在不使用.Add（）的情况下动态添加项目到列表<int>按索引

Question

I was trying to add items to list without using .Add() dynamically , Please notice I cant use alternative data structures like dictionary or simple array or even a HashSet due to the nature of my project. I need a workout solution to solve the below problem as it is :

List<int> initA = new List<int>();
for (int i = 0; i < m; i++)
     initA[i] = i;

UPDATE : However if anyone suggests me a better datastructure to implement the following idea then I can use it as alternative :

I have set such that A for example :

A[0] = (1,2,3,4,5...)
A[1] = (10,20,30,40 ...)

and I need direct access such that : A[0][1]=2 , A[1][3]=40 I don't know what is the size of the inner array (the actual size of set elements) thats why I implement this set as Array of Lists and I cant use Jagged Array for the inner array since I don't know its size. the set elements are added at the runtime.

Any Suggestions for solving the first Issue ? or even a suggestion to replace the strategy of represent such set ?

Please notice that I need effecient data strcture since A will store >1M sets.

This is very easy task in C++ where you can use pointers ... in fact this my work around for the problem since its risky to use unsafe code in C#

Answer 1

You can't.

A list data structure only allows you to add items with the Add method (or similar methods, eg AddRange ). The indexer can only be used to modify an entry at a specific position. It cannot be used to add entries to an initially empty list.

So, you definitely need to fill your list before you can modify an item at a specific position, eg:

List<int> initA = new List<int>(new int[m]);
for (int i = 0; i < m; i++)
     initA[i] = i;

Note that the first line is equivalent to adding m zeroes in a loop, so, technically, you did not avoid "adding" items.

Answer 2

You can work around with Dictionary:

Usage:

Vector<string> v = new Vector<string>();
v[0] = "item0";
v[10] = "item10";
string s = v[10]; //s will be "item10"

Code:

class Vector<T>
    {
        Dictionary<int,T> list;

        public Vector ()
        {
           this.list = new Dictionary<int,T>();
        }

        public T this [int index]{
            get
            {
                if (list.ContainsKey(index))
                    return list[index];
                else return default(T);
            }
            set {
                if (list.ContainsKey(index))
                    list[index] = value;
                else 
                    list.Add(index,value);
            }

        } 
    }

Answer 3

Obviously you can't. List is concrete object with concrete behavior which cannot be simply changed. A dirty workaround would be to pre-assign the list with default values:

var l = new List<int>(Enumerable.Repeat(0, numberOfValuesINeed));

or even:

var l = new List<int?>(Enumerable.Repeat((int?)null, numberOfValuesINeed));

So you are able to figure out which values have not been assigned.

A better option would be to implement your own IList (using List as the internal storage would make it pretty simple if you don't mind potential O(n) inserts).

But this is all syntactic sugar, so I would either use the language as intended or use a different language that offers the dynamic behavior required. I cannot think of a none academic scenario where these constraints would apply.

Answer 4

If the index is such important, you have to make sure that the element at that index actually exists or could be added.

List<int> initA = new List<int>();
for (int i = 0; i < m; i++)
     initA.SetValueAtIndex(i, i);


public static void SetValueAtIndex<T>(this IList<T> list, int index, T value)
{
  while (list.Count <= index + 1)
  {
    list.Add(default(T));
  }
  list[index] = value;
}

The weakness of these code is the .Add(default(T)) part. It adds items where the actual data isn't known. It this is not a problem in your case, don't mind. Another problem is that you could pass in a high number as index, which leads to a huge list. If this could be the case, you should definitively go with a dictionary.

Alternatively you can use an array when you know the number of items:

int[] initA = new int[m];
for (int i = 0; i < m; i++)
     initA[i] = i;