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Groups > comp.soft-sys.math.mathematica > #2814

Re: Evaluation control in Compile[]

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Am 30.05.2011 12:33, schrieb Ben:
> I'd like to compile a function that estimates (assuming a spherical
> earth) the length of a degree of longitude at any particular
> latitude.  Here's how I write it up and test its speed:
>
> In[1]:=  radiusOfEarth=6378100.;
> latitudeDegreeLength=2*Pi*radiusOfEarth/360
> Out[2]= 111319.
> In[3]:= X=Table[RandomReal[{0,90}],{100000}];
> In[4]:=
> GetLongitudeDegreeLength=Compile[{latitude},latitudeDegreeLength*Cos[latitude
> Degree]]
> Out[4]= CompiledFunction[{latitude},latitudeDegreeLength Cos[latitude
> =B0],-CompiledCode-]
> In[5]:= Timing[Map[GetLongitudeDegreeLength,X];]
> Out[5]:= {0.312,Null}
>
> Since the values of "latitudeDegreeLength" and "Degree" are constant,
> I'd prefer to use numerical approximations in the definition so the
> compiled code doesn't have to request the values of these global
> variables at runtime.  Like this:
>
> In[6]:= GetLongitudeDegreeLength=Compile[{latitude},
> 111318.*Cos[0.0174532925 latitude]]
> Out[6]:= CompiledFunction[{latitude},111318. Cos[0.0174533 latitude],-
> CompiledCode-]
> In[7]:= Timing[Map[GetLongitudeDegreeLength,X];]
> Out[7]:= {0.031,Null}
>
> Is there any way to tell Mathematica that I'd like these symbols to be
> evaluated before the compilation so that I don't have to copy and
> paste the numerical values into the definition?

there are several, but for this case I think With is probably the clearest:

radiusOfEarth=6378100.;
With[{latitudeDegreeLength=2*Pi*radiusOfEarth/360,deg=N[Degree]},
GetLongitudeDegreeLength=Compile[{latitude},
	latitudeDegreeLength*Cos[latitude*deg]]
]

hth,

albert

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Evaluation control in Compile[] Ben <benp84@gmail.com> - 2011-05-30 10:33 +0000
  Re: Evaluation control in Compile[] Albert Retey <awnl@gmx-topmail.de> - 2011-05-30 11:48 +0000
    Re: Evaluation control in Compile[] Ben <benp84@gmail.com> - 2011-05-31 11:46 +0000
  Re: Evaluation control in Compile[] Szabolcs Horvát <szhorvat@gmail.com> - 2011-05-31 11:49 +0000
  Re: Evaluation control in Compile[] Szabolcs Horvát <szhorvat@gmail.com> - 2011-06-01 08:30 +0000

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