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143 lines
5.7 KiB
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<META NAME="AUTHOR" CONTENT="pme@sources.redhat.com (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</TITLE>
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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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<UL>
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<LI>Template data structures such as <TT>valarray<></TT>
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and <TT>complex<></TT>.
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<LI>Template numerical functions such as <TT>accumulate</TT>;
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<TT>inner_product</TT>; <TT>partial_sum</TT> and
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<TT>adjacent_difference</TT>.
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</UL>
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All of the Standard C math functions are of course included in C++,
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and overloaded versions for <TT>long</TT>, <TT>float</TT>, and
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<TT>long double</TT> have been added for all of them.
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</P>
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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>
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<LI><A HREF="#2">Array Processing</A>
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<LI><A HREF="#3">Numerical Functions</A>
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</UL>
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<HR>
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<H2><A NAME="1">Complex Number Processing</A></H2>
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<P>Using <TT>complex<></TT> 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://home.flash.net/~dtribble/text/cdiffs.htm#C99.complex">here</A>.
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</P>
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<P><TT>complex<></TT> 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 <TT>op<<</TT>
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and <TT>op>></TT> that work with iostreams: <TT>op<<</TT>
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prints <TT>(u,v)</TT> and <TT>op>></TT> can read <TT>u</TT>,
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<TT>(u)</TT>, and <TT>(u,v)</TT>.
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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++. C99 adds a new
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keyword, <TT>restrict</TT>, to apply to individual pointers. The C++
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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 <TT>valarray</TT>, 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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<UL>
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<LI><TT>accumulate</TT>
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<LI><TT>inner_product</TT>
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<LI><TT>partial_sum</TT>
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<LI><TT>adjacent_difference</TT>
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</UL>
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</P>
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<P>Here is a simple example of the two forms of <TT>accumulate</TT>.
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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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The first call adds all the members of the array, using zero as an
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initial value for <TT>sum</TT>. The second does the same, but uses
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<TT>someval</TT> as the starting value (thus, <TT>sum_stuff == sum +
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someval</TT>). 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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<P CLASS="fineprint"><EM>
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Comments and suggestions are welcome, and may be sent to
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<A HREF="mailto:pme@sources.redhat.com">Phil Edwards</A> or
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<A HREF="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</A>.
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<BR> $Id: howto.html,v 1.7 2000/12/03 23:47:48 jsm28 Exp $
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</EM></P>
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