Changing a 2D array to jagged array

Question

In my code, I've using a 2D multidimensional array to represent a grid (not always of equal sizes, eg 10x15 or 21x7). After reading about how jagged arrays are faster and are generally considered better, I decided I would change my 2D array into a jagged array.

This is how I declared the multidimensional array:

int[,] array = new int[10, 10];

I'm trying to figure out how to declare and then initialise the same thing, but using jagged arrays.

Edit This code is inside a class, and in the constructor I already have:

class ProceduralGrid
{
    private int[][] grid;

    private int _columns;
    private int _rows;

    public ProceduralGrid(int rows, int columns)
    {
        _rows = rows;             //For getters
        _columns = columns;

        //Create 2D grid
        int x, y;
        grid = new int[rows][];

        for (x = 0; x < grid.Length; x++)
        {
            grid[x] = new int[10];
        }
    }

    public int GetXY(int rows, int columns)
    {
        if (rows >= grid.GetUpperBound(0) + 1)
        {

            throw new ArgumentException("Passed X value (" + rows.ToString() +
                ") was greater than grid rows (" + grid.GetUpperBound(0).ToString() + ").");
        }
        else
        {
            if (columns >= grid.GetUpperBound(1) + 1)
            {

                throw new ArgumentException("Passed Y value (" + columns.ToString() +
                    ") was greater than grid columns (" + grid.GetUpperBound(1).ToString() + ").");
            }
            else
            {
                return grid[rows][columns];
            }
        }
    }
}

And in another method I'm simply doing:

    Console.WriteLine(grid.GetXY(5, 5).ToString());

Error message I'm getting:

Unhandled Exception: System.IndexOutOfRangeException: Array does not have that m
any dimensions.
   at System.Array.GetUpperBound(Int32 dimension)
   at ProcGen.ProceduralGrid.GetXY(Int32 rows, Int32 columns) in C:\Users\Lloyd\
documents\visual studio 2010\Projects\ProcGen\ProcGen\ProceduralGrid.cs:line 115
   at ProcGen.Program.Main(String[] args) in C:\Users\Lloyd\documents\visual stu
dio 2010\Projects\ProcGen\ProcGen\Program.cs:line 27

What am I doing wrong and how should I be doing it?

Not sure what GetLength(0) is supposed to do, but you could just use grid.Length. — Codecat
– Codecat, Commented Aug 11, 2013 at 21:33

dtb · Accepted Answer · 2013-08-12 08:36:57Z

4

Since you're dealing with one-dimensional arrays, you can simply use the Length Property to get the length of the first dimension:

int[][] grid = new int[10][];

for (int x = 0; x < grid.Length; x++)
{
    grid[x] = new int[10];
}

(Using the GetLength Method works as well:)

int[][] grid = new int[10][];

for (int x = 0; x < grid.GetLength(0); x++)
{
    grid[x] = new int[10];
}

The problem with your code is that you're calling grid.GetUpperBound(1) where grid is a one-dimensional array -- it doesn't have a second dimension (index 1) that you could get the upper bound of.

Your GetXY method should look like this:

public int GetXY(int x, int y)
{
    if (x < 0 || x >= grid.Length)
    {
        throw ...
    }

    int[] items = grid[x];

    if (y < 0 || y >= items.Length)
    {
        throw ...
    }

    return items[y];
}

Note that jagged arrays are not magic that makes your code faster – measure if they actually do!

edited Aug 12, 2013 at 8:36

answered Aug 11, 2013 at 21:35

dtb

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5 Comments

user9993 Over a year ago

To clarify, this code is in a class, and in the class constructor I already have private int[][] grid; I forgot to mention that bit.

dtb Over a year ago

Then I'm not sure where your error messages comes from. Can you add the code for a short but complete program that demonstrates the problem? What's the error message you're receiving exactly?

user9993 Over a year ago

Ok, I have added the class.

user9993 Over a year ago

Ok thanks, that work's great! I also have a SetXY value that is identical to the GetXY that I posted, how would I go about changing that?

dtb Over a year ago

Replace return items[y]; with items[y] = value;.

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