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MathGroup Archive 2004

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Re: matrix times a vector

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49050] Re: matrix times a vector
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Tue, 29 Jun 2004 04:50:37 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

On 6/28/04 at 4:13 AM, david.salomon at csun.edu (David Salomon) wrote:

>The input {{a,b}, {c,d}}.{x,y} generates the output {a x+b y,c x+d
>y}.

>How can I get the output x{a,b}+y{c,d}  ?

In[1]:=Plus @@ Flatten[MapThread[Times, {{{a, b},{c, d}},{x, y}}]]
Out[1]= a*x + b*x + c*y + d*y

Here I am interpreting your x{a,b}+y{c,d} to be a*x + b*x + c*y + d*y

>This is a special case of lists with two elements. How can I extend
>this computation in an elegant way to lists with N elements?

This will work for any length list. All that is required is the first dimension of your matrix to have the same value as the length of your vector for this particular method to work.
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