Re: [Boost-users] graph: PropertyMap needs lvalue for vertex_index

21 Sep 2010

      ...
On Tue, 21 Sep 2010, Jens Weller wrote:
...
Hi,
I've been playing arround with boost::graph (and the layout algorithms
in special).
Now I encountered a little problem, I'm trying to get Kamada Kawai
Spring to work, but I do get an compile error when calling put for vertex_index.
So, this problem has nothing to do with KKS.
My Code:
typedef boost::adjacency_list >
                        // Edge properties
,boost::property
...
Graph;
Graph g(scene->getNodeCount());
boost::graph_traits<Graph>::vertex_iterator vi,vi_end;
int i = 0;
for (boost::tie(vi, vi_end) = boost::vertices(g); vi != vi_end; ++vi)
   boost::put(boost::vertex_index, g, *vi, i++);
put throws now a compile Error:
..\..\..\cpp\boost_1_43/boost/property_map/property_map.hpp:361: error:
lvalue required as left operand of assignment
The last argument of put does throw this error.
I expected the code to work, as I have copied it out of the layout_tests
file:
http://www.boost.org/doc/libs/1_43_0/libs/graph/test/layout_test.cpp
So, where is my error here?
I tried some different versions and some casts, but none made this code
work.
And I don't think that something that trivial is broken in the
testcases.
An adjacency_list with vecS as the vertex container already contains a 
built-in, read-only vertex_index map.  It is filled in correctly, so you 
should not need to modify it.
Ah, yes, thats infact one of the few differences, I decided to use vecS instead of listS.
Would be listS better?

Also I then have the question, why Kamada Kawai Spring fails in my case:

template<class Graph>
bool doKamadaKawaiSpring(Graph& g, double radius)
{
    // this call works
    circle_graph_layout(g, boost::get(vertex_position, g), radius/2);
    //returns false;
    return kamada_kawai_spring_layout(g,
                                           boost::get(vertex_position, g),
                                           boost::get(boost::edge_weight, g),
                                           boost::square_topology<>(radius * 2.0),
                                           boost::side_length(radius * 2.0));
}

called from:
Graph g(scene->getNodeCount());

boost::graph_traits<Graph>::edge_descriptor ed;
for(size_t i =0,s = edges.size(); i < s; ++i)
{
    ed = boost::add_edge(edges[i].first,edges[i].second,g).first;
    boost::put(boost::edge_weight, g, ed, 1.0);
}
    if(!doKamadaKawaiSpring(g,std::min(ui->graphicsView->height()/2-20.0,ui->graphicsView->width()/2-20.0)))
        QMessageBox::about(this,"Layout Failed","could not calculate Kamanda Kawai Spring Layout fully");

All I know is it returns false.

Documentation says:
...
Returns: true if layout was successful or false if a negative weight cycle was detected or the graph is disconnected.
so, what can I do to get this working?

My Current Graph:
A -> B
A -> C
A -> D
B -> C
B -> D

maybe the Graph is wrong ;)

regards,

Jens Weller
D -> E

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