RE: [boost] RE: math constant - generic function circle_area example.

From: Paul A. Bristow (boost_at_[hidden])
Date: 2003-01-24 07:39:21

Please send me your amended code.  I had thought that the Kenniston stuff was
included in the zip uploaded, but I will check. Apologies if not.

Thanks

Paul
> -----Original Message-----
> From: boost-bounces_at_[hidden]
> [mailto:boost-bounces_at_[hidden]]On Behalf Of Joerg Walter
> Sent: Friday, January 24, 2003 12:27 AM
> To: Boost mailing list
> Subject: Re: [boost] RE: math constant - generic function circle_area
> example.
>
>
> Hi Paul,
>
> you wrote:
>
> > > I've been looking into an earlier version of the proposed math constants
> > > before and asked myself how to implement a generic function like
> > >
> > > template<class T>
> > > T circle_area (const T &radius) {
> > >     return math_constants<T>::pi * radius * radius;
> > > }
> > >
> > > How should this be done?
> > >
> > > Thanks,
> > >
> > > Joerg
> >
> > Attached is an example using Michael Kenniston's Kunning Function
> constants.
> >
> > Briefly
> >
> > template <typename T>
> > T circle_area(const T& radius)
> > {
> >   // Usage example: circle_area<double>( 2.) // Explicit type double.
> >   // or             circle_area(2.F) // Implicit type float.
> > return boost::math::constant< T, boost::math::pi_tag >() * radius *
> radius;
> > }
>
> OK. I've tried to compile this with GCC 3.2.1 and after applying some
> remedies like
>
> <for example>
> #ifdef LATER
>   // Define constant is namespaces to hold three builtin floating-point
> representations.
>   namespace float_constants
>   {
>    constant< float, pi_tag > const pi;
>   }
>   namespace double_constants
>   {
>    constant< double, pi_tag > const pi;
>   }
>   namespace long_double_constants
>   {
>    constant< long double, pi_tag > const pi;
>   }
> #endif
> </for example>
>
> it compiled and executed giving the expected results. How is this solution
> related to your latest (Dec. 12) upload at groups.yahoo.com/group/boost?
>
> > It compiles with MSVC 7.0 (but not 6 - see MK's original example for why
> not).
> > (long double == double for MSVC, so long double not fully
> testable/useful).
> >
> > It seems to do the trick, without too many surprises.  I have displayed
> the
> > output using the 17 significant digits for double so one can see the
> difference
> > between a float pi 3.1415927410125732 and a double pi 3.1415926535897931
> (even
> > though only 9 are really significant for float).
> >
> > cout << "circle_area<float>(1.) = "  << circle_area<float>(1.) << endl; //
> > Explicit type float.
> > cout << "circle_area<double>(1.) = "  << circle_area<double>(1.) << endl;
> //
> > Explicit type double.
> > cout << "circle_area(1.F) = "  << circle_area(1.F) << endl; // Implicit
> type
> > float.
> > cout << "circle_area(1.) = "  << circle_area(1.) << endl; // Implicit type
> > double.
> > cout << "circle_area(1.L) = "  << circle_area(1.L) << endl; // implicit
> type
> > long double.
> > cout << "circle_area<double>(1.F) = "  << circle_area<double>(1.F) <<
> endl; //
> > Explicit over-rides implicit.
> >
> > And silly types like int fail helpfully.
> >   // cout << "circle_area(1) = "  << circle_area(1)<< endl; // Implicit
> int -
> > does not link!
> >   // cout << "circle_area<int>(1.) = "  << circle_area<int>(1.)<< endl; //
> > Explicit int - does not compile!
> >
> > Output is
> >
> > Test test_circle_area.cpp Thu Jan 23 00:06:28 2003
> > float pi  = 3.14159274
> > double pi = 3.1415926535897931
> > float pi  = 3.1415927410125732
> > circle_area<float>(1.) = 3.1415927410125732
> > circle_area<double>(1.) = 3.1415926535897931
> > circle_area(1.F) = 3.1415927410125732
> > circle_area(1.) = 3.1415926535897931
> > circle_area(1.L) = 3.1415926535897931
> > circle_area<double>(1.F) = 3.1415926535897931
> > circle_area<long double>(1.) = 3.1415926535897931
> > circle_area<float>(2.) = 12.566370964050293
> > circle_area<double>(2.) = 12.566370614359172
> > circle_area(2.F) = 12.566370964050293
> > circle_area(2.) = 12.566370614359172
> > circle_area(2.L) = 12.566370614359172
> > circle_area<double>(2.F) = 12.566370614359172
> > boost::math::constant< float, boost::math::pi_tag >() 3.1415927410125732
> >
> > I haven't looked at any assembler to check on efficiency, but believe/hope
> from
> > previous examples that it will be optimal if inlined.
> >
> > Does this look sensible/useful?
>
> Only if I learn to see the relation to your latest upload (which I'm
> assuming your review request is related to).
>
> Best,
>
> Joerg
>
>
>
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