Chapter 26:  Numerics
Chapter 26 deals with building block abstractions to aid in
numerical computing:
Template data structures such as
valarray<>
and
complex<>
.
Template numerical functions such as
accumulate
,
inner_product
,
partial_sum
, and
adjacent_difference
.
All of the Standard C math functions are of course included in C++,
and overloaded versions for
long
,
float
, and
long double
have been added for all of them.
Contents
#1
Complex Number Processing
#2
Array Processing
#3
Numerical Functions
#4
C99
Complex Number Processing
Using
complex<>
becomes even more comple- er, sorry,
complicated
, with the not-quite-gratuitously-incompatible
addition of complex types to the C language.  David Tribble has
compiled a list of C++98 and C99 conflict points; his description of
C's new type versus those of C++ and how to get them playing together
nicely is
http://david.tribble.com/text/cdiffs.htm#C99-complex
here
.
complex<>
is intended to be instantiated with a
floating-point type.  As long as you meet that and some other basic
requirements, then the resulting instantiation has all of the usual
math operators defined, as well as definitions of
op<<
and
op>>
that work with iostreams:
op<<
prints
(u,v)
and
op>>
can read
u
,
(u)
, and
(u,v)
.
Return
#top
to top of page
or
../faq/index.html
to the FAQ
.
Array Processing
One of the major reasons why FORTRAN can chew through numbers so well
is that it is defined to be free of pointer aliasing, an assumption
that C89 is not allowed to make, and neither is C++98.  C99 adds a new
keyword,
restrict
, to apply to individual pointers.  The
C++ solution is contained in the library rather than the language
(although many vendors can be expected to add this to their compilers
as an extension).
That library solution is a set of two classes, five template classes,
and "a whole bunch" of functions.  The classes are required
to be free of pointer aliasing, so compilers can optimize the
daylights out of them the same way that they have been for FORTRAN.
They are collectively called
valarray
, although strictly
speaking this is only one of the five template classes, and they are
designed to be familiar to people who have worked with the BLAS
libraries before.
Some more stuff should go here once somebody has time to write it.
Return
#top
to top of page
or
../faq/index.html
to the FAQ
.
Numerical Functions
There are four generalized functions in the <numeric> header
that follow the same conventions as those in <algorithm>.  Each
of them is overloaded:  one signature for common default operations,
and a second for fully general operations.  Their names are
self-explanatory to anyone who works with numerics on a regular basis:
accumulate
inner_product
partial_sum
adjacent_difference
Here is a simple example of the two forms of
accumulate
.
int   ar[50];
int   someval = somefunction();
// ...initialize members of ar to something...
int  sum       = std::accumulate(ar,ar+50,0);
int  sum_stuff = std::accumulate(ar,ar+50,someval);
int  product   = std::accumulate(ar,ar+50,1,std::multiplies<int>());
The first call adds all the members of the array, using zero as an
initial value for
sum
.  The second does the same, but uses
someval
as the starting value (thus,
sum_stuff == sum +
someval
).  The final call uses the second of the two signatures,
and multiplies all the members of the array; here we must obviously
use 1 as a starting value instead of 0.
The other three functions have similar dual-signature forms.
Return
#top
to top of page
or
../faq/index.html
to the FAQ
.
C99
In addition to the other topics on this page, we'll note here some
of the C99 features that appear in libstdc++-v3.
The C99 features depend on the
--enable-c99
configure flag.
This flag is already on by default, but it can be disabled by the
user.  Also, the configuration machinery will disable it if the
necessary support for C99 (e.g., header files) cannot be found.
As of GCC 3.0, C99 support includes classification functions
such as
isnormal
,
isgreater
,
isnan
, etc.
The functions used for 'long long' support such as
strtoll
are supported, as is the
lldiv_t
typedef.  Also supported
are the wide character functions using 'long long', like
wcstoll
.
Return
#top
to top of page
or
../faq/index.html
to the FAQ
.
See
../17_intro/license.html
license.html
for copying conditions.
Comments and suggestions are welcome, and may be sent to
mailto:libstdc++@gcc.gnu.org
the libstdc++ mailing list
.
