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1 | <HTML> |
| 2 | <!-- | |
| 3 | Copyright (c) The Trustees of Indiana University | |
| 4 | ||
| 5 | Use, modification and distribution is subject to the Boost Software | |
| 6 | License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at | |
| 7 | http://www.boost.org/LICENSE_1_0.txt) | |
| 8 | ||
| 9 | Authors: Douglas Gregor | |
| 10 | Andrew Lumsdaine | |
| 11 | --> | |
| 12 | <Head> | |
| 13 | <Title>Boost Graph Library: Power Law Out Degree (PLOD) Generator</Title> | |
| 14 | <script language="JavaScript" type="text/JavaScript"> | |
| 15 | <!-- | |
| 16 | function address(host, user) { | |
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| 18 | var thingy = user+atchar+host; | |
| 19 | thingy = '<a hre' + 'f=' + "mai" + "lto:" + thingy + '>' + user+atchar+host + '</a>'; | |
| 20 | document.write(thingy); | |
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| 23 | </script> | |
| 24 | </head> | |
| 25 | ||
| 26 | <BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" | |
| 27 | ALINK="#ff0000"> | |
| 28 | <IMG SRC="../../../boost.png" | |
| 29 | ALT="C++ Boost" width="277" height="86"> | |
| 30 | ||
| 31 | <tt>plod_iterator</tt> | |
| 32 | ||
| 33 | <br> | |
| 34 | ||
| 35 | <PRE> | |
| 36 | template<typename RandomGenerator, typename Graph> | |
| 37 | class plod_iterator | |
| 38 | { | |
| 39 | public: | |
| 40 | typedef std::input_iterator_tag iterator_category; | |
| 41 | typedef std::pair<vertices_size_type, vertices_size_type> value_type; | |
| 42 | typedef const value_type& reference; | |
| 43 | typedef const value_type* pointer; | |
| 44 | typedef void difference_type; | |
| 45 | ||
| 46 | plod_iterator(); | |
| 47 | plod_iterator(RandomGenerator& gen, vertices_size_type n, | |
| 48 | double alpha, double beta, bool allow_self_loops = false); | |
| 49 | // Iterator operations | |
| 50 | reference operator*() const; | |
| 51 | pointer operator->() const; | |
| 52 | plod_iterator& operator++(); | |
| 53 | plod_iterator operator++(int); | |
| 54 | bool operator==(const plod_iterator& other) const; | |
| 55 | bool operator!=(const plod_iterator& other) const; | |
| 56 | }; | |
| 57 | </PRE> | |
| 58 | ||
| 59 | <p> This class template implements a generator for scale-free graphs | |
| 60 | using the Power Law Out Degree (PLOD) algorithm | |
| 61 | [<a href="bibliography.html#palmer2000">63</a>], suitable for | |
| 62 | initializing an <a | |
| 63 | href="adjacency_list.html"><tt>adjacency_list</tt></a> or other graph | |
| 64 | structure with iterator-based initialization. A scale-free graph | |
| 65 | typically has a very skewed degree distribution, where few vertices | |
| 66 | have a very high degree and a large number of vertices have a very | |
| 67 | small degree. Many naturally evolving networks, such as the World | |
| 68 | Wide Web, are scale-free graphs, making these graphs a good model for | |
| 69 | certain networking problems.</p> | |
| 70 | ||
| 71 | <p>The Power Law Out Degree (PLOD) algorithm generates a scale-free | |
| 72 | graph from three parameters, <em>n</em>, <em>alpha</em>, and | |
| 73 | <em>beta</em>, by allocating a certain number of degree "credits" to | |
| 74 | each vertex, drawn from a power-law distribution. It then creates | |
| 75 | edges between vertices, deducting a credit from each involved vertex | |
| 76 | (in the undirected case) or the source vertex (in the directed | |
| 77 | case). The number of credits assigned to a vertex is: | |
| 78 | <em>beta*x<sup>-alpha</sup></em>, where <em>x</em> is a random value | |
| 79 | between 0 and <em>n-1</em>. The value of <em>beta</em> controls the | |
| 80 | y-intercept of the curve, so that increasing <em>beta</em> increases | |
| 81 | the average degree of vertices. The value of <em>alpha</em> controls | |
| 82 | how steeply the curve drops off, with larger values indicating a | |
| 83 | steeper curve. The web graph, for instance, has <em>alpha ~ | |
| 84 | 2.72</em>.</p> | |
| 85 | ||
| 86 | <h3>Where Defined</h3> | |
| 87 | ||
| 88 | <a href="../../../boost/graph/plod_generator.hpp"><tt>boost/graph/plod_generator.hpp</tt></a> | |
| 89 | ||
| 90 | <h3>Constructors</h3> | |
| 91 | ||
| 92 | <a name="default-constructor"/> | |
| 93 | <pre>plod_iterator();</pre> | |
| 94 | <blockquote> | |
| 95 | Constructs a past-the-end iterator. | |
| 96 | </blockquote> | |
| 97 | ||
| 98 | <pre> | |
| 99 | plod_iterator(RandomGenerator& gen, vertices_size_type n, | |
| 100 | double alpha, double beta, bool allow_self_loops = false); | |
| 101 | </pre> | |
| 102 | <blockquote> | |
| 103 | Constructs a PLOD generator iterator that creates a | |
| 104 | graph with <tt>n</tt> vertices. Probabilities are drawn from the | |
| 105 | random number generator <tt>gen</tt>. Self-loops are permitted only | |
| 106 | when <tt>allow_self_loops</tt> is <tt>true</tt>. | |
| 107 | </blockquote> | |
| 108 | ||
| 109 | <H3>Example</H3> | |
| 110 | ||
| 111 | <pre> | |
| 112 | #include <boost/graph/adjacency_list.hpp> | |
| 113 | #include <boost/graph/plod_generator.hpp> | |
| 114 | #include <boost/random/linear_congruential.hpp> | |
| 115 | ||
| 116 | typedef boost::adjacency_list<> Graph; | |
| 117 | typedef boost::plod_iterator<boost::minstd_rand, Graph> SFGen; | |
| 118 | ||
| 119 | int main() | |
| 120 | { | |
| 121 | boost::minstd_rand gen; | |
| 122 | // Create graph with 100 nodes | |
| 123 | Graph g(SFGen(gen, 100, 2.5, 1000), SFGen(), 100); | |
| 124 | return 0; | |
| 125 | } | |
| 126 | </pre> | |
| 127 | ||
| 128 | <br> | |
| 129 | <HR> | |
| 130 | <TABLE> | |
| 131 | <TR valign=top> | |
| 132 | <TD nowrap>Copyright © 2005</TD><TD> | |
| 133 | <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> | |
| 134 | <A HREF="http://www.osl.iu.edu/~lums">Andrew Lumsdaine</A>, | |
| 135 | Indiana University (<script language="Javascript">address("osl.iu.edu", "lums")</script>) | |
| 136 | </TD></TR></TABLE> | |
| 137 | ||
| 138 | </BODY> | |
| 139 | </HTML> |