Graph Implementation with Adjacency Matrix and Strings

Question

I'm trying to change my graph from accepting an integers as Vertices to strings. The folllowing is my Graph and Edge structures:

typedef struct GraphRep {
    int **edges;    // adjacency matrix storing positive weights
                    // 0 if nodes not adjacent
    int nV;         // #vertices
    int nE;         // #edges
} GraphRep;

typedef struct GraphRep *Graph;

typedef char *Vertex;

typedef struct Edge {
    Vertex v;
    Vertex w;
    int weight;
} Edge;

The following functions were written previously now having problems because I no longer can do indexing...

// check if vertex is valid in a graph
int validV(Graph g, Vertex v) {
    return (g != NULL && v >= 0 && v < g->nV);
}


void insertEdge(Graph g, Edge e) {
    assert(g != NULL && validV(g,e.v) && validV(g,e.w));

    if (g->edges[e.v][e.w] == 0) {   // edge e not in graph
        g->edges[e.v][e.w] = e.weight;
        g->nE++;
    }
}

I'm really stuck and not sure where to go from here. Could you please suggest what should I do to make it work? Also it would be appreciated if you can point me to an example of Graph implementation with vertices are strings.

Thank you very much!