Difficulty accessing a 2D array index

Unity Engine
1

Historically, I seem to have a lot of problems with arrays. Here’s my current problem… I’m setting up a 2D built-in array of custom objects, and whenever I attempt to access one of the indexes, I get a null reference exception. Here’s my whole code dump. Line 28 is indicative of the problem as trying to view a value in index[0,0] of the array is just erroring out. Any help would be greatly appreciated!

using UnityEngine;
using System.Collections;

public class ArrayController : MonoBehaviour
{
	private GridArray p1Grid;
	private GridArray p2Grid;
	
	public class GridArray
	{
		public Cell[,] cells = new Cell[ 6, 12 ];
	
		public class Cell
		{
			public int cellX = 0;
			public int cellY = 0;
			
			public EmailPiece mail = null;
		}
	}
	
	// Use this for initialization
	void Start ()
	{	
		p1Grid = new GridArray();
		p2Grid = new GridArray();
		
		print(p1Grid.cells[0,0].cellX);
		
		for( int myY = 0; myY < 12; myY++ )
		{
			for( int myX = 0; myX < 6; myX++ )
			{
				// Set up the internal x and y values of the Grid
				// I'm not sure this is necessary, but what the hay
				
				p1Grid.cells[ myX, myY ].cellX = myX;
				p1Grid.cells[ myX, myY ].cellY = myY;
				
				p2Grid.cells[ myX, myY ].cellX = myX;
				p2Grid.cells[ myX, myY ].cellY = myY;
			}
		}
	}
	
	public void SetPiece ( GameObject mail1, GameObject mail2, int player )
	{
		GridArray playerGrid = new GridArray();
		EmailPiece piece1 = (EmailPiece) mail1.GetComponent("EmailPiece");
		EmailPiece piece2 = (EmailPiece) mail2.GetComponent("EmailPiece");
		
		if ( player == 1 )
		{
			playerGrid = p1Grid;
		}
		else if ( player == 2 )
		{
			playerGrid = p2Grid;
		}
		else
		{
			return;
		}
		
		// read the email piece's transform position
		int mailX = (int) mail1.transform.position.x + 8;
		int mailY = (int) mail1.transform.position.y;
		print("mailX: " + mailX + ". mailY: " + mailY);
		
		// find the corresponding index of the player grid
		// set the emailpiece of the index
		playerGrid.cells[ mailX, mailY ].mail = piece1;
		
		//repeat with second piece
		mailX = (int) mail2.transform.position.x;
		mailY = (int) mail2.transform.position.y;
		playerGrid.cells[ mailX, mailY ].mail = piece2;

	}
	
	// Update is called once per frame
	void Update () {
	
	}
}
2

You created the arrays but you didnt initilized the objects…

for every index you should create a Cell object with new Cell before accessing their properties and methods

i.e.

p1Grid.cells[ myX, myY ] = new Cell (); // This is missing!

p1Grid.cells[ myX, myY ].cellX = myX;
.
.
.

also with p2Grid’s cells

i recommend you that you do this initialization on the GridArray constructor since the bidimensional array is one of his properties.

3

Thanks Omar, that’s very helpful. I’ve got it working by creating a new Cell() when I first loop through the array. I’m interested in your suggestion of creating the Cell() class when the constructor is called. I did some experimenting but could not figure this out. Would you mind explaining a little more how this is done with a 2D array? It’d be much appreciated!

4

You could also declare the Cell type as a struct instead of a class. This way you would not have to call new Cell to initialize it as it will be a value type stored directly in your array. (As opposed to the current one where new Cell[6,2] creates an array of uninitialized references to Cell objects.)