Create a linked vector and lists

Question

I want to create vector which stores pointers to lists as shown in image. I don't know how many lists will be needed here. So, I want to write function like this

vector<node*> address; //node is class.
if ((int)address.size()<number)  //number is integer taken as input to function.
    { while((int)address.size()!=number)
        {address.push(/*here I want some function which will generate
 empty list and return pointer to head node or any information 
that will help to access list later*/)
        }
else
{    address[number-1].push_back(name); //name has type string;
//this line may be wrong for syntax but idea is A[i] gives me 
   // list where I want to put name.

}

Preferably using STL library.

Answer 1

If you want to use STL library, then just use

std::vector<std::list<node>> address; //node is class (note that you can pass size here)

// Using your code in your code:
if ((int)address.size() < number)  //number is integer taken as input to function.
{
        address.resize(number);
}
else
{    address.back().push_back(name); //name has type string;

}

Please note that the node is type of the elements you want to push into the vector. As @john said, if you want to keep a list of strings then declare address as:

 std::vector<std::list<std::string>> address;

Also, if you get errors because of >> , either compile this as C++11, or write address as:

 std::vector<std::list<std::string> > address;

Answer 2

It is not much clear, but I guess you want an automatic resize container (as javascript vectors) where lists indexes are 1 based (ie no list at address 0) and a method to insert at the end of one list at a given index. Basically something as this:

struct MyCollection: public std::vector<std::list<std::string> > {
    void push_at(size_t index, const std::string& item) {
        resize(std::max(size(),index - 1);
        at(index - 1).push_back(item);
    }
};

Anything else you might want from such container is likely to be already implemented in the vector and list template classes (check stl documentation to see whats available), for example:

MyCollection a; //declares an empty collection
a.push_at(6,"hello"); //create 6 lists and inserts "hello" at the end of the last one
a[5]; //gets 6th list (method [] is 0-based on vector)
a.push_at(6,"hi"); // since list already exists it just adds "hi" at the end
a[5].front() //"hello"
a[5].back() //"hi"

Other suggestions:

• if you plan to put a lot of items in there and do not need to have all lists object at consecutive addresses (ie compatibility of vector with C arrays), I suggest you use a deque instead of a vector , or provide a proper size hint with reserve , otherwise you might be find yourself wondering why sometimes adding a single string in a list that does not exists yet is so slow.
• STL uses 0-based indexes all around. You will only get confused by defining a 1-based one, so count your lists from 0 (just replace index - 1 with index in the example above) and do your math on application logic)