Re: Lists and rules
- To: mathgroup at smc.vnet.net
- Subject: [mg66508] Re: Lists and rules
- From: "Ray Koopman" <koopman at sfu.ca>
- Date: Wed, 17 May 2006 03:30:04 -0400 (EDT)
- References: <e4bj1m$1b9$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
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
Is this what you want (but with 100 instead of 10)?
In[1]:= t = Table[Random[Integer,{1,3}],{10}]
Out[1]= {2,1,1,2,1,3,3,2,1,2}
In[2]:= s = Table[#==j&/@t,{j,3}]
Out[2]= {{False,True,True,False,True,False,False,False,True,False},
{True,False,False,True,False,False,False,True,False,True},
{False,False,False,False,False,True,True,False,False,False}}
You could also do this:
In[3]:= sa = SparseArray[Thread[Flatten[Table[{j,#}&/@
Flatten@Position[t,j],{j,3}],1]->True],{3,10},False]
Out[3]= SparseArray[<10>,{3,10},False]
In[4]:= sa == s
Out[4]= True