ];
    temp = Map[#1[[2]] & , temp, {2}];
    maxlength = Max[Length /@ temp];
    (PadRight[#1, maxlength, 0.] & ) /@ temp]

blist = {0.2, 0.6, 1.2, -0.2, 0.5, 0.3, 0.7, -0.2, -0.6};
clist = {1, 2, 3, 1, 2, 1, 2, 1, 1};

generateAList[blist, clist]
{{-0.6, -0.2, -0.2, 0.2, 0.3}, {0.5, 0.6, 0.7, 0., 0.}, {1.2, 0., 0., 0.,
0.}}

You seem to have dropped the last case for the first vector, or else there
is something about your algorithm that you have not explained.

Here is a longer test case with timing. I divided a 50,000 length list into
200 "vectors".

blisttest = Table[Random[], {50000}];
clisttest = Table[Random[Integer, {1, 200}], {50000}];

(alisttest = generateAList[blisttest, clisttest];) // Timing
{1.54 Second, Null}

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/



> From: James Jones [mailto:j.k.jones at dl.ac.uk]
To: mathgroup at smc.vnet.net
>
> Hi,
>
> 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 ;->
>
> Again, any help much appreciated,
>
> James Jones
> Daresbury Laboratory
>
>
>



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