Prepend to a C# Array

If you are only going to perform this operation once then there isn't a whole lot of choices. The code provided by Monroe's answer should do just fine.

byte[] newValues = new byte[values.Length + 1];
newValues[0] = 0x00;                                // set the prepended value
Array.Copy(values, 0, newValues, 1, values.Length); // copy the old values

If, however, you're going to be performing this operation multiple times you have some more choices. There is a fundamental problem that prepending data to an array isn't an efficient operation, so you could choose to use an alternate data structure.

A LinkedList can efficiently prepend data, but it's less efficient in general for most tasks as it involves a lot more memory allocation/deallocation and also looses memory locallity, so it may not be a net win.

A double ended queue (known as a deque) would be a fantastic data structure for you. You can efficiently add to the start or the end, and efficiently access data anywhere in the structure (but you can't efficiently insert somewhere other than the start or end). The major problem here is that .NET doesn't provide an implementation of a deque. You'd need to find a 3rd party library with an implementation.

You can also save yourself a lot when copying by keeping track of "data that I need to prepend" (using a List/Queue/etc.) and then waiting to actually prepend the data as long as possible, so that you minimize the creation of new arrays as much as possible, as well as limiting the number of copies of existing elements.

You could also consider whether you could adjust the structure so that you're adding to the end, rather than the start (even if you know that you'll need to reverse it later). If you are appending a lot in a short space of time it may be worth storing the data in a List (which can efficiently add to the end) and adding to the end. Depending on your needs, it may even be worth making a class that is a wrapper for a List and that hides the fact that it is reversed. You could make an indexer that maps i to Count-i, etc. so that it appears, from the outside, as though your data is stored normally, even though the internal List actually holds the data backwards.


Ok guys, let's take a look at the perfomance issue regarding this question. This is not an answer, just a microbenchmark to see which option is more efficient.

So, let's set the scenario:

  • A byte array of 1,000,000 items, randomly populated
  • We need to prepend item 0x00

We have 3 options:

  1. Manually creating and populating the new array
  2. Manually creating the new array and using Array.Copy (@Monroe)
  3. Creating a list, loading the array, inserting the item and converting the list to an array

Here's the code:

    byte[] byteArray = new byte[1000000];

    for (int i = 0; i < byteArray.Length; i++)
    {
        byteArray[i] = Convert.ToByte(DateTime.Now.Second);
    }

    Stopwatch stopWatch = new Stopwatch();

    //#1 Manually creating and populating a new array;

    stopWatch.Start();

    byte[] extendedByteArray1 = new byte[byteArray.Length + 1];

    extendedByteArray1[0] = 0x00;

    for (int i = 0; i < byteArray.Length; i++)
    {
        extendedByteArray1[i + 1] = byteArray[i];
    }

    stopWatch.Stop();
    Console.WriteLine(string.Format("#1: {0} ms", stopWatch.ElapsedMilliseconds));
    stopWatch.Reset();

    //#2 Using a new array and Array.Copy

    stopWatch.Start();

    byte[] extendedByteArray2 = new byte[byteArray.Length + 1];
    extendedByteArray2[0] = 0x00;                                
    Array.Copy(byteArray, 0, extendedByteArray2, 1, byteArray.Length);

    stopWatch.Stop();
    Console.WriteLine(string.Format("#2: {0} ms", stopWatch.ElapsedMilliseconds));
    stopWatch.Reset();

    //#3 Using a List

    stopWatch.Start();

    List<byte> byteList = new List<byte>();
    byteList.AddRange(byteArray);
    byteList.Insert(0, 0x00);

    byte[] extendedByteArray3 = byteList.ToArray();

    stopWatch.Stop();
    Console.WriteLine(string.Format("#3: {0} ms", stopWatch.ElapsedMilliseconds));
    stopWatch.Reset();

    Console.ReadLine();

And the results are:

#1: 9 ms
#2: 1 ms
#3: 6 ms

I've run it multiple times and I got different numbers, but the proportion is always the same: #2 is always the most efficient choice.

My conclusion: arrays are more efficient then Lists (although they provide less functionality), and somehow Array.Copy is really optmized (would like to understand that, though).

Any feedback will be appreciated.

Best regards.

PS: this is not a swordfight post, we are at a Q&A site to learn and teach. And learn.


As you surmised, the fastest way to do this is to create new array of length + 1 and copy all the old values over.

If you are going to be doing this many times, then I suggest using a List<byte> instead of byte[], as the cost of reallocating and copying while growing the underlying storage is amortized more effectively; in the usual case, the underlying vector in the List is grown by a factor of two each time an addition or insertion is made to the List that would exceed its current capacity.

...

byte[] newValues = new byte[values.Length + 1];
newValues[0] = 0x00;                                // set the prepended value
Array.Copy(values, 0, newValues, 1, values.Length); // copy the old values

Tags:

C#

Arrays

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