C# - How to add values to a 3D array?

Question

I'm building an interactive map using Mapbox and would like to draw a polygon over a specific area like is shown here. For this I need to dynamically fill a 3D array with the X,Y and Z coordinates from a database. The array structure I want to achieve is:

[
  [
    [
      [xCoordinate1, yCoordinate1, zCoordinate1],
      [xCoordinate2, yCoordinate2, zCoordinate2],
      [xCoordinate3, yCoordinate3, zCoordinate3]
    ]
  ]
]

I have been trying to accomplish this with C#. In my application I initialized a 3D list, like so:

List<List<List<List<double>>>> coordinates = new List<List<List<List<double>>>>();

Next I iterated over the coordinates that are coming from the database so they would be added to the array:

foreach (var coordinate in Coordinates) {
  coordinates.Add({ coordinate.X, coordinate.Y, coordinate.Z })
}

However this doesn't add the values at the disired position and throws an IndexOutOfBounds exception. I have also tried to initialize the array, like so:

double[, , ,] coordinates = {
  { 
    { 
      { coordinate.X, coordinate.Y, coordinate.Z }, 
      { coordinate.X, coordinate.Y, coordinate.Z }, 
      { coordinate.X, coordinate.Y, coordinate.Z }
    }
  }
};

With this approach i was also unable to format my array the way it should be formatted. Can someone show me how to work with a complex 3D array so that it gets the structure I'm looking for?

To sum up:

int[,,,] array3D = new int[,,,] { 
  { 
    {
      { 1, 2, 3 },
      { 4, 5, 6 } 
      //How can I add more here dynamically?
    }
  }
};

array3D[0, 0, 0, 3] = { 7, 8, 8 }; //This doesn't do the trick :(

Try changing the var to the actual type. You need to add a list of lists of lists to coordinate — doctorlove
– doctorlove, Commented May 20, 2019 at 16:29
1. The linked example has a) only 3 levels and b) only 2D coordinates. Why do you want 4 levels and 3D coordinates? 2. Please, don't use the same name for two different variables (Coordinates and coordinates). That's confusing. — Thomas Weller
– Thomas Weller, Commented May 20, 2019 at 16:38
1. You are right, I think I misunderstood the difference between a 4D array and an array with 3 levels and 2D coordinates. I would like to achieve the structure from the first snippet. 2. Thanks for you tip, I think it's pretty helpful. — Mitch
– Mitch, Commented May 20, 2019 at 17:12

John Alexiou · Accepted Answer · 2019-05-21 14:21:27Z

You cannot change the size of a multidimensional array, but that is ok because your JSON really represents an array of arrays of sorts.

Start with the (expandable) list of coordinates

var coords = new List<double[]>
{
    new double[] { 1,2,3 },
    new double[] { 4,5,6 },
};
// later
coords.Add(new double[] { 7, 8, 9 });

Then convert to the JSON structure for export. You showed an array of array of array of coordinates (array).

var json = new double[][][][] {
    new double[][][] {
        coords.ToArray()
    }
};

This is how you recover the coordinates

foreach (var item in json[0][0])
{
    Debug.WriteLine($"Point=({item[0]}, {item[1]}, {item[2]})");
}

In the output window, you see

// Point=(1, 2, 3)
// Point=(4, 5, 6)
// Point=(7, 8, 9)

Ivan B · Accepted Answer · 2019-05-21 13:09:39Z

If I understand correctly, you can get away with just 2D list, where the first list contains sets of coordinates (i.e. (x,y,z)) and the second list simply contains a bunch of first lists, like so:

List<List<double>> coords = new List<List<double>>();
coords.Add(new List<double> { 24, 54, 46 });
coords.Add(new List<double> { 32, 45, 48 });
Console.WriteLine(coords[1][1]);
//Outputs 45.
//Note: xCoord = 24, yCoord = 54, zCoord = 46 in the first list entry

You can make it a separate method or an extension method where the coordinates are passed in as an arguments. It's also possible to loop through the lists to get particular x,y or z coordinate (if you need to search through them in your code).

Creating a list and later add that list to the other list did the trick! Thanks for pointing me in the right direction!
@Mitch It's great that we've solved the problem! Good luck with coding!

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