<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>network_simplex — NetworkX 1.8.1 documentation</title> <link rel="stylesheet" href="../../_static/networkx.css" type="text/css" /> <link rel="stylesheet" href="../../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../../', VERSION: '1.8.1', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: false }; </script> <script type="text/javascript" src="../../_static/jquery.js"></script> <script type="text/javascript" src="../../_static/underscore.js"></script> <script type="text/javascript" src="../../_static/doctools.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within NetworkX 1.8.1 documentation" href="../../_static/opensearch.xml"/> <link rel="top" title="NetworkX 1.8.1 documentation" href="../../index.html" /> <link rel="up" title="Flows" href="../algorithms.flow.html" /> <link rel="next" title="min_cost_flow_cost" href="networkx.algorithms.flow.min_cost_flow_cost.html" /> <link rel="prev" title="ford_fulkerson_flow_and_auxiliary" href="networkx.algorithms.flow.ford_fulkerson_flow_and_auxiliary.html" /> </head> <body> <div style="color: black;background-color: white; font-size: 3.2em; text-align: left; padding: 15px 10px 10px 15px"> NetworkX </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="networkx.algorithms.flow.min_cost_flow_cost.html" title="min_cost_flow_cost" accesskey="N">next</a> |</li> <li class="right" > <a href="networkx.algorithms.flow.ford_fulkerson_flow_and_auxiliary.html" title="ford_fulkerson_flow_and_auxiliary" accesskey="P">previous</a> |</li> <li><a href="http://networkx.github.com/">NetworkX Home </a> | </li> <li><a href="http://networkx.github.com/documentation.html">Documentation </a>| </li> <li><a href="http://networkx.github.com/download.html">Download </a> | </li> <li><a href="http://github.com/networkx">Developer (Github)</a></li> <li><a href="../index.html" >Reference</a> »</li> <li><a href="../pdf_reference.html" >Reference</a> »</li> <li><a href="../algorithms.html" >Algorithms</a> »</li> <li><a href="../algorithms.flow.html" accesskey="U">Flows</a> »</li> </ul> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="networkx.algorithms.flow.ford_fulkerson_flow_and_auxiliary.html" title="previous chapter">ford_fulkerson_flow_and_auxiliary</a></p> <h4>Next topic</h4> <p class="topless"><a href="networkx.algorithms.flow.min_cost_flow_cost.html" title="next chapter">min_cost_flow_cost</a></p> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="network-simplex"> <h1>network_simplex<a class="headerlink" href="#network-simplex" title="Permalink to this headline">¶</a></h1> <dl class="function"> <dt id="networkx.algorithms.flow.network_simplex"> <tt class="descname">network_simplex</tt><big>(</big><em>G</em>, <em>demand='demand'</em>, <em>capacity='capacity'</em>, <em>weight='weight'</em><big>)</big><a class="headerlink" href="#networkx.algorithms.flow.network_simplex" title="Permalink to this definition">¶</a></dt> <dd><p>Find a minimum cost flow satisfying all demands in digraph G.</p> <p>This is a primal network simplex algorithm that uses the leaving arc rule to prevent cycling.</p> <p>G is a digraph with edge costs and capacities and in which nodes have demand, i.e., they want to send or receive some amount of flow. A negative demand means that the node wants to send flow, a positive demand means that the node want to receive flow. A flow on the digraph G satisfies all demand if the net flow into each node is equal to the demand of that node.</p> <table class="docutils field-list" frame="void" rules="none"> <col class="field-name" /> <col class="field-body" /> <tbody valign="top"> <tr class="field-odd field"><th class="field-name">Parameters :</th><td class="field-body"><p class="first"><strong>G</strong> : NetworkX graph</p> <blockquote> <div><p>DiGraph on which a minimum cost flow satisfying all demands is to be found.</p> </div></blockquote> <p><strong>demand: string</strong> :</p> <blockquote> <div><p>Nodes of the graph G are expected to have an attribute demand that indicates how much flow a node wants to send (negative demand) or receive (positive demand). Note that the sum of the demands should be 0 otherwise the problem in not feasible. If this attribute is not present, a node is considered to have 0 demand. Default value: ‘demand’.</p> </div></blockquote> <p><strong>capacity: string</strong> :</p> <blockquote> <div><p>Edges of the graph G are expected to have an attribute capacity that indicates how much flow the edge can support. If this attribute is not present, the edge is considered to have infinite capacity. Default value: ‘capacity’.</p> </div></blockquote> <p><strong>weight: string</strong> :</p> <blockquote> <div><p>Edges of the graph G are expected to have an attribute weight that indicates the cost incurred by sending one unit of flow on that edge. If not present, the weight is considered to be 0. Default value: ‘weight’.</p> </div></blockquote> </td> </tr> <tr class="field-even field"><th class="field-name">Returns :</th><td class="field-body"><p class="first"><strong>flowCost: integer, float</strong> :</p> <blockquote> <div><p>Cost of a minimum cost flow satisfying all demands.</p> </div></blockquote> <p><strong>flowDict: dictionary</strong> :</p> <blockquote> <div><p>Dictionary of dictionaries keyed by nodes such that flowDict[u][v] is the flow edge (u, v).</p> </div></blockquote> </td> </tr> <tr class="field-odd field"><th class="field-name">Raises :</th><td class="field-body"><p class="first"><strong>NetworkXError</strong> :</p> <blockquote> <div><p>This exception is raised if the input graph is not directed, not connected or is a multigraph.</p> </div></blockquote> <p><strong>NetworkXUnfeasible</strong> :</p> <blockquote> <div><dl class="docutils"> <dt>This exception is raised in the following situations:</dt> <dd><ul class="first last simple"> <li>The sum of the demands is not zero. Then, there is no flow satisfying all demands.</li> <li>There is no flow satisfying all demand.</li> </ul> </dd> </dl> </div></blockquote> <p><strong>NetworkXUnbounded</strong> :</p> <blockquote class="last"> <div><p>This exception is raised if the digraph G has a cycle of negative cost and infinite capacity. Then, the cost of a flow satisfying all demands is unbounded below.</p> </div></blockquote> </td> </tr> </tbody> </table> <div class="admonition-see-also admonition seealso"> <p class="first admonition-title">See also</p> <p class="last"><a class="reference internal" href="networkx.algorithms.flow.cost_of_flow.html#networkx.algorithms.flow.cost_of_flow" title="networkx.algorithms.flow.cost_of_flow"><tt class="xref py py-obj docutils literal"><span class="pre">cost_of_flow</span></tt></a>, <a class="reference internal" href="networkx.algorithms.flow.max_flow_min_cost.html#networkx.algorithms.flow.max_flow_min_cost" title="networkx.algorithms.flow.max_flow_min_cost"><tt class="xref py py-obj docutils literal"><span class="pre">max_flow_min_cost</span></tt></a>, <a class="reference internal" href="networkx.algorithms.flow.min_cost_flow.html#networkx.algorithms.flow.min_cost_flow" title="networkx.algorithms.flow.min_cost_flow"><tt class="xref py py-obj docutils literal"><span class="pre">min_cost_flow</span></tt></a>, <a class="reference internal" href="networkx.algorithms.flow.min_cost_flow_cost.html#networkx.algorithms.flow.min_cost_flow_cost" title="networkx.algorithms.flow.min_cost_flow_cost"><tt class="xref py py-obj docutils literal"><span class="pre">min_cost_flow_cost</span></tt></a></p> </div> <p class="rubric">Notes</p> <p>This algorithm is not guaranteed to work if edge weights are floating point numbers (overflows and roundoff errors can cause problems).</p> <p class="rubric">References</p> <p>W. J. Cook, W. H. Cunningham, W. R. Pulleyblank and A. Schrijver. Combinatorial Optimization. Wiley-Interscience, 1998.</p> <p class="rubric">Examples</p> <p>A simple example of a min cost flow problem.</p> <div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">networkx</span> <span class="kn">as</span> <span class="nn">nx</span> <span class="gp">>>> </span><span class="n">G</span> <span class="o">=</span> <span class="n">nx</span><span class="o">.</span><span class="n">DiGraph</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'a'</span><span class="p">,</span> <span class="n">demand</span> <span class="o">=</span> <span class="o">-</span><span class="mi">5</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'d'</span><span class="p">,</span> <span class="n">demand</span> <span class="o">=</span> <span class="mi">5</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'a'</span><span class="p">,</span> <span class="s">'b'</span><span class="p">,</span> <span class="n">weight</span> <span class="o">=</span> <span class="mi">3</span><span class="p">,</span> <span class="n">capacity</span> <span class="o">=</span> <span class="mi">4</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'a'</span><span class="p">,</span> <span class="s">'c'</span><span class="p">,</span> <span class="n">weight</span> <span class="o">=</span> <span class="mi">6</span><span class="p">,</span> <span class="n">capacity</span> <span class="o">=</span> <span class="mi">10</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'b'</span><span class="p">,</span> <span class="s">'d'</span><span class="p">,</span> <span class="n">weight</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="n">capacity</span> <span class="o">=</span> <span class="mi">9</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'c'</span><span class="p">,</span> <span class="s">'d'</span><span class="p">,</span> <span class="n">weight</span> <span class="o">=</span> <span class="mi">2</span><span class="p">,</span> <span class="n">capacity</span> <span class="o">=</span> <span class="mi">5</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">flowCost</span><span class="p">,</span> <span class="n">flowDict</span> <span class="o">=</span> <span class="n">nx</span><span class="o">.</span><span class="n">network_simplex</span><span class="p">(</span><span class="n">G</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">flowCost</span> <span class="go">24</span> <span class="gp">>>> </span><span class="n">flowDict</span> <span class="go">{'a': {'c': 1, 'b': 4}, 'c': {'d': 1}, 'b': {'d': 4}, 'd': {}}</span> </pre></div> </div> <p>The mincost flow algorithm can also be used to solve shortest path problems. To find the shortest path between two nodes u and v, give all edges an infinite capacity, give node u a demand of -1 and node v a demand a 1. Then run the network simplex. The value of a min cost flow will be the distance between u and v and edges carrying positive flow will indicate the path.</p> <div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">G</span><span class="o">=</span><span class="n">nx</span><span class="o">.</span><span class="n">DiGraph</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_weighted_edges_from</span><span class="p">([(</span><span class="s">'s'</span><span class="p">,</span><span class="s">'u'</span><span class="p">,</span><span class="mi">10</span><span class="p">),</span> <span class="p">(</span><span class="s">'s'</span><span class="p">,</span><span class="s">'x'</span><span class="p">,</span><span class="mi">5</span><span class="p">),</span> <span class="gp">... </span> <span class="p">(</span><span class="s">'u'</span><span class="p">,</span><span class="s">'v'</span><span class="p">,</span><span class="mi">1</span><span class="p">),</span> <span class="p">(</span><span class="s">'u'</span><span class="p">,</span><span class="s">'x'</span><span class="p">,</span><span class="mi">2</span><span class="p">),</span> <span class="gp">... </span> <span class="p">(</span><span class="s">'v'</span><span class="p">,</span><span class="s">'y'</span><span class="p">,</span><span class="mi">1</span><span class="p">),</span> <span class="p">(</span><span class="s">'x'</span><span class="p">,</span><span class="s">'u'</span><span class="p">,</span><span class="mi">3</span><span class="p">),</span> <span class="gp">... </span> <span class="p">(</span><span class="s">'x'</span><span class="p">,</span><span class="s">'v'</span><span class="p">,</span><span class="mi">5</span><span class="p">),</span> <span class="p">(</span><span class="s">'x'</span><span class="p">,</span><span class="s">'y'</span><span class="p">,</span><span class="mi">2</span><span class="p">),</span> <span class="gp">... </span> <span class="p">(</span><span class="s">'y'</span><span class="p">,</span><span class="s">'s'</span><span class="p">,</span><span class="mi">7</span><span class="p">),</span> <span class="p">(</span><span class="s">'y'</span><span class="p">,</span><span class="s">'v'</span><span class="p">,</span><span class="mi">6</span><span class="p">)])</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'s'</span><span class="p">,</span> <span class="n">demand</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'v'</span><span class="p">,</span> <span class="n">demand</span> <span class="o">=</span> <span class="mi">1</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">flowCost</span><span class="p">,</span> <span class="n">flowDict</span> <span class="o">=</span> <span class="n">nx</span><span class="o">.</span><span class="n">network_simplex</span><span class="p">(</span><span class="n">G</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">flowCost</span> <span class="o">==</span> <span class="n">nx</span><span class="o">.</span><span class="n">shortest_path_length</span><span class="p">(</span><span class="n">G</span><span class="p">,</span> <span class="s">'s'</span><span class="p">,</span> <span class="s">'v'</span><span class="p">,</span> <span class="n">weight</span> <span class="o">=</span> <span class="s">'weight'</span><span class="p">)</span> <span class="go">True</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">([(</span><span class="n">u</span><span class="p">,</span> <span class="n">v</span><span class="p">)</span> <span class="k">for</span> <span class="n">u</span> <span class="ow">in</span> <span class="n">flowDict</span> <span class="k">for</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">flowDict</span><span class="p">[</span><span class="n">u</span><span class="p">]</span> <span class="k">if</span> <span class="n">flowDict</span><span class="p">[</span><span class="n">u</span><span class="p">][</span><span class="n">v</span><span class="p">]</span> <span class="o">></span> <span class="mi">0</span><span class="p">])</span> <span class="go">[('s', 'x'), ('u', 'v'), ('x', 'u')]</span> <span class="gp">>>> </span><span class="n">nx</span><span class="o">.</span><span class="n">shortest_path</span><span class="p">(</span><span class="n">G</span><span class="p">,</span> <span class="s">'s'</span><span class="p">,</span> <span class="s">'v'</span><span class="p">,</span> <span class="n">weight</span> <span class="o">=</span> <span class="s">'weight'</span><span class="p">)</span> <span class="go">['s', 'x', 'u', 'v']</span> </pre></div> </div> <p>It is possible to change the name of the attributes used for the algorithm.</p> <div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">G</span> <span class="o">=</span> <span class="n">nx</span><span class="o">.</span><span class="n">DiGraph</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'p'</span><span class="p">,</span> <span class="n">spam</span> <span class="o">=</span> <span class="o">-</span><span class="mi">4</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'q'</span><span class="p">,</span> <span class="n">spam</span> <span class="o">=</span> <span class="mi">2</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'a'</span><span class="p">,</span> <span class="n">spam</span> <span class="o">=</span> <span class="o">-</span><span class="mi">2</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'d'</span><span class="p">,</span> <span class="n">spam</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'t'</span><span class="p">,</span> <span class="n">spam</span> <span class="o">=</span> <span class="mi">2</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_node</span><span class="p">(</span><span class="s">'w'</span><span class="p">,</span> <span class="n">spam</span> <span class="o">=</span> <span class="mi">3</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'p'</span><span class="p">,</span> <span class="s">'q'</span><span class="p">,</span> <span class="n">cost</span> <span class="o">=</span> <span class="mi">7</span><span class="p">,</span> <span class="n">vacancies</span> <span class="o">=</span> <span class="mi">5</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'p'</span><span class="p">,</span> <span class="s">'a'</span><span class="p">,</span> <span class="n">cost</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="n">vacancies</span> <span class="o">=</span> <span class="mi">4</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'q'</span><span class="p">,</span> <span class="s">'d'</span><span class="p">,</span> <span class="n">cost</span> <span class="o">=</span> <span class="mi">2</span><span class="p">,</span> <span class="n">vacancies</span> <span class="o">=</span> <span class="mi">3</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'t'</span><span class="p">,</span> <span class="s">'q'</span><span class="p">,</span> <span class="n">cost</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="n">vacancies</span> <span class="o">=</span> <span class="mi">2</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'a'</span><span class="p">,</span> <span class="s">'t'</span><span class="p">,</span> <span class="n">cost</span> <span class="o">=</span> <span class="mi">2</span><span class="p">,</span> <span class="n">vacancies</span> <span class="o">=</span> <span class="mi">4</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'d'</span><span class="p">,</span> <span class="s">'w'</span><span class="p">,</span> <span class="n">cost</span> <span class="o">=</span> <span class="mi">3</span><span class="p">,</span> <span class="n">vacancies</span> <span class="o">=</span> <span class="mi">4</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">G</span><span class="o">.</span><span class="n">add_edge</span><span class="p">(</span><span class="s">'t'</span><span class="p">,</span> <span class="s">'w'</span><span class="p">,</span> <span class="n">cost</span> <span class="o">=</span> <span class="mi">4</span><span class="p">,</span> <span class="n">vacancies</span> <span class="o">=</span> <span class="mi">1</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">flowCost</span><span class="p">,</span> <span class="n">flowDict</span> <span class="o">=</span> <span class="n">nx</span><span class="o">.</span><span class="n">network_simplex</span><span class="p">(</span><span class="n">G</span><span class="p">,</span> <span class="n">demand</span> <span class="o">=</span> <span class="s">'spam'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">capacity</span> <span class="o">=</span> <span class="s">'vacancies'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">weight</span> <span class="o">=</span> <span class="s">'cost'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">flowCost</span> <span class="go">37</span> <span class="gp">>>> </span><span class="n">flowDict</span> <span class="go">{'a': {'t': 4}, 'd': {'w': 2}, 'q': {'d': 1}, 'p': {'q': 2, 'a': 2}, 't': {'q': 1, 'w': 1}, 'w': {}}</span> </pre></div> </div> </dd></dl> </div> </div> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="networkx.algorithms.flow.min_cost_flow_cost.html" title="min_cost_flow_cost" >next</a> |</li> <li class="right" > <a href="networkx.algorithms.flow.ford_fulkerson_flow_and_auxiliary.html" title="ford_fulkerson_flow_and_auxiliary" >previous</a> |</li> <li><a href="http://networkx.github.com/">NetworkX Home </a> | </li> <li><a 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