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MathGroup Archive 2004

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RE: Alternative to defining 'operator' function?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg47559] RE: [mg47505] Alternative to defining 'operator' function?
  • From: "David Park" <djmp at earthlink.net>
  • Date: Fri, 16 Apr 2004 05:20:24 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hywel,

What about...

Rest@FoldList[Dot[#2, #1] &, v, {R1, R2, R3, R4}]
{R1.v, R2.R1.v, R3.R2.R1.v, R4.R3.R2.R1.v}

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/ 

From: Owen, HL (Hywel) [mailto:H.L.Owen at dl.ac.uk]
To: mathgroup at smc.vnet.net

Hi folks,

I often have programming problem where I'd like to calculate a set of dot
products, e.g. applying a list of square matrices {R1,R2,R3...} to a vector
v to obtain:

{R1.v,R2.R1.v,R3.R2.R1.v,...}

or other functions like that.

The method I've been using is to define an 'operator' function, e.g.

DotOperator[M_] := Dot[M, #] &

Then we have:

In: DotOperator[R][v]
Out: R.v

as wanted, so that we can define a ComposeList as

In: Rest[ComposeList[DotOperator[#] & /@ {R1, R2, R3}, v]]
Out: {R1.v, R2.R1.v, R3.R2.R1.v}

to obtain the result we want.

Is there a simpler way than this that doesn't involve defining functions
like DotOperator?

Thanks,

Hywel




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