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MathGroup Archive 2001

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RE: Nested Lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg28392] RE: [mg28373] Nested Lists
  • From: "tgarza01 at prodigy.net.mx" <tgarza01 at prodigy.net.mx>
  • Date: Sun, 15 Apr 2001 00:13:44 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

This is a good exercise on list manipulation. Suppose your data  in an Excel file (call it ptest.txt, for example) is as follows:

{{1},{222,345,256},{121,321,43},{234,111,221},{684,322,345},{2},{324,356,
    246},{322,543,553},{123,324,232},{78,543,456},{3},{432,567,345},{543,654,
    789},{222,432,876},{567,890,321},{4},{321,325,654},{234,432,321},{567,332,
    357},{345,543,222}}

i.e. a time-stamp and a set of (in this example) 4 3D points. You read them in with

In[1]:=
a=ReadList["ptest.txt",
    Number, RecordLists->True];

This will take the fourth reading in each time-stamp:

In[2]:=
object4=#[[4]]&/@
    Partition[Partition[Flatten[Rest[#]&/@Partition[Flatten[a],13]],3],4]
Out[60]=
{{684,322,345},{78,543,456},{567,890,321},{345,543,222}}

Tomas Garza
Mexico City


Original Message:
-----------------
From:  rlove at neosoft.com (Robert Love)
To: mathgroup at smc.vnet.net
Subject: [mg28392] [mg28373] Nested Lists


I'm confused about nested lists and how to select/extract data from them.

I have a file of data I can use ReadList to read.  The data is laid out
like this:

t1
x y z
x y z
x y z
x y z
t2
x y z
x y z
x y z
x y z
t3 
etc.

The data consists of a timestamp on a line and then the 3D position of 
each of the N objects for that time.  N is always the same number
so I read it it like this:

ReadList["file",{Number, Table[Number, Number, Number,{N}]}]

I'm doing this from memory since the computer w/Mathematica is at work.
So I may not have every  detail of the ReadList correct but what I end up
with is

{{t1,{{x,y,z},{x,y,z},{x,y,z},{x,y,z}},
 {t2,{{x,y,z}...


My question is now  do I extract this  suitable for graphing, that is, how
do I turn this into something like a set of all the object #4 positions
along with the time?  I'm sure there is some combination of Flatten, Take,
Transpose and Map that  will do what I want.  What is it?  Also, is there
a better way to use  ReadList?  I'm stuck with the file format but I can
change how I read the data.

All advice appreciated. 

Bob Love

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