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MathGroup Archive 2004

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Re: Extracting Matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg45410] Re: Extracting Matrix
  • From: drbob at bigfoot.com (Bobby R. Treat)
  • Date: Tue, 6 Jan 2004 04:17:24 -0500 (EST)
  • References: <btb8oi$jp4$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

This is related to an ongoing thread on computing Transpose (and other
functions) of MatrixForm objects. There should be a better answer, but
here's one in the meantime:

data = Array[{# + Random[], # - Random[]} &, {100}];
z = Regress[data, {1, x, x^2}, x, RegressionReport ->
CovarianceMatrix]
X = CovarianceMatrix /. (z /. MatrixForm -> Identity)
X[[2, 2]]

It's strange that THIS doesn't work:

X = CovarianceMatrix /. z /. MatrixForm -> Identity

Bobby

"Bruce W. Colletti" <bcolletti at compuserve.com> wrote in message news:<btb8oi$jp4$1 at smc.vnet.net>...
> Re Mathematica 5.0 running under WinXP Home.
> 
> Am using...
> 
>     z = Regress[...RegressionReport->CovarianceMatrix];
>     X = CovarianceMatrix /. z;
> 
> ...to extract the given matrix. 
> 
> However, when I use X[[k,k]] to access the k'th diagonal element, I get 
> a row instead!  The culprit is that MatrixForm is the X-head (as 
> FullForm reveals).
> 
> How can I strip away this head and make X a list of lists?
> 
> Thankx.
> 
> Bruce


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