Assigning members of a list of list to a 2D array in C#

Question

I create a list of list:

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

Then assign some values to it. Assume the assignment is as follows:

            List<string> B = new List<string>();
            for (int j = 0; j < x; j++)
            {
                B.Clear();
                for (int i = 0; i < y; i++)
                    B.Add(i.ToString());           
                C.Add(B);
             }

x and y vary at each run of the code. So, I do not know them beforehand. (At this point of the code, I do not know how to find the size of C. I mean x and y, not x*y.)

I would like to assign/copy C to a 2D array, D[,]. Before the declaration of D, size of D (i.e., rows and columns that correspond to x and y) should have been calculated in some way by using C. How can I make the assignment?

Do you realize you're only adding strings to a single list (`B`) and then adding that same list over and over to `C`? — Rufus L, Mar 20 '23 at 21:44
Now it is even more clear that @RufusL was right - you have only one array assigned to all rows... Please review https://stackoverflow.com/questions/2156482/why-does-adding-a-new-value-to-list-overwrite-previous-values-in-the-list — Alexei Levenkov, Mar 20 '23 at 22:16

score 1 · Accepted Answer · answered Mar 20 '23 at 21:45

You can't just assign a jagged array value to a 2D array because these structures store data differently. A jagged array is an array each element of which stores a reference to another array, while a two-dimensional array is a series of elements stored in memory. See also "Multidimensional Arrays (C# Programming Guide)" article.

A jagged array is an array whose elements are arrays, possibly of different sizes. A jagged array is sometimes called an "array of arrays." The following examples show how to declare, initialize, and access jagged arrays.

Also, you cannot simply assign the value of a list of lists to a variable of two-dimensional array or jagged array type, because these are different data types that require an explicit conversion.

So you need to copy each element to the new array.

List<List<string>> C = FillListWithSomeValues();

int numRows = C.Count;
int numCols = C[0].Count;

string[,] D = new string[numRows, numCols];

for (int row = 0; row < numRows; row++)
    for (int col = 0; col < numCols; col++) 
        D[row, col] = C[row][col];

score 1 · Answer 2 · answered Mar 20 '23 at 21:48

1

You can find the length of the longest list for the 2D array definition:

int longDimension = C.Max(list => list.Count);

answered Mar 20 '23 at 21:48

Rufus L

36,127
5
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score -1 · Answer 3 · answered Mar 20 '23 at 21:46

-1

You must, first of all, asign values to x and y and declare and initialize b before. Then assign the value of C to D. At least thats one way to do it.

Good luck!

answered Mar 20 '23 at 21:46

Victorio Gilistro

19
2

He already stated that `x` and `y` values change over the course of the code. The question is how to find the correct bounds for assigning the jagged array to a 2D array. – Rufus L Mar 20 '23 at 21:47

Assigning members of a list of list to a 2D array in C#

3 Answers3