Services & ResourcesWolfram ForumsMathGroup Archive

[Date Index] [Thread Index] [Author Index]

Re: extracting elements from an expression

• To: mathgroup at smc.vnet.net
• Subject: [mg27675] Re: [mg27663] extracting elements from an expression
• From: Jean-Marie THOMAS <jmt at agat.net>
• Date: Sun, 11 Mar 2001 04:04:19 -0500 (EST)
• References: <200103100549.AAA15809@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Union@Flatten@m
does the work, unless I misunderstood the problem.


On Saturday 10 March 2001 06:49, Albert Maydeu-Olivares wrote:
Dear all,

I've spent several frustrating hours attempting to do something in principle
very simple.
After repeated failures I'm resorting to the group for help.

Consider this simple expression

m = {{{{1, 2}, {2, 3}}}, {{{4, 5}, {8, 9}}, {{6, 7}, {7, 10}}},
{{{11, 12}, {17, 18}}, {{13, 14}, {19, 20}}, {{15, 16}, {16, 21}}}};
m // TableForm

I'm simply trying to obtain to obtain this list

{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21}

using the positions' elements in m. This will do it the hard way

Join[m[[1, 1, 1]],
  Drop[m[[1, 1, 2]], 1],
  m[[2, 1, 1]],
  m[[2, 2, 1]],
  m[[2, 1, 2]],
  Drop[m[[2, 2, 2]], 1],
  m[[3, 1, 1]],
  m[[3, 2, 1]],
  m[[3, 3, 1]],
  m[[3, 1, 2]],
  m[[3, 2, 2]],
  Drop[m[[3, 3, 2]], 1]]

I've failed to do it the easy way. Here's one of my failed attemps

Table[If[i == j && k == 2, Drop[m[[i, j, k]], 1], m[[i, j, k]]],
{i, nvars}, {j, i}, {k, 2}] // Flatten

which fails because I don't know how to control the k subscript
appropriately
as you can see by doing

Flatten[Table[{i, j, k}, {i, nvars}, {j, i}, {k, 2}],2]

Any help would be very much appreciated.

Albert

• References:
  • extracting elements from an expression
    • From: "Albert Maydeu-Olivares" <amaydeu@tinet.fut.es>