Expert: Ralph McArdell - 11/4/2010

Question
I have to create a large number of double link list. Is there a way to create these in a function or Algorithm or a loop, or just create each link list separately? I can create a link list, or a double link list, but I can not find anything on creating multiple link list.

Thanks.
Ed

Answer
If you replace double linked list with say int and linked list with say primitive in your question so it read:

   "I have to create a large number of ints. Is there a way to create these in
    a function or Algorithm or a loop, or just create each primitive separately?
    I can create a primitive, or an int, but I can not find anything on creating
    multiple primitives."

Would you still have a problem?

Basically presuming your linked list or doubly linked list classes allow instances to be created in the usual ways then you create them as and when required, as you would with any other object (be they primitives such as ints, chars, floats or class types such as a linked list).

So let us presume you are using the doubly linked list provided by the C++ standard library. In fact this is a class template and you provide it with the element type to create specific doubly linked list classes.

The code fragments below show using the std::list class template to create a specialisation (class) to hold int values, and various ways we can create instances of them. Note that these are code fragments only and not complete programs and are for example use only and are in no way supposed to be meant as production quality. The do of course presume you are using a reasonably modern C++  implementation (i.e. late 1990s or newer) that has a reasonable implementation of the ISO C++ standard library.

   #include <list>

   typedef std::list<int>  ListOfInt;      // type alias for a std::list type that contains int elements.

   ...

   ListOfInt aListOfInt;
   ListOfInt anotherListOfInt;

   unsigned int const ArraySize(100);

   ListOfInt arrayOfListOfInt[ArraySize];             // creates ArraySize lists
   
   ListOfInt * arrayOfPointersToListOfInt[ArraySize]; // creates ArraySize _pointers_ to lists...

   for ( unsigned int i=0; i<ArraySize; ++i )         // .. so have to create lists for them to point to...
   {
       arrayOfPointersToListOfInt[i] = new ListOfInt;
   }

   ...

   for ( unsigned int i=0; i<ArraySize; ++i )         // ... and later remember to delete them
   {                                                  //     as they were created dynamically
       delete arrayOfPointersToListOfInt[i];
   }

We can even have a list of list of whatever in which each element is itself a list. For example following on from the previous examples we might want a list containing lists that contain ints:

   typedef std::list<int>        ListOfInt;
   typedef std::list<ListOfInt>  ListOfListOfInt;

   ...

   ListOfListOfInt  aListOfListOfInt;

Of course we can use other containers to hold lists - built in arrays for example, as in the original example. Maybe using a std::vector instead of a plain built in array:

   #include <list>
   #include <vector>

   typedef std::list<int>          ListOfInt;
   typedef std::vector<ListOfInt>  VectorOfListOfInt;

   ...

   VectorOfListOfInt aVectorOfListOfInt;

Or perhaps a mapping of strings to a list of values (although we could get a similar effect using a C++ library multimap type):

   #include <list>
   #include <map>
   #include <string>

   typedef std::list<int>                    ListOfInt;
   typedef std::map<std::string, ListOfInt>  MapStringToListOfInt;

   ...

   MapStringToListOfInt aMapStringToListOfInt;


So it is really up to you and your requirements as to how you use these building blocks and how exactly you put them together.

Hope this has given you some ideas.