How do you initialize a 2 dimensional array when you do not know the size

Question

Howdy,

I have a two dimensional array that I need to load data into. I know the width of the data (22 values) but I do not know the height (estimated around 4000 records, but variable).

I have it declared as follows:

float[,] _calibrationSet;
    ....
int calibrationRow = 0;
While (recordsToRead)
{
  for (int i = 0; i < SensorCount; i++)
   {
     _calibrationSet[calibrationRow, i] = calibrationArrayView.ReadFloat();
   }
   calibrationRow++;
}

This causes a NullReferenceException, so when I try to initialize it like this:

_calibrationSet = new float[,];

I get an "Array creation must have array size or array initializer."

Thank you, Keith

Answer 1

As far as I know, arrays must be initialized with a size.

Perhaps you could use a List<float[]> instead.

Answer 2

You can't use an array. Or rather, you would need to pick a size, and if you ended up needing more then you would have to allocate a new, larger, array, copy the data from the old one into the new one, and continue on as before (until you exceed the size of the new one...)

Generally, you would go with one of the collection classes - ArrayList, List<>, LinkedList<>, etc. - which one depends a lot on what you're looking for; List will give you the closest thing to what i described initially, while LinkedList<> will avoid the problem of frequent re-allocations (at the cost of slower access and greater memory usage).

Example:

List<float[]> _calibrationSet = new List<float[]>();

// ...

while (recordsToRead)
{
    float[] record = new float[SensorCount];
    for (int i = 0; i < SensorCount; i++)
    {
     record[i] = calibrationArrayView.ReadFloat();
    }
    _calibrationSet.Add(record);
}

// access later: _calibrationSet[record][sensor]

Oh, and it's worth noting (as Grauenwolf did), that what i'm doing here doesn't give you the same memory structure as a single, multi-dimensional array would - under the hood, it's an array of references to other arrays that actually hold the data. This speeds up building the array a good deal by making reallocation cheaper, but can have an impact on access speed (and, of course, memory usage). Whether this is an issue for you depends a lot on what you'll be doing with the data after it's loaded... and whether there are two hundred records or two million records.

Answer 3

You can't create an array in .NET (as opposed to declaring a reference to it, which is what you did in your example) without specifying its dimensions, either explicitly, or implicitly by specifying a set of literal values when you initialize it. (e.g. int[,] array4 = { { 1, 2 }, { 3, 4 }, { 5, 6 }, { 7, 8 } };)

You need to use a variable-size data structure first (a generic list of 22-element 1-d arrays would be the simplest) and then allocate your array and copy your data into it after your read is finished and you know how many rows you need.

Answer 4

I generally use the nicer collections for this sort of work (List, ArrayList etc.) and then (if really necessary) cast to T[,] when I'm done.

Answer 5

you would either need to preallocate the array to a Maximum size (float[999,22] ) , or use a different data structure.

i guess you could copy/resize on the fly.. (but i don't think you'd want to)

i think the List sounds reasonable.

Answer 6

I would just use a list, then convert that list into an array.

You will notice here that I used a jagged array (float[][]) instead of a square array (float [,]). Besides being the "standard" way of doing things, it should be much faster. When converting the data from a list to an array you only have to copy [calibrationRow] pointers. Using a square array, you would have to copy [calibrationRow] x [SensorCount] floats.

        var tempCalibrationSet = new List<float[]>();
        const int SensorCount = 22;
        int calibrationRow = 0;

        while (recordsToRead())
        {
            tempCalibrationSet[calibrationRow] = new float[SensorCount];

            for (int i = 0; i < SensorCount; i++)
            {
                tempCalibrationSet[calibrationRow][i] = calibrationArrayView.ReadFloat();
            } calibrationRow++;
        }

        float[][] _calibrationSet = tempCalibrationSet.ToArray();

Answer 7

You could also use a two-dimensional ArrayList (from System.Collections) -- you create an ArrayList, then put another ArrayList inside it. This will give you the dynamic resizing you need, but at the expense of a bit of overhead.