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c#arraysperformance
George Johnston
George Johnston

Reputation: 32258

Extremely fast way to clone the values of a jagged array into a second array?

I am currently working on an application that is responsible for calculating random permutations of a jagged array.

Currently the bulk of the time in the application is spent copying the array in each iteration (1 million iterations total). On my current system, the entire process takes 50 seconds to complete, 39 of those seconds spent cloning the array.

My array cloning routine is the following:

    public static int[][] CopyArray(this int[][] source)
    {
        int[][] destination = new int[source.Length][];
        // For each Row
        for (int y = 0; y < source.Length; y++)
        {
            // Initialize Array
            destination[y] = new int[source[y].Length];
            // For each Column
            for (int x = 0; x < destination[y].Length; x++)
            {
                destination[y][x] = source[y][x];
            }
        }
        return destination;
    }

Is there any way, safe or unsafe, to achieve the same effect as above, much faster?

Upvotes: 11

Views: 11722

Answers (4)

Alexei Levenkov
Alexei Levenkov

Reputation: 100527

The faster way to copy objects is not to copy them at all - have you considered this option? If you just need to generate permutations you don't need to copy data on each one - just change the array. This approach will not work well if you need to keep on results of previous calls. In any case review your code to see if you are not copying data more times that you have to.

Upvotes: 0

Matthew Whited
Matthew Whited

Reputation: 22433

Either of these should work for you. They both run in about the same amount of time and are both much faster than your method.

// 100 passes on a int[1000][1000] set size

// 701% faster than original (14.26%)
static int[][] CopyArrayLinq(int[][] source)
{
    return source.Select(s => s.ToArray()).ToArray();
}

// 752% faster than original (13.38%)
static int[][] CopyArrayBuiltIn(int[][] source)
{
    var len = source.Length;
    var dest = new int[len][];

    for (var x = 0; x < len; x++)
    {
        var inner = source[x];
        var ilen = inner.Length;
        var newer = new int[ilen];
        Array.Copy(inner, newer, ilen);
        dest[x] = newer;
    }

    return dest;
}

Upvotes: 27

Pent Ploompuu
Pent Ploompuu

Reputation: 5414

You can use Array.Clone for the inner loop:

public static int[][] CopyArray(this int[][] source)
{
    int[][] destination = new int[source.Length][];
    // For each Row
    for(int y = 0;y < source.Length;y++)
    {
        destination[y] = (int[])source[y].Clone();
    }
    return destination;
}

Another alternative for the inner loop is Buffer.BlockCopy, but I haven't measured it's performance against Array.Clone - maybe it's faster:

destination[y] = new int[source[y].Length];
Buffer.BlockCopy(source[y], 0, destination[y], 0, source[y].Length * 4);

Edit: Buffer.BlockCopy takes the number for bytes to copy for the count parameter, not the number of array elements.

Upvotes: 1

Muad&#39;Dib
Muad&#39;Dib

Reputation: 29196

how about serializing/deserializing the array, if you use memory streams and binary serialization, I am thinking that it should be pretty quick.

Upvotes: 0

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