Services & ResourcesWolfram ForumsMathGroup Archive

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

extracting elements from an expression

• To: mathgroup at smc.vnet.net
• Subject: [mg27663] extracting elements from an expression
• From: "Albert Maydeu-Olivares" <amaydeu at tinet.fut.es>
• Date: Sat, 10 Mar 2001 00:49:52 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

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

• Follow-Ups:
  • Re: extracting elements from an expression
    • From: Jean-Marie THOMAS <jmt@agat.net>
  • Re: extracting elements from an expression
    • From: Ken Levasseur <Kenneth_Levasseur@uml.edu>