C++ Vector object question

focusdoit

Hi, I read a block of code, like:

class Graph
{
public:
int V; // number of vertices
vector *adj; //adjacency list

Graph(int V);
void addEdge(int x, int y);
bool isRoute(int x, int y);

};

// Constructor
Graph::Graph(int V)
{
this->V = V;
this->adj = new vector[V];
}

// add a directed edge from x to y
void Graph::addEdge(int x, int y) {
adj[x].push_back(y);
}

int main() {

Graph g(6);
g.addEdge(5, 2);
g.addEdge(5, 0);
g.addEdge(4, 0);
g.addEdge(4, 1);
g.addEdge(2, 3);
g.addEdge(3, 1);

}

I wonder the adj is a pointer that points to a Vector object, and a vector object I think it's like an one-dimensional array, but the addEdge() operation, adj[x].push_back(y),it make another array[x,y]. in Main(), the structure of the vector object, seems like: [5,2,0] [4,0,1] [2,3] [3,1] Then this is not an one-dimensional array.so this is a Vector object, or 4 vector objects? Thanks

Lost User

Not sure what any of that means. But why not use a POINT structure (or create your own), and then you can have a vector of points?

struct POINT
{
int x;
int y;
};
vector pointList;

markkuk

focusdoit wrote:

I wonder the adj is a pointer that points to a Vector object

No, it's an array of vectors with V elements. So it's a two-dimensional data structure.

samzcs

vector *adj; //adjacency list is it point to an Vector object?

Lost User

Yes, it's a pointer to a vector, but you are trying to assign an array of vectors to it. Why not use the suggestion I offered above?

samzcs

Thanks, I am reading the program. Just try to understand why a pointer to an array, be used as an array of vectors.

samzcs

is it similar to a pointer that points to an integer. like: int a = 0; int *ip = &a; then (ip+1) point to an integer too. but do we need to allocate memory space to it?

Lost User

It's not a pointer to an array, it's a pointer to a vector (singular). If you want a pointer to an array of vectors then you need something like:

vector *varray[];

Then each element of varray will need to be a vector*, that is, a pointer to a vector. A vector is an instance of the vector class so each pointer will point to a single vector.

Lost User

Yes, you must always allocate memory if you intend to read from or write to it. A pointer declaration does not allocate any memory, it is just a single variable that can be used to address individual items. But the memory space to hold the items must be allocated before you use the pointer.

markkuk

Memory is allocated in the constructor:

this->adj = new vector[V];

samzcs

Thanks, I understood