Re: Appending to Lists

• To: mathgroup at smc.vnet.net
• Subject: [mg27058] Re: Appending to Lists
• From: "Allan Hayes" <hay at haystack.demon.co.uk>
• Date: Sat, 3 Feb 2001 04:58:54 -0500 (EST)
• References: <95b686\$4mr@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```James,

b = Table[Random[], {10000}];
c = Table[Random[Integer, {1, 100}], {10000}];

bin1[b_, c_] :=
Block[{a = {}},
Do[AppendTo[a, Flatten[Part[b, #] & /@ Position[c, i]]], {i, 1,
Max[c]}];
a
]

(r1 = bin1[b, c]); // Timing

{59.26 Second, Null}

Two alternatives:

This give the same ressult as bin1:

bin2[b_, c_] :=
Part[b, #] & /@ ((Join @@ Position[c, #]) & /@ Range[1, Max[c]])

(r2 = bin2[b, c]); // Timing

{5.05 Second, Null}

This one sorts the bins:

bin3[b_, c_] :=
Split[Sort[Transpose[{c, b}]], #1[[1]] === #2[[1]] &][[All, All, -1]]

(r3 = bin3[b, c]); // Timing

{2.58 Second, Null}

Test

Sort /@ r1 === Sort /@ r2 === r3

True

The padding with zeros is quite quick for this example(few bins)

With[{n = Max[Length /@ mat]}, PadRight[#, n, 0.] & /@ mat];

{0.05 Second, Null}

Spot check

Short[Take[pr3, {2}], 3]

{{0.0126083, 0.0146036, 0.0247964,
0.038953, \[LeftSkeleton]121\[RightSkeleton], 0., 0., 0., 0.}}

--
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

"James Jones" <j.k.jones at dl.ac.uk> wrote in message
news:95b686\$4mr 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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