C# quickest way to shift array

Question

How can I quickly shift all the items in an array one to the left, padding the end with null?

For example, [0,1,2,3,4,5,6] would become [1,2,3,4,5,6,null]

Ed

Answer 1

Incorrect and slightly amusing answer (thanks, i'll be here all night !)

int?[] test = new int?[] {0,1,2,3,4,5,6 };

        int?[] t = new int?[test.Length];
        t = test.Skip(1).ToArray();
        t[t.Length - 1] = null; 

In the spirit of still using Skip (dont ask me, i know worst usage of LINQ extension methods ever), the only way I thought of rewriting it would be

        int?[] test = new int?[] { 0, 1, 2, 3, 4, 5, 6 };

        int?[] t = new int?[test.Length];
        Array.Copy(test.Skip(1).ToArray(), t, t.Length - 1);

But it's in NO WAY faster than the other options.

Answer 2

Can't you

• allocate the array with an extra 1000 elements

• have an integer variable int base = 0

• instead of accessing a[i] access a[base+i]

• to do your shift, just say base++

Then after you've done this 1000 times, copy it down and start over.
That way, you only do the copy once per 1000 shifts.

---

Old joke:
Q: How many IBM 360s does it take to shift a register by 1 bit?
A: 33. 32 to hold the bits in place, and 1 to move the register. (or some such...)

Answer 3

I know this is an old question but coming from Google there was no simple example so thanks to this is the easiest way to reorder a list, and you don't have to supply the type it will work it out at runtime,

   private static List<T> reorderList<T>(List<T> list){
       List<T> newList = new List<T>();

       list.ForEach(delegate(T item)
       {
           newList.Add(item);
       });

       return newList;
   }

Answer 4

Couldn't you use a System.Collections.Generic.Queue instead of an array ?

I feel like you need to perform actions on your value the discard it, thus using a queue seems to be more appropriate :

// dummy initialization
        System.Collections.Generic.Queue<int> queue = new Queue<int>();
        for (int i = 0; i < 7; ++i ) { queue.Enqueue(i); }// add each element at the end of the container

        // working thread
        if (queue.Count > 0)
            doSomething(queue.Dequeue());// removes the last element of the container and calls doSomething on it

Answer 5

Try this! using Linq. No need of second Array.

        var i_array = new int?[] {0, 1, 2, 3, 4, 5, 6 };

        i_array = i_array.Select((v, k) => new { v = v, k = k }).
            Where(i => i.k > 0).Select(i => i.v).ToArray();

        Array.Resize(ref i_array, i_array.Length + 1);

Output: [0,1,2,3,4,5,6] would become [1,2,3,4,5,6,null]

Answer 6

The best and most efficient method I believe is using Buffer.BlockCopy function. You will set both source and destination to your array, the offset of the source is 1. Depending on your array type (I assume it is int), 1 int = 4 bytes, so you must pass in 4 as the second parameter of this function. Note that the offset is byte offset.

So it looks like this:

int bytes2copy = yourArray.length - 4;
Buffer.BlockCopy(yourArray, 4, yourArray, 0, bytes2copy);
yourArray[yourArray.length-1] = null;