Re: Nested list data extraction bottleneck

• To: mathgroup at smc.vnet.net
• Subject: [mg107621] Re: [mg107601] Nested list data extraction bottleneck
• From: Leonid Shifrin <lshifr at gmail.com>
• Date: Fri, 19 Feb 2010 07:34:00 -0500 (EST)
• References: <201002190834.DAA00850@smc.vnet.net>

```Hi,

In[175]:=
LS = {{{0, 533}, {0, 549}, {0, 554}, {0, 547}}, {{0, 533}, {0,
536}, {0, 541}, {0, 533}}};

The following is probably as fast as it gets:

In[176]:= LS[[All, -1]]

Out[176]= {{0, 547}, {0, 533}}

In[177]:= LS[[All, -1, -1]]

Out[177]= {547, 533}

In[183]:= LS[[All, All, 2]]

Out[183]= {{533, 549, 554, 547}, {533, 536, 541, 533}}

If you deal with numbers (reals in particular), you may consider converting
your lists to PackedArray form with Developer`ToPackedArray (if it is not
already in that form) - this may speed up many operations on your data.

Hope this helps.

Regards,
Leonid

On Fri, Feb 19, 2010 at 11:34 AM, Mumtaz <x64x58 at gmail.com> wrote:

> (* BEGIN the beguine simple code *)
>
> ClearAll["Global`*"]
>
> (* prototype data list *)
> LS = {{{0,533},{0,549},{0,554},{0,547}},{{0,533},{0,536},{0,541},
> {0,533}}};
>
> (* list LS dimensions *)
> LSdim = Dimensions[LS]
> {2,4,2}
>
> (* list LS terminal values  -- major bottle neck for higher dimensions
> *)
> LSterminal = Table[Last[LS[[i]]], {i, LSdim[[3]]}]
> {{0,547},{0,533}}
>
> (* extract relevant list LS terminal values data *)
> LSterminal[[All,2]]
> {547,533}
>
> (* OK *)
> (* END simple code *)
>
>
> (* No doubt, there is a better way to extract terminal values from
> list LS since the data has already been generated; Table function for
> generating LSterminal should not be necessary *)
>
> (* Any ideas appreciated *)
>
> (* Further,  any ideas on extracting the following type stream from
> list LS would also be appreciated: *)
> (* SomeFunction[LS] operating on LS above will output: *)
> (* {{533, 549, 554, 547}, {533, 536, 541, 533}} *)
>
>
>
> (* Thanks in advance and best regards, M *)
>
> (* PS Please note the 0 element in list LS is a result of rounding for
> legibility. Parsing the list LS on that basis would not be optimal.
> All numbers are Real *)
>
>

```

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