Re: Can anyone see a faster way to compute quantities for a pair or
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- Subject: [mg127425] Re: Can anyone see a faster way to compute quantities for a pair or
- From: daniel.lichtblau0 at gmail.com
- Date: Mon, 23 Jul 2012 19:56:52 -0400 (EDT)
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On Monday, July 23, 2012 12:04:19 AM UTC-5, W Craig Carter 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
Could use Compile to get a speed improvement. below shows one way to go about this.
In[445]:= myArcTanC2 = Compile[{{xy, _Real, 1}},
With[{x = xy[[1]], y = xy[[2]]},
If[x == 0. && y == 0., 0., ArcTan[x, y]]]];
In[452]:= psiFieldFinder = Compile[{{gfield, _Real, 3}},
Map[myArcTanC2, Transpose[gfield, {3, 1, 2}], {2}],
CompilationOptions -> {"ExpressionOptimization" -> True,
"InlineCompiledFunctions" -> True},
CompilationTarget -> "C"];
In[453]:= Do[psiField2 = psiFieldFinder[gradfield], {100}] // Timing
Out[453]= {0.85, Null}
If you intend to do this on the same array then pre-transposing, and removing that step from the compiled function, might give a further slight improvement.
For norms you might get better speed using Sqrt[arg1.arg2].
Daniel Lichtblau
Wolfram Research