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