140 lines
4.7 KiB
HTML
140 lines
4.7 KiB
HTML
<HTML>
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<!--
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Copyright (c) The Trustees of Indiana University
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Use, modification and distribution is subject to the Boost Software
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License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)
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Authors: Douglas Gregor
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Andrew Lumsdaine
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-->
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<Head>
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<Title>Boost Graph Library: Power Law Out Degree (PLOD) Generator</Title>
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<script language="JavaScript" type="text/JavaScript">
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</head>
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<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
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ALINK="#ff0000">
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<IMG SRC="../../../boost.png"
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ALT="C++ Boost" width="277" height="86">
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<tt>plod_iterator</tt>
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<br>
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<PRE>
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template<typename RandomGenerator, typename Graph>
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class plod_iterator
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{
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public:
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typedef std::input_iterator_tag iterator_category;
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typedef std::pair<vertices_size_type, vertices_size_type> value_type;
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typedef const value_type& reference;
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typedef const value_type* pointer;
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typedef void difference_type;
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plod_iterator();
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plod_iterator(RandomGenerator& gen, vertices_size_type n,
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double alpha, double beta, bool allow_self_loops = false);
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// Iterator operations
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reference operator*() const;
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pointer operator->() const;
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plod_iterator& operator++();
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plod_iterator operator++(int);
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bool operator==(const plod_iterator& other) const;
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bool operator!=(const plod_iterator& other) const;
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};
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</PRE>
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<p> This class template implements a generator for scale-free graphs
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using the Power Law Out Degree (PLOD) algorithm
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[<a href="bibliography.html#palmer2000">63</a>], suitable for
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initializing an <a
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href="adjacency_list.html"><tt>adjacency_list</tt></a> or other graph
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structure with iterator-based initialization. A scale-free graph
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typically has a very skewed degree distribution, where few vertices
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have a very high degree and a large number of vertices have a very
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small degree. Many naturally evolving networks, such as the World
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Wide Web, are scale-free graphs, making these graphs a good model for
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certain networking problems.</p>
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<p>The Power Law Out Degree (PLOD) algorithm generates a scale-free
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graph from three parameters, <em>n</em>, <em>alpha</em>, and
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<em>beta</em>, by allocating a certain number of degree "credits" to
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each vertex, drawn from a power-law distribution. It then creates
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edges between vertices, deducting a credit from each involved vertex
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(in the undirected case) or the source vertex (in the directed
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case). The number of credits assigned to a vertex is:
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<em>beta*x<sup>-alpha</sup></em>, where <em>x</em> is a random value
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between 0 and <em>n-1</em>. The value of <em>beta</em> controls the
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y-intercept of the curve, so that increasing <em>beta</em> increases
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the average degree of vertices. The value of <em>alpha</em> controls
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how steeply the curve drops off, with larger values indicating a
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steeper curve. The web graph, for instance, has <em>alpha ~
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2.72</em>.</p>
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<h3>Where Defined</h3>
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<a href="../../../boost/graph/plod_generator.hpp"><tt>boost/graph/plod_generator.hpp</tt></a>
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<h3>Constructors</h3>
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<a name="default-constructor"/>
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<pre>plod_iterator();</pre>
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<blockquote>
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Constructs a past-the-end iterator.
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</blockquote>
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<pre>
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plod_iterator(RandomGenerator& gen, vertices_size_type n,
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double alpha, double beta, bool allow_self_loops = false);
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</pre>
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<blockquote>
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Constructs a PLOD generator iterator that creates a
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graph with <tt>n</tt> vertices. Probabilities are drawn from the
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random number generator <tt>gen</tt>. Self-loops are permitted only
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when <tt>allow_self_loops</tt> is <tt>true</tt>.
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</blockquote>
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<H3>Example</H3>
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<pre>
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#include <boost/graph/adjacency_list.hpp>
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#include <boost/graph/plod_generator.hpp>
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#include <boost/random/linear_congruential.hpp>
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typedef boost::adjacency_list<> Graph;
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typedef boost::plod_iterator<boost::minstd_rand, Graph> SFGen;
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int main()
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{
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boost::minstd_rand gen;
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// Create graph with 100 nodes
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Graph g(SFGen(gen, 100, 2.5, 1000), SFGen(), 100);
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return 0;
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}
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</pre>
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<br>
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<HR>
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<TABLE>
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<TR valign=top>
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<TD nowrap>Copyright © 2005</TD><TD>
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<A HREF="http://www.boost.org/people/doug_gregor.html">Doug Gregor</A>, Indiana University (<script language="Javascript">address("cs.indiana.edu", "dgregor")</script>)<br>
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<A HREF="https://homes.cs.washington.edu/~al75">Andrew Lumsdaine</A>,
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Indiana University (<script language="Javascript">address("osl.iu.edu", "lums")</script>)
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</TD></TR></TABLE>
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