Determining size of an array of arrays in C#?

Question

I am creating an array of arrays such that:

var arrNewArray = new string[arrOldArray.Length][7];

arrOldArray is an array of arrays such that it's [X][4], meaning the length of the 1st array or "outside" array can change, but the length of the "inside" array is ALWAYS 4, or hold 4 strings ([0][1][2][3]).

Why won't the compiler accept my statement above?

Essentially, I'm trying to take arrOldArray and expand it, or add a few more "columns" by increasing the [4] in the old array to a [7] in the new array and then copy the contents over. Perhaps I'm not doing it the best/efficient way, so any guidance would be appreciated thanks.

Answer 1

I think you want a two dimensional array:

var arrNewArray = new string[arrOldArray.Length, 7];

You would access it like this: arrNewArray[x, y] .

This is better than a jagged array, because it clearly communicates that the number of "columns" is the same for every row.

If you want to continue using a jagged array, you need to do it like this:

var arrNewArray = new string[arrOldArray.Length][];
for(int i = 0; i < arrOldArray.Length; ++i)
    arrNewArray[i] = new string[7];

The reason for this convoluted way is: With a jagged array, each "row" can have a different number of "columns". A short-hand syntax for the case where each "row" has the same number of "columns" doesn't exist. That's why your code doesn't compile.
A jagged array is essential an array of arrays, so you need to create a new array instance for each "row" of the outer array and explicitly assign it. That's what the for loop is doing.

You can't use Array.Copy with jagged arrays. Each child-array is it's own instance and Array.Copy doesn't make a deep copy, it merely copies the references from one array to another. The effect would be, that both arrays would point to the same items and changing an item in one array would be seen from the other.

Answer 2

You are not creating the jagged array properly. The proper way is to create the first dimension of the jagged array and then loop through the items of the first dimension to create the nested arrays and copy the data from the old arrays. Here's an example:

int newSize = 7;
string[][] newArray = new string[oldArray.Length][];
for (int i = 0; i < oldArray.Length; i++)
{
    newArray[i] = new string[newSize];
    Array.Copy(oldArray[i], newArray[i], oldArray[i].Length);
}

Answer 3

你会想要

var arrNewArray = new string[arrOldArray.Length, 7];

Answer 4

var arrNewArray = new[] {new string[7]};//array of arrays
var arrNewArray = new string[arrOldArray.Length, 7];//two-dimensional array

Answer 5

Using Linq:

int[][] jaggedArray2 = new int[][] 
{
    new int[] {1,3,5,7,9},
    new int[] {0,2,4,6},
    new int[] {11,22}
};
int length = jaggedArray.Sum(a => a.Length);

Answer 6

I don't believe what you're asking is directly possible. Because the syntax that you are using is for a jagged array, and what you are doing is effectively asking it to create a multi-dimensional array.

The syntax is confusing since it reads like what you really want is a multi-dimensional array (although I'm aware that's not the case.)

I don't believe you could store your arrays in the newly allocated array either due to a size change. You would need to build a custom copy method to move the data into the larger array.