Question

UPDATE:
The following code gives me an error

Graph.cpp: In function ‘std::ostream&
operator<<(std::ostream&, const
Graph&)’: Graph.cpp:43: error: passing
‘const std::map >,
std::less,
std::allocator > > > >’ as
‘this’ argument of ‘_Tp&
std::map<_Key, _Tp, _Compare,
_Alloc>::operator[](const _Key&) [with _Key = long int, _Tp = std::vector >, _Compare =
std::less, _Alloc =
std::allocator > > >]’
discards qualifiers Graph.cpp:44:
error: passing ‘const std::map >,
std::less,
std::allocator > > > >’ as
‘this’ argument of ‘_Tp&
std::map<_Key, _Tp, _Compare,
_Alloc>::operator[](const _Key&) [with _Key = long int, _Tp = std::vector >, _Compare =
std::less, _Alloc =
std::allocator > > >]’
discards qualifiers make[2]: *
[build/Debug/GNU-MacOSX/Graph.o] Error
1

class Graph {
public:
    Graph();
    Graph(const Graph& orig);
    virtual ~Graph();

    void clear();
    void rgg(lint, double);
    bool is_directed() { return directed; }
    friend ostream& operator<< (ostream&, const Graph&); 


private:
    map< lint, vector<lint> > adjList;
    vector< pair<double, double> > xy;
    double radius;
    MTRand Rand;
    bool directed;
};


void Graph::rgg(lint n, double r) {
    radius = r; directed = false;

    clear(); 

    for(lint i = 0; i < n; i++)
        xy.push_back(pair<double, double>(Rand.rand(), Rand.rand()));
}

ostream& operator<< (ostream& os, const Graph& inGraph) {
    for(lint i = 0; i < inGraph.nodes; i++) {
        os << i << " ";
        if( inGraph.adjList.find(i) != inGraph.adjList.end() ) {
            for(lint idx = 0; idx < (inGraph.adjList[i]).size(); idx++ )
                os << inGraph.adjList[i].at(idx) << " ";

        }
        os << endl;
    }
}

Thank you in advance,

Answer 1

The cause of your problems is that map’s operator[] is a mutable operation (if the key doesn’t exist, it will be added to the map).

You will have to use the iterator returned from find():

map< lint, vector<lint> >::const_iterator it = inGraph.adjList.find(i);
if (it != inGraph.adjList.end();
for(lint idx = 0; idx < it->size(); idx++ )
    os << it->at(idx) << " ";