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Re: Lists and rules


Thomas Schmelzer wrote:
> Experts,
> I would like to produce a decent piece of code for one of my projects.
> I am constructing a list of sets
> 
> S_1, S_2 and S_3
> 
> where S_j = { m  ,   T[[1,m]] == j, m running from 1 to 100}
> 
> T[[1,*]] is a row vector containing only 1, 2 and 3.
> 
> I could setup a for loop and work with append. I have done that, but I 
> believe this approach is neither efficient nor elegant. Any hints for a 
> newbie?
> 
> Best,
> Thomas
> 
> 
> 
Hi Thomas,

You could use *Table* [1]. For instance

Subscript[S, 2] = Table[{m, T[[1,m]] == 2}, {m, 100}]

generates S_2 as an 100 by 2 matrix where the first column ranges from 1 
to 100 and the second contains the boolean values True or False 
depending on whether the value of T[[1,m]] is equal to 2.

HTH,
Jean-Marc

[1] http://documents.wolfram.com/mathematica/functions/Table


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