 # Boost :

From: Paul A Bristow (boost_at_[hidden])
Date: 2003-06-20 10:14:52

It may helpful to those unfamiliar to the Boost Interval library
to see some exactly representable values of pi
(from test_pi_interval.cpp)

// Float 24 bit significand, 32 bit float ////////////////
// static const float pi_f_l = 13176794.0f/(1 << 22);
// static const float pi_f_u = 13176795.0f/ (1 << 22);

// Exactly representable values calculated using NTL.
static const float pi_f_l = 3.141592502593994140625F;
static const float pi_f_u = 3.1415927410125732421875F;

cout << "pi_f_l = " << pi_f_l << endl; // pi_f_l = 3.1415925
cout << "pi_f_u = " << pi_f_u << endl; // pi_f_u = 3.14159274

// double 53-bit significand, 64 bit double //////////////
cout.precision(17); // significant digits10
// static const double pi_d_u = 3537118876014221.0f/(1 << 51);
// compiler chokes :-( - divide by zero! so need a cunning
trick:
| static const double pi_d_l = (3373259426.0 + 273688.0 / (1<<21)) /
(1<<30);
// Or the NTL calculated exact representation:
static const double pi_d_l =
3.141592653589793115997963468544185161590576171875;
static const double pi_d_u =
3.141592653589793560087173318606801331043243408203125;
cout << "pi_d_l = " << pi_d_l << endl; // pi_d_l =
3.1415926535897931
cout << "pi_d_u = " << pi_d_u << endl; // pi_d_u =
3.141592653589794

// Long double 64 bit significand values, 80 bit ///////////////
cout.precision(21); // significant digits10

//static const long double pi_l_l = 7244019458077122842.0f/(1 <<
62)
//static const long double pi_l_u = 7244019458077122843.0f/(1 <<
62);
// Compiler will choke! or an even more cunning trick will be
needed.
static const long double pi_l_l =
3.14159265358979323829596852490908531763125210004425048828125L;
static const long double pi_l_u =
3.141592653589793238729649393903287091234233203524017333984375L;

cout << "pi_l_l = " << pi_l_l << endl; // 3.1415926535897931
cout << "pi_l_u = " << pi_l_u << endl; // 3.1415926535897931

and there are also 128 bit too, but I won't bore you further :-)

| [*] It is not even true. Due to "double rounding" troubles,
| using a higher precision can lead to a value that is not the
| nearest number.

Is this true even when you have a few more digits than necessary?
Kahan's article suggested to me that adding two guard decimal digits
avoids this problem. This why 40 was chosen.

Consistency is also of practical importance - in practice, don't all
compilers read decimal digit strings the same way and will end up with
the same internal representation (for the same floating point format),
and thus calculations will be as portable as is possible? This is
what causes most trouble in practice - one gets a slightly different
result and wastes much time puzzling why.

| So maybe the interface should provide four
| values for each constant at a given
| precision: an approximation, the nearest value, a lower
| bound, and an upper bound.

Possible, but yet more complexity?

Paul

| -----Original Message-----
| From: boost-bounces_at_[hidden]
| [mailto:boost-bounces_at_[hidden]] On Behalf Of
| Guillaume Melquiond
| Sent: 20 June 2003 13:28
| To: Boost mailing list
| Subject: Re: [boost] Advanced match constants scheme
|
|
| On Thu, 19 Jun 2003, Augustus Saunders wrote:
|
| > >PS I'd like to hear more views on this -
| > >previous review comments were quite different,
|
| I didn't react to this review at first because I was a bit
| disappointed by the content of the library. It was more like
| some questions about the best way to represent constants in
| a C++ library. And since I already had given my thoughts
| about that, I didn't feel the need to speak about it again.
|
| > Disclaimer: I am neither a mathemetician nor a scientist
| (I don't even
| > play one one TV). I do find the prospect of writing natural,
| > effecient, and precise code for solving various equations
| a worthwhile
| > goal. So, since you asked for comments, here's my non-expert
| > thoughts.
| >
| > As I understand it, the original proposal's goal was to provide
| > conveniently accessible mathematical constants with
| precision greater
| > than current hardware floating point units without any unwanted
| needed to work
| > with the interval library easily. To work around some compilers'
| > failure to remove unused constants or poor optimization,
| we wound up
| > discussing function call and macro interfaces. Nobody,
| however, is
| > thrilled with polluting the global namespace, so unless Paul
| > Mensonides convinces the world that macro namespaces are a
| good thing,
| > some of us need convincing that macros are really the way to go.
|
| I am not really interested in macros. I would prefer for the
| library to only provide one kind of interface. There could
| then be other headers on top of it to provide other
| interfaces to access the constants.
|
| The standard interface should provide a way to access a
| constant at a given precision and an enclosing interval of
| it. For example, this kind of scheme would be enough for me:
| "constant<pi, double>::lower()". I'm not suggesting that
| such a notation should be adopted; it's just a way to show
| what I consider important in a constant.
|
| If a particular precision is not available, the library
| should be able to infer it thanks to the value of the
| constant for other precisions. For example, if the only
| available precisions are "float" and "long double" for a
| particular architecture and/or constant, and if the user
| needs "double", the library should be able to do such conversions:
|
| constant<pi, double>::value() <-> constant<pi, long
| double>::value()
| constant<pi, double>::lower() <-> constant<pi, float>::lower()
|
| Please note that for the value of a constant, a higher
| precision constant can be used instead [*]; but for the
| lower and upper bound, it must be a lower precision
| constant. So it is a bit more complicated than just
| providing 40 digits constants.
|
| It is the reason why I was rooting for a library specialized
| in constants. It would provide an interface able to hide the
| conversion problems. The library would have to know the
| underlying format of floating-point numbers since the
| precision of the formats is not fixed (there are 80-bits and
| 128-bits long double for example).
|
| The Interval library defines three constants: pi, 2*pi and
| pi/2. They are needed in order to compute interval
| trigonometric functions. At the time we designed the
| library, it was not easy task to correctly define these
| constants. Here is the example of one of the 91 lines of the
|
| static const double pi_d_l = (3373259426.0 + 273688.0 / (1<<21))
| / (1<<30);
|
| Using such a formula was (in our opinion) necessary in order
| for the compiler to correctly deal with these constants. I
| would be happy to remove such an header and use another
|
| > In the course of discussion, a more ambitions plan was proposed.
| > Instead of just providing a big list of constants, IIUC it was
| > suggested that an expression template library be used to
| allow common
| > constant combinations like 2*pi or pi/2 to be expressed
| with normal
| > operators. This seems good, it provides a natural syntax
| and reduces
| > namespace clutter and is easier to remember. However,
| since the idea
| > was to use a special math program to generate high precision
| > constants, I'm not sure whether an ETL can eliminate the need to
| > compute things like 2*pi with the third party program. So
| I'd like to
| > know:
| >
| > does
| >
| > 1) 2*pi ==> BOOST_2_PI
| >
| > where BOOST_2_PI is a constant already defined, or does
| >
| > 2) 2*pi ==> BOOST_PP_MULT( 2, BOOST_PI )
| >
| > using high precision preprocessor math (or something) to
| sidestep the
| > need for defining BOOST_2_PI in the first place?
| >
| > If this was implemented the first way, then I would see
| > scheme as being a layer on top of the actual constant
| library, to give
| > it a more convenient interface. The second way might
| actually impact
| > what constants get defined in the first place, in which
| case we should
| > talk it out enough to know what constants should be
| defined. But I'm
| > not sure the possibility of an advanced scheme should
| prevent us from
| > defining the basic constants--an expression framework
| could be another
| > library, right?
|
| I agree. I don't think the library should deal with
| expressions like "two
| * pi" in the first place. It is more like a layer on top of
| it, some kind of expression template library. The constant
| library should only define a common interface for accessing
| constants and choosing their precision. Then another library
| could be built on top of it and deal with expressions
| involving these constants.
|
| So, I'm not asking much from a constant library, I just want
| it to provide
|
| Guillaume
|