Convert List<double> to double[n,1]

user_7283981 0
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I need to convert a large List of length n into a double[n,1] array. What is the fastest way to make the conversion?

For further background this is to pass into an set Excel object's Range.Value which requires a two dimensional array.

arrays c#
2 Answers

I'm writing this on the assumption that you really want the most efficient way to do this. Extreme performance almost always comes with a trade-off, usually code readability.

I can still substantially optimize one part of this as the comments note, but I didn't want to go overboard using dynamic methods on first pass.

const int TEST_SIZE = 100 * 1000;
//Test data setup
var list = new List<double>();
for (int i = 0; i < TEST_SIZE; i++)
    list.Add(i);
//Grab the list's underlying array, which is not public
//This can be made MUCH faster with dynamic methods if you want me to optimize
var underlying = (double[])typeof(List<double>)
    .GetField("_items", BindingFlags.NonPublic | BindingFlags.Instance)
    .GetValue(list);
//We need the actual length of the list because there can be extra space in the array
//Do NOT use "underlying.Length"
int underlyingLength = list.Count;
//Benchmark it
var sw = Stopwatch.StartNew();
var twodarray = new double[underlyingLength, 1];
Buffer.BlockCopy(underlying, 0, twodarray, 0, underlyingLength * sizeof(double));
var elapsed = sw.Elapsed;
Console.WriteLine($"Elapsed: {elapsed}");

Output:

Elapsed: 00:00:00.0001998

Hardware used:

AMD Ryzen 7 3800X @ 3.9 Ghz 32 GB DDR4 3200 RAM

user_39803310

I think this is what you want.

This operation will take no more than a few milliseconds even on a slow core. So why bother? How many times will you do this conversion? If millions of times, than try to find a better approach. But if you do this when the end-user presses a button...

Criticize the answer, but please providing metrics if about efficiency.

// Populate a List with 100.000 doubles
Random r = new Random();
List<double> dList = new List<double>();
int i = 0;
while (i++ < 100000) dList.Add(r.NextDouble());

// Convert to double[100000,1]
Stopwatch chrono = Stopwatch.StartNew();

// Conversion:
double[,] ddArray = new double[dList.Count, 1];
int dIndex = 0;
dList.ForEach((x) => ddArray[dIndex++, 0] = x);

Console.WriteLine("Completed in: {0}ms", chrono.Elapsed);

Outputs: (10 repetitions) - Maximum: 2.6 ms

Completed in: 00:00:00.0020677ms
Completed in: 00:00:00.0026287ms
Completed in: 00:00:00.0013854ms
Completed in: 00:00:00.0010382ms
Completed in: 00:00:00.0019168ms
Completed in: 00:00:00.0011480ms
Completed in: 00:00:00.0011172ms
Completed in: 00:00:00.0013586ms
Completed in: 00:00:00.0017165ms
Completed in: 00:00:00.0010508ms

Edit 1.

double[,] ddArray = new double[dList.Count, 1];
foreach (double x in dList) ddArray[dIndex++, 0] = x;

seems just a little bit faster, but needs more testing:

Completed in: 00:00:00.0020318ms
Completed in: 00:00:00.0019077ms
Completed in: 00:00:00.0023162ms
Completed in: 00:00:00.0015881ms
Completed in: 00:00:00.0013692ms
Completed in: 00:00:00.0022482ms
Completed in: 00:00:00.0015960ms
Completed in: 00:00:00.0012306ms
Completed in: 00:00:00.0015039ms
Completed in: 00:00:00.0016553ms
user_54972070
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