       • To: mathgroup at smc.vnet.net
• Subject: [mg90749] Re: Threading over matrices
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Wed, 23 Jul 2008 05:55:50 -0400 (EDT)
• Organization: Uni Leipzig
• References: <g64487\$djo\$1@smc.vnet.net>

```Hi,

x = Table[Random[], {3}, {4}];
y = Table[Random[], {3}, {4}];
a = 0.5;

a) If[x > y, 1/x, x - y]

with

MapThread[If[#1 > #2, 1/#1, #1 - #2] &, {x, y}, 2]

Piecewise[{{1, #1 == a}, {#1^2, #1 > a}}, #1 #2^2] &, {x, y}, 2]

Regards
Jens

"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 or
> more usefully how to do what I would like them to do.
>
> I have searched Help on all the likely commands (I think: Map,
> Thread, Apply, Distribute, ...) and this archive, where there
> are similar enquiries but none that match.  Perhaps I'm looking
> in the wrong place - I expect there's someone who can help.
>
> Robert
>
>

```

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