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Re: extracting elements from an expression

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27695] Re: extracting elements from an expression
  • From: "Drago Ganic" <drago.ganic at in2.hr>
  • Date: Sun, 11 Mar 2001 04:04:38 -0500 (EST)
  • References: <98cgas$fod@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Albert,
This is the solotion - if I haven not overseen something -

Union@Flatten@m

Greetings from Croatia,
Drago Ganic


"Albert Maydeu-Olivares" <amaydeu at tinet.fut.es> wrote in message news:98cgas$fod at smc.vnet.net...
> 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
> 
> 
> 



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