demo/lp_maxflow_demo.cc
author alpar
Tue, 12 Jul 2005 16:16:19 +0000
changeset 1547 dd57a540ff5f
parent 1435 8e85e6bbefdf
child 1560 01707a8a4ca6
permissions -rw-r--r--
Improve doc
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#ifdef HAVE_CONFIG_H
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#include <config.h>
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#endif
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#include<lemon/graph_reader.h>
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#include<lemon/list_graph.h>
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#ifdef HAVE_GLPK
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#include <lemon/lp_glpk.h>
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#elif HAVE_CPLEX
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#include <lemon/lp_cplex.h>
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#endif
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using namespace lemon;
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#ifdef HAVE_GLPK
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typedef LpGlpk LpDefault;
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#elif HAVE_CPLEX
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typedef LpCplex LpDefault;
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#endif
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template<class G,class C>
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double maxFlow(const G &g,const C &cap,typename G::Node s,typename G::Node t)
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{
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  LpDefault lp;
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  typedef G Graph;
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  typedef typename G::Node Node;
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  typedef typename G::NodeIt NodeIt;
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  typedef typename G::Edge Edge;
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  typedef typename G::EdgeIt EdgeIt;
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  typedef typename G::OutEdgeIt OutEdgeIt;
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  typedef typename G::InEdgeIt InEdgeIt;
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  //Define a map on the edges for the variables of the LP problem
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  typename G::template EdgeMap<LpDefault::Col> x(g);
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  lp.addColSet(x);
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  //Nonnegativity and capacity constraints
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  for(EdgeIt e(g);e!=INVALID;++e) {
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    lp.colUpperBound(x[e],cap[e]);
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    lp.colLowerBound(x[e],0);
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  }
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  //Flow conservation constraints for the nodes (except for 's' and 't')
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  for(NodeIt n(g);n!=INVALID;++n) if(n!=s&&n!=t) {
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    LpDefault::Expr ex;
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    for(InEdgeIt  e(g,n);e!=INVALID;++e) ex+=x[e];
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    for(OutEdgeIt e(g,n);e!=INVALID;++e) ex-=x[e];
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    lp.addRow(ex==0);
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  }
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  //Objective function: the flow value entering 't'
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  {
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    LpDefault::Expr ex;
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    for(InEdgeIt  e(g,t);e!=INVALID;++e) ex+=x[e];
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    for(OutEdgeIt e(g,t);e!=INVALID;++e) ex-=x[e];
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    lp.setObj(ex);
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  }
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  //Maximization
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  lp.max();
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#ifdef HAVE_GLPK
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  lp.presolver(true);
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  lp.messageLevel(3);
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#endif
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  //Solve with the underlying solver
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  lp.solve();
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  return lp.primalValue();
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}
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int main() 
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{
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  ListGraph g;
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  ListGraph::Node s;
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  ListGraph::Node t;
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  ListGraph::EdgeMap<double> cap(g);
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  GraphReader<ListGraph> reader(std::cin,g);
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  reader.readNode("source",s).readNode("target",t)
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    .readEdgeMap("capacity",cap).run();
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  // std::ifstream file("../test/preflow_");
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//   readDimacs(file, g, cap, s, t);
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  std::cout << "Max flow value = " << maxFlow(g,cap,s,t) << std::endl;
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}