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Re: issue generating table of random numbers

  • To: mathgroup at smc.vnet.net
  • Subject: [mg81904] Re: issue generating table of random numbers
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Sat, 6 Oct 2007 04:40:22 -0400 (EDT)
  • References: <fe4v33$1kh$1@smc.vnet.net>

John Molina wrote:
> Hi, I'm running Mathematica 6 on OsX (10.4) and have been experiencing difficulties generating a table of random numbers (a table of random matrices actually). 
> matrix = Table[RandomChoice[{-1, 1}, {4, 4}], {250}]
> for a table size less than 250 it seems to work fine but for larger sizes mathematica is flattening out the lists:
> matrix[[i]] = {-1,1,1,-1}
> and not
>             = {{-1,1},{1,-1}}
> As far as I can tell this is only happening with the RandomChoice command (RandomInteger and RandomReal work fine). Does anyone know what is happening? is this a bug?
> thanks
> 

Wow, this looks like a nasty bug to me.  It must be some kind of trouble 
with packed arrays, since Table switches to a packed array 
representation at size 250:

In[1]:= <<Developer`

In[2]:= PackedArrayQ[Table[1, {249}]]
Out[2]= False

In[3]:= PackedArrayQ[Table[1, {250}]]
Out[3]= True

As a workaround you could use RandomChoice[{-1, 1}, {250, 4, 4}]

-- 
Szabolcs


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