New Blank 2D array without size

Question

I need to create an array for which I do not know the size in advance. I used the following, but it gives an index out of bounds error in my for -loop.

string[] arr = s.Split('\n');
int[] indices = new int[arr.Length];

string[][] new_arr = new string[arr.Length][];

for (int i = 0; i < arr.Length; i++)
{
    if (arr[i].StartsWith("21")) 
    {
        indices[i] = i;
    }
}

indices = indices.Where(val => val != 0).ToArray(); //contains indices of records beginning with 21

for (int i = 0; i < indices.Length - 1; i++)
{

    new_arr[0][i] = arr[i];
} //first n elements

The error is in the second for-loop. It says

Object reference not set to an instance of an object.

But I did instantiate the string at the beginning?

If you don't know the size, you can use a List instead of an array which will grow on demand — vc 74, Aug 15 '17 at 14:41
It's a [`jagged array`](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/arrays/jagged-arrays) — Jakub Dąbek, Aug 15 '17 at 14:47
@RicheekDey Yes, you can make an array of lists or a list of lists to make it multi-dimensional — Jakub Dąbek, Aug 15 '17 at 14:51
Ah, but I'm new to C#. hence I wanna keep it simple. Arrays I know. Lists...not really. — Richeek Dey, Aug 15 '17 at 14:53
Do you need to have a multidimensional array? What are you trying to achieve? — Jakub Dąbek, Aug 15 '17 at 14:55
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/151970/discussion-between-richeek-dey-and-jakub-dabek). — Richeek Dey, Aug 15 '17 at 14:56
Possible duplicate of [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Camilo Terevinto, Aug 15 '17 at 14:58
Looks like you just need a single dimensional array, since you are using only `[0]` — Vikhram, Aug 15 '17 at 16:05

score 0 · Answer 1 · answered Aug 15 '17 at 14:45

0

You just need to initialize each of the arrays within the jagged array:

indices = indices.Where(val => val != 0).ToArray(); //contains indices of records beginning with 21

// create second-level array 
new_arr[0] = new string[indices.Length] ;

for (int i = 0; i < indices.Length - 1; i++)
{
    new_arr[0][i] = arr[i];  // why 0 here?
}

answered Aug 15 '17 at 14:45

D Stanley

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So what I am trying to do is to create a new column (column 0) with the first n elements from a 1D array. I do not know the "n" (it is dynamic) hence I can't hard-code it. The 0 is to access the 0th column ( it is [i][0], i typed that by mistake) – Richeek Dey Aug 15 '17 at 14:49
Regardless, you need to create each sub-array with a fixed size (or use a list and add elements. I'm not familiar with your process to know what the size of the sub-array should be, though. – D Stanley Aug 15 '17 at 15:07

score 0 · Answer 2 · answered Aug 15 '17 at 14:54

For a 2 dimensional array, you will have to specify the size of the first dimension. As I see your code, it is using what is called the jagged array (array containing array as elements) and not a pure 2 dimensional array.

var array1 = new int[2,10]; // this is a 2 dimensional array
var array2 = new int[2][]; // this is an array of 2 elements where each element is an `int[]`

As you notice, we don't need to specify the size of the inside element int[]

Now you understand the jagged array, you can see that you are not initializing the inside element, but simply accessing it - and since it is null by default, you hit the exception.

for (int i = 0; i < indices.Length - 1; i++)
{
    new_arr[0][i] = arr[i];
} //first n elements

In the code above, you should have new_arr[i][0] = arr[i] and also before that line you need to do new_arr[i] = new int[x] where x is teh size that you want for the inside array.

New Blank 2D array without size

2 Answers2