Question

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c# Leaner way of initializing int array

Basically I would like to know if there is a more efficent code than the one shown below

    private static int[] GetDefaultSeriesArray(int size, int value)
    {
        int[] result = new int[size];
        for (int i = 0; i < size; i++)
        {
            result[i] = value;
        }
        return result;
    }

where size can vary from 10 to 150000. For small arrays is not an issue, but there should be a better way to do the above.
I am using VS2010(.NET 4.0)

Answer 1

One way that you can improve speed is by utilizing Array.Copy. It’s able to work at a lower level in which it’s bulk assigning larger sections of memory.

By batching the assignments you can end up copying the array from one section to itself.

On top of that, the batches themselves can be quite effectively paralleized.

Here is my initial code up. On my machine (which only has two cores) with a sample array of size 10 million items, I was getting a 15% or so speedup. You’ll need to play around with the batch size (try to stay in multiples of your page size to keep it efficient) to tune it to the size of items that you have. For smaller arrays it’ll end up almost identical to your code as it won’t get past filling up the first batch, but it also won’t be (noticeably) worse in those cases either.

private const int batchSize = 1048576;
private static int[] GetDefaultSeriesArray2(int size, int value)
{

    int[] result = new int[size];

    //fill the first batch normally
    int end = Math.Min(batchSize, size);
    for (int i = 0; i < end; i++)
    {
        result[i] = value;
    }

    int numBatches = size / batchSize;

    Parallel.For(1, numBatches, batch =>
    {
        Array.Copy(result, 0, result, batch * batchSize, batchSize);
    });

    //handle partial leftover batch
    for (int i = numBatches * batchSize; i < size; i++)
    {
        result[i] = value;
    }

    return result;
}