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Re: How to get sub-list elements at certain position in a long list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg123082] Re: How to get sub-list elements at certain position in a long list
  • From: "Oleksandr Rasputinov" <oleksandr_rasputinov at hmamail.com>
  • Date: Wed, 23 Nov 2011 07:03:52 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jafuf7$rs7$1@smc.vnet.net>

On Tue, 22 Nov 2011 10:44:55 -0000, Gy Peng <hitphyopt at gmail.com> wrote:

> Dear Experts,
>
> I have a list:
>
> {{{{2.5}, {3.8, 31}, {54.23, 46.263, 45.987}, *{-82.1476,** 5.17782,
> -205.36}*}, {{2.5}, {4.2, 30}, {233.835, 46.469,
>     46.819}, *{93.8625, -13.9818, -198.778}*}}, {{{2.5}, {4.1,30},
> {139.845, 15.719, 155.87}, *{93.6633,**4.46398, -205.432}*}, {{2.5},  
> {2.6,
> 32}, {113.381, 40.988,85.09}, *{90.1206, 16.9792, -205.314}*}}, {{{2.5},
> {3.1,30}, {113.858, 34.709, 87.567}, *{55.3298,**17.7214, -211.648}*},
> {{2.5}, {2.7, 30}, {110.54, 31.595,85.601}, *{-25.8558, 175.588,  
> -121.105}*
> }}}
>
> How do I get each 4th sublist element in the last level? I mean, I want  
> to
> get a list looks like:
> {*{-82.1476, **5.17782, -205.36}, **{93.8625, -13.9818,  
> -198.778},**{90.1206,
> 16.9792, -205.314},**{55.3298,**17.7214, -211.648},**{-25.8558, 175.588,
> -121.105}}*
> which only includes every 4th element in the last level. Do I describe my
> question clearly?
>
> Better not to use any "For", simple Map or Thread method are preferred.
>
> Thank you very much!!!
> *
> *

I'm not sure where all the superfluous *'s have come from, but these  
render your expression syntactically invalid, so I have removed them.

lst = {{
  {{2.5},{3.8, 31},{54.23, 46.263, 45.987},{-82.1476, 5.17782, -205.36}},
  {{2.5},{4.2, 30},{233.835, 46.469, 46.819},{93.8625, -13.9818,  
-198.778}}},
  {{{2.5},{4.1, 30},{139.845, 15.719, 155.87},{93.6633, 4.46398, -205.432}},
  {{2.5},{2.6, 32},{113.381, 40.988, 85.09},{90.1206, 16.9792, -205.314}}},
  {{{2.5},{3.1, 30},{113.858, 34.709, 87.567},{55.3298, 17.7214, -211.648}},
  {{2.5},{2.7, 30},{110.54, 31.595, 85.601},{-25.8558, 175.588, -121.105}}
}}

You can use Part:

lst[[All, All, 4]]

{{{-82.1476, 5.17782, -205.36}, {93.8625, -13.9818, -198.778}},
  {{93.6633, 4.46398, -205.432}, {90.1206, 16.9792, -205.314}},
  {{55.3298, 17.7214, -211.648}, {-25.8558, 175.588, -121.105}}}

Or, if you insist on a solution with Map,

Map[#[[4]] &, lst, {2}]

{{{-82.1476, 5.17782, -205.36}, {93.8625, -13.9818, -198.778}},
  {{93.6633, 4.46398, -205.432}, {90.1206, 16.9792, -205.314}},
  {{55.3298, 17.7214, -211.648}, {-25.8558, 175.588, -121.105}}}



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