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MathGroup Archive 2004

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Re: [Outer [Times, , ] ] Question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg45824] Re: [Outer [Times, , ] ] Question
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Mon, 26 Jan 2004 01:53:33 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

On 1/25/04 at 3:04 AM, MichaelSMcCain at cs.com wrote:

> I want to take a Column Vector and multiply it by a Row Vector to create an 
> "Outer" Matrix. 
 
> In other words, I want to take a 5x1 Column Vector and multiply it by a 1x4 
> Row Vector and output a 5x4 Matrix.
 
> I then want to add several of these Matrices together and obtain a "final" 
> Matrix.

> I tried the following:

> x1 = {1, -1, -1, 1, 1};
> y1 = {1, 1, -1, -1};

<snip>

> m1 = MatrixForm[Outer [Times, x1, y1] ]

The problem is your usage of MatrixForm here. MatrixForm adds a wrapper to the matrix. But that means Mathematica no longer sees m1 as a matrix. That is

MatrixQ[m1]

False

if you get rid of the MatrixForm all will work as you want. If you want to display the matrix as a traditional matrix you can either set the default format type as Traditional Form or do the following

MatrixForm[m1=Outer[Times, x1, y1]]

As an aside you don't need Outer to perform this task. All you need is

Transpose at {x1}.{y1}

That is define the two things to be multiplied as matrices and use the built in matrix multiplication operator ".".
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