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<meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
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<meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" />
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<meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 26." />
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<meta name="GENERATOR" content="vi and eight fingers" />
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<title>libstdc++-v3 HOWTO: Chapter 26: Numerics</title>
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<link rel="Start" href="../documentation.html" type="text/html"
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title="GNU C++ Standard Library" />
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<link rel="Prev" href="../25_algorithms/howto.html" type="text/html"
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title="Algorithms" />
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<link rel="Next" href="../27_io/howto.html" type="text/html"
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title="Input/Output" />
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<link rel="Help" href="../faq/index.html" type="text/html" title="F.A.Q." />
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</head>
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<body>
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<h1 class="centered"><a name="top">Chapter 26: Numerics</a></h1>
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<p>Chapter 26 deals with building block abstractions to aid in
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numerical computing:
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</p>
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<ul>
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<li>Template data structures such as <code>valarray<></code>
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and <code>complex<></code>.
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</li>
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<li>Template numerical functions such as <code>accumulate</code>,
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<code>inner_product</code>, <code>partial_sum</code>, and
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<code>adjacent_difference</code>.
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</li>
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</ul>
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<p>All of the Standard C math functions are of course included in C++,
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and overloaded versions for <code>long</code>, <code>float</code>, and
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<code>long double</code> have been added for all of them.
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</p>
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<!-- ####################################################### -->
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<hr />
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<h1>Contents</h1>
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<ul>
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<li><a href="#1">Complex Number Processing</a></li>
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<li><a href="#2">Array Processing</a></li>
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<li><a href="#3">Numerical Functions</a></li>
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<li><a href="#4">C99</a></li>
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</ul>
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<hr />
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<!-- ####################################################### -->
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<h2><a name="1">Complex Number Processing</a></h2>
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<p>Using <code>complex<></code> becomes even more comple- er, sorry,
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<em>complicated</em>, with the not-quite-gratuitously-incompatible
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addition of complex types to the C language. David Tribble has
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compiled a list of C++98 and C99 conflict points; his description of
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C's new type versus those of C++ and how to get them playing together
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nicely is
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<a href="http://david.tribble.com/text/cdiffs.htm#C99-complex">here</a>.
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</p>
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<p><code>complex<></code> is intended to be instantiated with a
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floating-point type. As long as you meet that and some other basic
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requirements, then the resulting instantiation has all of the usual
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math operators defined, as well as definitions of <code>op<<</code>
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and <code>op>></code> that work with iostreams: <code>op<<</code>
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prints <code>(u,v)</code> and <code>op>></code> can read <code>u</code>,
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<code>(u)</code>, and <code>(u,v)</code>.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="2">Array Processing</a></h2>
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<p>One of the major reasons why FORTRAN can chew through numbers so well
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is that it is defined to be free of pointer aliasing, an assumption
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that C89 is not allowed to make, and neither is C++98. C99 adds a new
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keyword, <code>restrict</code>, to apply to individual pointers. The
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C++ solution is contained in the library rather than the language
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(although many vendors can be expected to add this to their compilers
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as an extension).
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</p>
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<p>That library solution is a set of two classes, five template classes,
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and "a whole bunch" of functions. The classes are required
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to be free of pointer aliasing, so compilers can optimize the
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daylights out of them the same way that they have been for FORTRAN.
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They are collectively called <code>valarray</code>, although strictly
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speaking this is only one of the five template classes, and they are
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designed to be familiar to people who have worked with the BLAS
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libraries before.
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</p>
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<p>Some more stuff should go here once somebody has time to write it.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="3">Numerical Functions</a></h2>
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<p>There are four generalized functions in the <numeric> header
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that follow the same conventions as those in <algorithm>. Each
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of them is overloaded: one signature for common default operations,
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and a second for fully general operations. Their names are
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self-explanatory to anyone who works with numerics on a regular basis:
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</p>
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<ul>
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<li><code>accumulate</code></li>
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<li><code>inner_product</code></li>
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<li><code>partial_sum</code></li>
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<li><code>adjacent_difference</code></li>
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</ul>
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<p>Here is a simple example of the two forms of <code>accumulate</code>.
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</p>
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<pre>
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int ar[50];
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int someval = somefunction();
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// ...initialize members of ar to something...
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int sum = std::accumulate(ar,ar+50,0);
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int sum_stuff = std::accumulate(ar,ar+50,someval);
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int product = std::accumulate(ar,ar+50,1,std::multiplies<int>());
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</pre>
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<p>The first call adds all the members of the array, using zero as an
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initial value for <code>sum</code>. The second does the same, but uses
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<code>someval</code> as the starting value (thus, <code>sum_stuff == sum +
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someval</code>). The final call uses the second of the two signatures,
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and multiplies all the members of the array; here we must obviously
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use 1 as a starting value instead of 0.
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</p>
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<p>The other three functions have similar dual-signature forms.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="4">C99</a></h2>
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<p>In addition to the other topics on this page, we'll note here some
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of the C99 features that appear in libstdc++-v3.
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</p>
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<p>The C99 features depend on the <code>--enable-c99</code> configure flag.
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This flag is already on by default, but it can be disabled by the
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user. Also, the configuration machinery will disable it if the
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necessary support for C99 (e.g., header files) cannot be found.
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</p>
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<p>As of GCC 3.0, C99 support includes classification functions
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such as <code>isnormal</code>, <code>isgreater</code>,
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<code>isnan</code>, etc.
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The functions used for 'long long' support such as <code>strtoll</code>
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are supported, as is the <code>lldiv_t</code> typedef. Also supported
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are the wide character functions using 'long long', like
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<code>wcstoll</code>.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<!-- ####################################################### -->
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<hr />
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<p class="fineprint"><em>
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See <a href="../17_intro/license.html">license.html</a> for copying conditions.
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Comments and suggestions are welcome, and may be sent to
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<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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</em></p>
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</body>
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