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MathGroup Archive 2006

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Re: Multiplying Elements of arrays: Inner product

  • To: mathgroup at smc.vnet.net
  • Subject: [mg68523] Re: [mg68497] Multiplying Elements of arrays: Inner product
  • From: "Adriano Pascoletti" <pascolet at dimi.uniud.it>
  • Date: Wed, 9 Aug 2006 04:18:20 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Gopinath Venkatesan wrote ..
> I have to dot multiply cd (3x3 matrix) by ab (3x1 column vector). Also
> I defined values for the first row and the diagonal elements of cd matrix,
> as well as defined the column vector ab with values. Now ab = {1, 2, 3},
> and cd = {{1, 2, 3}, {b[2,1], 1, b[2,3]}, {b[3,1], b[3,2], 1}}.
> 
> I expect the ef to be {14, 2+b[2,1]+3b[2,3], 3+b[3,1]+2b[3,2]} column vector.
> But it returns the first element in unevaluated form as 1.1+2.2+3.3. Why
> is this so? How to make it evaluate them? Thanks for the suggestions. Please
> see below for my example.
> 
> ab = Array[a, 3];
> cd = Array[b, {3, 3}];
> ef = Array[c, 3];
> Do[cd[[i, i]] = 1, {i, 1, 3}];
> cd[[1, 2]] = 2;
> cd[[1, 3]] = 3;
> Do[ab[[i]] = i, {i, 1, 3}];
> Do[ef[[i]] = 0, {i, 1, 3}];
> Do[ef[[i]] = ef[[i]] + cd[[i, k]].ab[[k]], {i, 1, 3}, {k, 1, 3}];
> 
> Gopinath
> 
The dot . means dot product, not product: replace the dot in cd[[i, k]].ab[[k]] with * (or nothing) to get the desired result.

Adriano Pascoletti 


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