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Re: Can anyone see a faster way to compute quantities for a pair or

• To: mathgroup at smc.vnet.net
• Subject: [mg127430] Re: Can anyone see a faster way to compute quantities for a pair or
• From: Ray Koopman <koopman at sfu.ca>
• Date: Tue, 24 Jul 2012 04:15:38 -0400 (EDT)
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• References: <juim0j$k9a$1@smc.vnet.net>

On Jul 22, 10:04 pm, W Craig Carter <ccar... at MIT.EDU> wrote:
> Hello,
> I am computing the gradient on a grid, then computing the gradient's
> angle, and its magnitude. The computations below are the bottleneck for
> a longer bit of code.  I would be grateful for any insights on how to
> speed these up.
>
> (*
> Let gradfield be the gradient that I have computed and placed in two matrices. Here I will just use random numbers as a proxy:
> *)
>
> (*i.e., df/dx, df/dy*)
> gradfield =  { RandomReal[{-1, 1}, {256, 256}], RandomReal[{-1, 1}, {256, 256}]};
>
> (*my gradients has many zeroes, so I need to handle these*)
>
> SetAttributes[myArcTan, {Listable, NumericFunction}];
> myArcTan[0.0, 0.0] = 0.0;
> myArcTan[x_, y_] := ArcTan[x, y]
>
> (*the angles, this is slow*)
> psiField = MapThread[myArcTan, gradfield, 2];
>
> (*the magnitudes, this is slower*)
> magfield =  MapThread[Norm[{#}] &, gradfield, 2];
>
> (*examples*)
> Do[psiField = MapThread[myArcTan, gradfield, 2], {100}] // Timing
> Do[magfield =  MapThread[Norm[{#}] &, gradfield, 2], {100}] // Timing
>
> W Craig Carter
> Professor of Materials Science, MIT

My previous post should have said

   psifield2 = ArcTan@@{gradfield[[1]] + UnitStep[-magfield2],
                        gradfield[[2]]}

(ArcTan with two arguments is not like atan2. Old habits die hard!)

• Follow-Ups:
  • Re: Can anyone see a faster way to compute quantities for a pair or
    • From: Ray Koopman <koopman@sfu.ca>
  • Re: Can anyone see a faster way to compute quantities for a pair or
    • From: W Craig Carter <ccarter@MIT.EDU>