How to correctly initialize a 2D array in a called class

Question

This is throwing an error that EdgeList is not initialized. Here is the class (the pertinent part is way at the bottom):

public class TacLineStruct
{
    // The number of unit groups in the Army
    public int NumGroups
    {
        get
        {
            return _NumGroups;
        }
        set
        {
            _NumGroups = value;
        }
    }
    private int _NumGroups;

    // The number of edges, 
    public int NumEdges
    {
        get
        {
            return _NumEdges;
        }
        set
        {
            _NumEdges = value;
        }
    }
    private int _NumEdges;

    // The number of units below the threshold 
    public int NumBelowThreshold
    {
        get
        {
            return _NumBelowThreshold;
        }
        set
        {
            _NumBelowThreshold = value;
        }
    }
    private int _NumBelowThreshold;

    // The specific Group that a unit belongs to
    public int[] GroupID
    {
        get;
        set;
    }

    // The list of all the edges
    public int[][] EdgeList
    {
        get;
        set;
    }

    // The list of all the edge weights
    public float[] EdgeWeight
    {
        get;
        set;
    }

    // The geographical center of each group
    public Point[] GroupCenter
    {
        get;
        set;
    }

    public TacLineStruct(int arrayLength)
    {
        GroupID = new int[arrayLength];
        int[,] EdgeList = new int[(arrayLength * arrayLength),2];
        EdgeWeight = new float[arrayLength * arrayLength];
        GroupCenter = new Point[arrayLength];
    }
}

And this is how I'm calling and initializing it (snippet):

TacLineStruct TLS = new TacLineStruct(Army.Count);

for (int i = 0; i <= Army.Count; i++)
{
    for (int j = i + 1; j <= Army.Count; j++)
    {
        TLS.EdgeList[NumEdges][0] = i;     /* first vertex of edge */
        TLS.EdgeList[NumEdges][1] = j;     /* second vertex of edge */
        
        // ...
    }
}

I'm getting a runtime error that EdgeList is not initialized. My best guess is that I'm not doing something correctly with a 2D array with the length set at runtime.

In the constructor you initialize a local variable and not the class member (EdgeList). In addition, I think your loop needs to be < Army.Count. Also, you keep setting the same array cell again and again inside the loop. — Metheny
– Metheny, Commented Apr 25, 2019 at 18:45
I tried to indicate that I cut out a lot of stuff in the loop that wasn't relevant to the problem. NumEdges gets incremented further down — zetar
– zetar, Commented Apr 25, 2019 at 18:51

adjan · Accepted Answer · 2019-04-25 18:55:35Z

2

In your constructor, you are doing:

int[,] EdgeList = new int[(arrayLength * arrayLength), 2];

which creates a new (local) variable with the same name as the field. Instead you should do:

this.EdgeList = new int[(arrayLength * arrayLength), 2];

You could omit the this, but it can prevent you from making this mistake again.

Further, you should change the field declaration to

public int[,] EdgeList

Then you can set individual fields in the array via:

 EdgeList[i,j] = value;

edited Apr 25, 2019 at 18:55

answered Apr 25, 2019 at 18:42

adjan

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6 Comments

zetar Over a year ago

This throws a compiler error: Invalid rank specifier.

Metheny Over a year ago

Your class member is defined differently (as a jagged 2d array) You need to decide which type of a 2d array you're using

adjan Over a year ago

Well you had the field declared as a jagged array (array of arrays), you should rather be using a 2D array. I've edited my answer

zetar Over a year ago

Thanks! That's the ticket. What's the difference between arrays like [][[] and [,]?

Metheny Over a year ago

@zetar: [,] can only be a "rectangular" array, which means that every row has the same amount of columns (e.g. 2x3, 2x2, 4x2 etc). [][] means that every row can have a different number of columns (i.e. jagged).

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