I am boost noob. I am wondering why compilation fails in the following code. I am creating a set of vertices, and trying to assign my own vertex indices and vertex names. (I am following this page: http://fireflyblue.blogspot.com/2008/01/boost-graph-library.html. )
I understand that vertS
vertex lists in Boost
does not need explicit vertex id creations, and I have also seen this very related question in Stackoverflow (how provide a vertex_index property for my graph) which discusses how to use an associative_property_map
to assign vertex indices. The following though - getting the vertex_index map, and assigning the key value pairs - seems a fairly straightforward thing to do, and I would like to understand why it fails. Any help is greatly appreciated!
The compile error is as below:
error: expression is not assignable
vertIndx[v] = i;
//Define graph
typedef boost::property<boost::vertex_name_t, std::string> sv_namePty;
typedef boost::property<boost::vertex_index_t, int, sv_namePty > sv_indx_n_name_pty;
typedef boost::property<boost::edge_weight_t, int> se_weightPty;
typedef boost::adjacency_list<boost::vecS, boost::vecS, boost::undirectedS,
sv_indx_n_name_pty, se_weightPty> ScafGraph;
//descriptors
typedef boost::graph_traits<ScafGraph>::vertex_descriptor SV;
typedef boost::graph_traits<ScafGraph>::edge_descriptor SE;
//Graph Object
ScafGraph SG;
//property accessors
boost::property_map<ScafGraph,
boost::vertex_name_t>::type vertName = boost::get(boost::vertex_name, SG);
boost::property_map<ScafGraph,
boost::vertex_index_t>::type vertIndx = boost::get(boost::vertex_index, SG);
boost::property_map<ScafGraph,
boost::edge_weight_t>::type edgeWeight = boost::get(boost::edge_weight, SG);
//Populate Graph
std::vector<SV> svlist;
for(int i=0; i<4; i++) {
SV v = boost::add_vertex(SG);
svlist.push_back(v);
vertName[v] = std::to_string(i);
vertIndx[v] = i;
}
The expression vertIndx[v]
returns a Vertex by value. Thus you get the error because it's not an lvalue when you try to assign to it.
Furthermore, it actually returns v
. Here's the code run by vertIndx[v]
:
inline value_type operator[](key_type v) const { return v; }
Here's a version that is hopefully clear about how it works:
#include <boost\graph\adjacency_list.hpp>
int main()
{
//Define graph
typedef boost::adjacency_list
<
boost::vecS //! edge list
, boost::vecS //! vertex list
, boost::undirectedS //! undirected graph
, boost::property<boost::vertex_name_t, std::string> //! vertex properties : name
, boost::property<boost::edge_weight_t, int> //! edge properties : weight
> ScafGraph;
//descriptors
typedef boost::graph_traits<ScafGraph>::vertex_descriptor SV;
typedef boost::graph_traits<ScafGraph>::edge_descriptor SE;
//Graph Object
ScafGraph SG;
//property accessors
boost::property_map<ScafGraph,
boost::vertex_name_t>::type vertName = boost::get(boost::vertex_name, SG);
boost::property_map<ScafGraph,
boost::vertex_index_t>::type vertIndx = boost::get(boost::vertex_index, SG);
boost::property_map<ScafGraph,
boost::edge_weight_t>::type edgeWeight = boost::get(boost::edge_weight, SG);
//Populate Graph
std::vector<SV> svlist;
for (int i = 0; i < 4; i++) {
SV v = boost::add_vertex(ScafGraph::vertex_property_type(std::to_string(i)), SG);
svlist.push_back(v);
assert(vertName[v] == std::to_string(i));
assert(vertIndx[v] == i);
}
return 0;
}
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