dynamoc array size

Question

Im writing a class library and i want it to handle any type of data.
Q1: do I decelare data members as varName;
Q2: how do i declare an arrya of the above?
is it varName[]   or [] varName
Q3: these arrary are unknown size at design time - how to i allow handle arrays that wont be sized untill the user enters there data?

Vulpes · Accepted Answer

A dynamic array is best dealt with by declaring a generic List. For example to declare a List of ints:
List list = new List ();
list.Add(1);
list.Add(42);
int count = list.Count; // 2
You can declare a generic List of any type you like - even object.
List list2 = new List ();
list2.Add("hello"); // add a string
list2.Add(23.5); // add a double
list2.Add(list); // add a List
int count2 = list2.Count; // 3
To retrieve elements of the generic List just use indexing as you would for an ordinary array:
int i = list[1]; // 42 string s = (string)list2[0]; // "hello"

Vulpes · Answer

Yes, it's possible (and, indeed, common) to have a list of lists. You index them in the same way as 'jagged' two-dimensional arrays (see later). For example:
List > list2d = new List >();     List list = new List ();     list.Add(2);     list2d.Add(list);     int element = list2d[0][0];      Console.WriteLine(element); // 2
You can also do what you suggested, though this would be unusual:
List myList = new  List ();
for(int i = 0; i     {         List newList = new List ();         myList.Add(newList);     }
Each declaration does not need a new name. The local variable newList is recreated each time through the loop and points to a different List instance.
By definition a matrix is rectangular so all its rows are the same length and all its columns are the same length but not necessarily the same as the rows. They would usually be represented by a normal two-dimensional array.
You can also have jagged two-dimensional arrays where the rows can be of different (although fixed) length. For example:
int[][] array2d = new int[2][];     array2d[0] = new int[2]{1, 2};     array2d[1] = new int[3]{3, 4, 5};     int element2 = array2d[1][2];      Console.WriteLine(element2); // 5
These, of course, are analogous to lists of lists execept the latter are not in fixed in length.

Richard Brennan · Answer

how about undimensioned 2d arrays - is it practical or indeed possible to have a list of lists.  and how would you index them.
Is this possible
List myList = new  List ();
for(int i = 0; i newList = new List ();
myList.Add(newList);
}
woule the newList decleration give the compiler a wobbly.  Does each decleartion need a new name - and if so how do u assign the names.
Again it is unknown at design time how long the list of lists may need to be.
I suppose this is similar to a matrix - how can u declare an undimensioned matrix (where each row [or column] can be different lengths).

Vulpes · Answer

Taking those questions in the same order:
1. No, it just uses an ordinary array as its backing store.
This starts off with an initial length of four (IIRC) and then the length is doubled as elements are added to the List and further storage is needed. Each time the length is doubled, a new array is created and the existing elements are copied over from the old one.
Although there is a generic LinkedList class in .NET, it's not used much. This is because linked lists are not very efficient in .NET because of the way that memory for new objects is allocated.
In contrast the generic List is only marginally less efficient than an ordinary array and significantly more efficient than an ArrayList for value types. This is because the latter stores everything internally as of type 'object' and so value types need to be 'boxed' on the heap and then 'unboxed' when they are retrieved which is a slow operation. In contrast the generic List is 'strongly typed', elements are stored as their actual type and so no boxing or casting is necessary.
2. Well, the class in the dll will contain a method, property or field which is of a generic List type. You won't usually need to create an instance of the List yourself because the class will have already done it for you. Sometimes the method or property might return 'null' if no List instance has yet been created for some reason.
Having obtained a reference to the List instance, you could add elements to it either singly (using the Add() method) or plurally (using the AddRange() method). For example if 'ia' is an array of integers:
List list = someObject.GetList(); list.AddRange(ia); int count = list.Count; // ia.Length + the number of elements already in the list
3. If you use the Remove() or RemoveAt() methods then, yes, all the subsequent elements in the list are moved down one and therefore have an index one less than they previously did. You don't have to do anything yourself.

Richard Brennan · Answer

if i have  a list say its called myList
If i do:  myList.RemoeAt(2)   //delete an item
does all the data in postion 3+ automatically move 1 place left to fill the space or do i have to handle that my self?

Richard Brennan · Answer

if the decleartion for the list is in a class library and if i want a constructor for it how could it be done;
1) one constructor to set the list to null
2) could i user constructor(params object varName) and us this to pass an array of data to put in the list. (could i use varNAme.length to count the elements of varNAme?

Richard Brennan · Answer

thanx - by the way does the List use an implementation of a linked list?