How create many list as array?

Question

I have a problem on creating a set of list in array

this is my coding but it's wrong one, any correction for this?

List<string>[] item = new List<string>[10]();

I want to create 10 list in string named as item but I can't do it

and then how can I store more than 1 element in each of the 10 item list??

item[1].add(a);     //when I want to print that a I use item[1][0]
item[1].add(b);     //when I want to print that b I use item[1][1]

item[2].add(aa);
item[2].add(bb);

but how can I store element in each of the list?

You're probably looking for List<string[]> item = new List<new string[10]>(); or something similar — Tim
– Tim, Commented Jun 17, 2015 at 3:18
It's I can ultilize the list like this? item[1] or item[5]................ — Green Hill
– Green Hill, Commented Jun 17, 2015 at 3:20
No. You'd have to do item[1][1]; or similar syntax. Or just use a simple List<string>, unless you really want a List that contains 10 element string arrays, where each element in the List<T> is a 10 element string array. — Tim
– Tim, Commented Jun 17, 2015 at 3:21
No, I want 10 list that can contain many element, 10 item with dynamic space — Green Hill
– Green Hill, Commented Jun 17, 2015 at 3:24
A List<T> can contain any number of elements - it's dynamic. Are you saying you want 10 separate instances of a List<T>, or just one with a dynamic number of elements? — Tim
– Tim, Commented Jun 17, 2015 at 3:25

David L · Accepted Answer · 2015-06-17 13:20:30Z

1

If you KNOW for certain that you want exactly ten lists, you can use an array instantiated to 10 items, each a list of string.

List<string> [] items = new List<string> [10];

Each List is not initialized, so you need to initialize your list before you can use it and each list can be accessed via normal indexer syntax..

if (items[0] == null)
    items[0] = new List<string>();

Once initialized, you can fill in your data.

items[0].Add("another string");

If you wanted to pre-initialize each list so that you do not get a NullReferenceException, do so in a loop.

for (var i = 0; i < items.Length; i++)
    items[i] = new List<string>();

However, if you think that your items may need to hold more List<string> down the road, just simply use a list of lists.

List<List<string>> items = new List<List<string>>();

List wraps array and gives you some nice syntactic sugar and optimizations for expanding arrays which makes your life a lot easier. You can still use indexer syntax to access each list within your list.

if (items[0] == null)
    items[0] = new List<string>();

items[0].Add("another string").

edited Jun 17, 2015 at 13:20

answered Jun 17, 2015 at 3:32

David L

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2 Comments

Green Hill Over a year ago

items[0].Add("another string"); when I use this, it's got error and say that it's null

David L Over a year ago

@GreenHill It's because you are not initializing that list instance. In both techniques I initialize the list instance inside of the parent array/list before using. You cannot miss this crucial step! I also added a quick example of how to initialize each list when using the array parent.

Hari Prasad · Accepted Answer · 2015-06-17 03:37:47Z

1

As per your comments

"I want 10 separete list that have dynamic space"

You can define your collection as follows.

List<string> [] collection= new List<string> [10];

for(int i=0; i<10; i++)
collection[i] = new List<string>();

or, if you don't care size of the array then you can use this.

List<List<string>> collection = new List<List<string>>();

answered Jun 17, 2015 at 3:37

Hari Prasad

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1 Comment

Jagannath Over a year ago

I like the List<List<string>> approach.

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