Convert from List to double[,]

Question

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

That all depends how long you want your line to be :) (I suspect a few more than 80 chars). — leppie, Aug 23 '10 at 09:00
@abatishchev: In the old days, it was, nowadays, I think people prefer 120. — leppie, Aug 23 '10 at 09:26
@leppie: Never understood such small sizes. 19" monitors has wide spread occurrence. Many developers has monitors much larger that 19 - and 21", and 25". So 140-240 chars is easy to look over, imo — abatishchev, Aug 23 '10 at 09:30
@abatishchev: In the old days text terminals were limited to 80 x 25 - 50 (vertical lines). — leppie, Aug 23 '10 at 09:33
@leppie: Old days - 80x-early 90x - no problem to understand, technical limitation, etc. That's that. But follow such standards in 00x - is a nonsense imo. Specially when it turns into `throw new Exception( + + "some long " + + "text");` — abatishchev, Aug 23 '10 at 09:46

Dirk Vollmar · Accepted Answer · 2010-08-23T09:23:56.697

5

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();

edited Aug 23 '10 at 09:23

answered Aug 23 '10 at 09:18

Dirk Vollmar

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You also can add trailing zeroes if array isn't of the required size – abatishchev Aug 23 '10 at 09:23
@abatishchev: Yes, good point. This would require an additional pass to find the maximum length entry in the list first so you can determine the array size needed. – Dirk Vollmar Aug 23 '10 at 09:25
Finding the longest array in the list first is better to use list[0]. – Cheng Chen Aug 23 '10 at 09:39
@Danny: It's rather better to iterate one time more I think than trim all arrays longer than first – abatishchev Aug 23 '10 at 09:42
1

@Danny: `new List().Max(a => a.Lenght)` – abatishchev Aug 23 '10 at 10:06

score 3 · Answer 2 · edited Aug 23 '10 at 09:31

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
}

You could use `int rows = source.Count(); int columns = source.Max(r => r.Count());` directly without creating a jagged array first. — Dirk Vollmar, Aug 23 '10 at 09:57
Yes, but I need the jagged array afterwards to access the data (I could use the ElementAt method but it's wouldn't work for enumerables that can be enumerated only once) — Thomas Levesque, Aug 23 '10 at 11:14

score 0 · Answer 3 · answered Dec 13 '13 at 02:00

0

This syntax should work: return new List{new double[] { minX, minY }, new double[] { maxX, maxY }};

answered Dec 13 '13 at 02:00

Sam Saarian

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score 0 · Answer 4 · answered Aug 23 '10 at 09:13

0

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.

answered Aug 23 '10 at 09:13

Miserable Variable

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abatishchev · Answer 5 · 2010-08-23T10:55:47.140

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));

Convert from List to double[,]

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