Convert from List<double[]> to double[,]

Question

Is there an one-liner (no looping) that converts List<double[]> to double[,]?

Answer 1

If a 2 dim array is to be created from List of 1 dim array then looping is required, though it may not look like that at the call-site.

Answer 2

Converting to double[,] can only be done by looping through the list and requires that all arrays contained in the list are of same size:

double[,] arr = new double[list.Count, list[0].Length];
for (int i = 0; i < list.Count; i++)
{
    for (int j = 0; j < list[0].Length; j++)
    {
        arr[i, j] = list[i][j];
    }
}

Of course, you can easily create a jagged double[][] array of arrays by calling .ToArray():

double[] array = new double[] { 1.0, 2.0, 3.0 };
double[] array1 = new double[] { 4.0, 5.0, 6.0 };

List<double[]> list = new List<double[]>();
list.Add(array);
list.Add(array1);

double[][] jaggedArray = list.ToArray();

Answer 3

Well, you probably can't implement it without loops, but you can make the usage a one-liner :

double[,] array = list.To2DArray();

To2DArray is an extension method implemented as follows:

public static class ExtensionMethods
{
    public static T[,] To2DArray<T>(this IEnumerable<IEnumerable<T>> source)
    {
        var jaggedArray = source.Select(r => r.ToArray()).ToArray();
        int rows = jaggedArray.GetLength(0);
        int columns = jaggedArray.Max(r => r.Length);
        var array = new T[rows, columns];
        for (int i = 0; i < rows; i++)
        {
            for (int j = 0; j < jaggedArray[i].Length; j++)
            {
                array[i, j] = jaggedArray[i][j];
            }
        }
        return array;
    }
}

Note that it will only work in C# 4, since earlier versions don't support covariance. This variant should work in C# 3 but it is more specific:

public static T[,] To2DArray<T>(this IEnumerable<T[]> source)
{
    var jaggedArray = source.ToArray();
    // same code from here
}

Answer 4

public static T[,] ToMultidimensional<T>(this T[][] arr, int maxSize)
{
    T[,] md = (T[,])Array.CreateInstance(typeof(double), arr.Length, maxSize);

    for (int i = 0; i < arr.Length; i++)
    for (int j = 0; j < arr[i].Length; j++)
        md[i, j] = arr[i][j];

    return md;
}

var arr = new List<double[]>
{
    new double[] { 1, 2, 3 },
    new double[] { 4, 5 }
}
.ToArray();

var j = arr.ToMultidimensional(arr.Max(a => a.Length));