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Re: Threading over matrices

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90771] Re: [mg90741] Threading over matrices
  • From: Sseziwa Mukasa <mukasa at jeol.com>
  • Date: Wed, 23 Jul 2008 05:59:55 -0400 (EDT)
  • References: <200807220757.DAA13797@smc.vnet.net>

On Jul 22, 2008, at 3:57 AM, "Robert <"@frank-exchange-of- 
views.oucs.ox.ac.uk wrote:

> How can I get evaluations to thread over matrices with
> conditional functions?
> Here's examples that show the behaviour that's really
> frustrating me.
> Create a couple of matrices:
>
> x = Table[Random[],{3},{4}];
> y = Table[Random[],{3},{4}];
> a=0.5;
>
> (These are example values I would like the following
> to apply to lists of any dimension.)
> When you add them they create a result with the same
> dimensions where each element corresponds to the
> input elements
>
> x + a y
>
> And some functions do similar
>
> Cos[x] + Sin[a y]
>
> But some don't, e.g.
>
> x > y
> x > a
>
> I would have liked those to produce a matrix of corresponding
> True and False results, and then something like:
>
> If[x > y, 1/x, x - y]
> Piecewise[{{1,x==a},{x^2,x>a}},x y^2]
>
> to produce a matrix of results corresponding to each element.
>
> They don't - I haven't managed to find out why they don't

Because they don't have the attribute Flat.

> or
> more usefully how to do what I would like them to do.

There are a variety of ways to do this, and I'm sure you'll see a lot  
of varying responses, personally I use MapThread in such situations:

MapThread[>,{x,y},2]

or

MapThread[>,{x,a},2]

Regards,

Ssezi


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