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MathGroup Archive 2004

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Extracting elements of lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg45458] Extracting elements of lists
  • From: sashan <nothing at important.com>
  • Date: Fri, 9 Jan 2004 05:20:26 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Hi

I'm trying to extract a elements from a list and return another list 
with those elements. I've tried doing it using a while loop and 
recursion and haven't been able to do it.

The list I'm working with is a list of lists where each inner list 
contains 4 elements. Example:
{{0, 1, 0, 0}, {0.04, 1, -0.0008, 0}, {0.1, 0.999987, -0.00499995,
     0}, {0.15, 0.999937, -0.0112494, 0}, {0.18, 0.999869, -0.0161983, 
0},....

This is my recursive attempt:

listoftimex[l_] := {{First[l][[1]], First[l][[2]]}, listoftimex[Rest[l]]}

{{0, 1}, {{0.04,
    1}, {{0.1, 0.999987}, {{
     0.15, 0.999937}, {{0.18, 0.999869}, {{0.23, 0.99965}, {{0.26, 
0.999429}, \
{{0.31, 0.998847}, {{0.35, 0.998128}, {{0.39,

It extracts the 1st 2 elements of the list and puts them in a list but 
it nests lists too deep (too many { ) and it errors with 'Recursion 
depth of 256 exceeded'.

This is my attempt using a while loop:
timex[l_] := Module[{i =
     1, ans = {}}, While[i < Length[l], Append[ans, {l[[i,
        1]], l[[i, 2]]}]; i = i + 1]; Return[ans]];

This doesn't work and returns an empty list {}.

I'm going to use python + reportlab now ....



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