COIN-OR::LEMON - Graph Library

source: lemon-0.x/demo/lp_maxflow_demo.cc @ 1571:68ce4302fb0b

Last change on this file since 1571:68ce4302fb0b was 1571:68ce4302fb0b, checked in by Alpar Juttner, 14 years ago

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1/* -*- C++ -*-
2 * demo/lp_maxflow_demo.cc - Part of LEMON, a generic C++ optimization library
3 *
4 * Copyright (C) 2005 Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport
5 * (Egervary Research Group on Combinatorial Optimization, EGRES).
6 *
7 * Permission to use, modify and distribute this software is granted
8 * provided that this copyright notice appears in all copies. For
9 * precise terms see the accompanying LICENSE file.
10 *
11 * This software is provided "AS IS" with no warranty of any kind,
12 * express or implied, and with no claim as to its suitability for any
13 * purpose.
14 *
15 */
16
17///\ingroup demos
18///\file
19///\brief Max flow problem solved with an LP solver (demo).
20///
21///This demo program shows how to solve a maximum (or maximal) flow
22///problem using the LEMON LP solver interface. We would like to lay
23///the emphasis on the simplicity of the way one can formulate the LP
24///constraints with LEMON that arise in graph theory.
25
26#ifdef HAVE_CONFIG_H
27#include <config.h>
28#endif
29
30#include<lemon/graph_reader.h>
31#include<lemon/list_graph.h>
32
33#include <fstream>
34#include <iostream>
35
36
37#ifdef HAVE_GLPK
38#include <lemon/lp_glpk.h>
39#elif HAVE_CPLEX
40#include <lemon/lp_cplex.h>
41#endif
42
43using namespace lemon;
44
45#ifdef HAVE_GLPK
46typedef LpGlpk LpDefault;
47#elif HAVE_CPLEX
48typedef LpCplex LpDefault;
49#endif
50
51
52template<class G,class C>
53double maxFlow(const G &g,const C &cap,typename G::Node s,typename G::Node t)
54{
55  LpDefault lp;
56 
57  typedef G Graph;
58  typedef typename G::Node Node;
59  typedef typename G::NodeIt NodeIt;
60  typedef typename G::Edge Edge;
61  typedef typename G::EdgeIt EdgeIt;
62  typedef typename G::OutEdgeIt OutEdgeIt;
63  typedef typename G::InEdgeIt InEdgeIt;
64 
65  //Define a map on the edges for the variables of the LP problem
66  typename G::template EdgeMap<LpDefault::Col> x(g);
67  lp.addColSet(x);
68 
69  //Nonnegativity and capacity constraints
70  for(EdgeIt e(g);e!=INVALID;++e) {
71    lp.colUpperBound(x[e],cap[e]);
72    lp.colLowerBound(x[e],0);
73  }
74
75
76  //Flow conservation constraints for the nodes (except for 's' and 't')
77  for(NodeIt n(g);n!=INVALID;++n) if(n!=s&&n!=t) {
78    LpDefault::Expr ex;
79    for(InEdgeIt  e(g,n);e!=INVALID;++e) ex+=x[e];
80    for(OutEdgeIt e(g,n);e!=INVALID;++e) ex-=x[e];
81    lp.addRow(ex==0);
82  }
83 
84  //Objective function: the flow value entering 't'
85  LpDefault::Expr obj;
86  for(InEdgeIt  e(g,t);e!=INVALID;++e) obj+=x[e];
87  for(OutEdgeIt e(g,t);e!=INVALID;++e) obj-=x[e];
88  lp.setObj(obj);
89
90
91  //Maximization
92  lp.max();
93
94#ifdef HAVE_GLPK
95  lp.presolver(true);
96  lp.messageLevel(3);
97#endif
98
99  //Solve with the underlying solver
100  lp.solve();
101
102  return lp.primalValue();
103}
104
105int main(int argc, char *argv[])
106{
107  if(argc<2)
108  {
109      std::cerr << "  USAGE: lp_maxflow_demo <input_file.lgf>" << std::endl;
110      std::cerr << "  The file 'input_file.lgf' has to contain a max "
111                << "flow instance in\n"
112                << "  LEMON format (e.g. sample.lgf is such a file)."
113                << std::endl;
114      return 0;
115  }
116
117
118  //input stream to read the graph from
119  std::ifstream is(argv[1]);
120
121
122  ListGraph g;
123  ListGraph::Node s;
124  ListGraph::Node t;
125 
126  ListGraph::EdgeMap<double> cap(g);
127 
128  GraphReader<ListGraph> reader(is,g);
129  reader.readNode("source",s).readNode("target",t)
130    .readEdgeMap("capacity",cap).run();
131 
132  std::cout << "Max flow value = " << maxFlow(g,cap,s,t) << std::endl;
133
134}
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