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

You might do it like this:

var items = new int?[] { 0, 1, 2, 3, 4, 5, 6 };  // Your array
var itemList = new List<int?>(items);  // Put the items in a List<>
itemList.RemoveAt(1); // Remove the item at index 1
itemList.Add(null); // Add a null to the end of the list
items = itemList.ToArray(); // Turn the list back into an array

Of course, it would be more efficient to get rid of the array entirely and just use a List<>. You could then forget the first line and last line and do it like this:

var itemList = new List<int?> { 0, 1, 2, 3, 4, 5, 6 };
itemList.RemoveAt(1); // Remove the item at index 1
itemList.Add(null); // Add a null to the end of the list

Answer 2

Array copying is an O(n) operation and creates a new array. While array copying can certainly be done quickly and efficiently, the problem you've stated can actually be solved in an entirely different way without (as you've requested) creating a new array/data structure and only creating one small wrapping object instance per array:

using System;
using System.Text;

public class ArrayReindexer
{
    private Array reindexed;
    private int location, offset;

    public ArrayReindexer( Array source )
    {
        reindexed = source;
    }

    public object this[int index]
    {
        get
        {
            if (offset > 0 && index >= location)
            {
                int adjustedIndex = index + offset;
                return adjustedIndex >= reindexed.Length ? "null" : reindexed.GetValue( adjustedIndex );
            }

            return reindexed.GetValue( index );
        }
    }

    public void Reindex( int position, int shiftAmount )
    {
        location = position;
        offset = shiftAmount;
    }

    public override string ToString()
    {
        StringBuilder output = new StringBuilder( "[ " );
        for (int i = 0; i < reindexed.Length; ++i)
        {
            output.Append( this[i] );
            if (i == reindexed.Length - 1)
            {
                output.Append( " ]" );
            }
            else
            {
                output.Append( ", " );
            }
        }

        return output.ToString();
    }
}

By wrapping and controlling access to the array in this manner, we can now demonstrate how the problem was solved with an O(1) method call...

ArrayReindexer original = new ArrayReindexer( SourceArray );
Console.WriteLine( "   Base array: {0}", original.ToString() );
ArrayReindexer reindexed = new ArrayReindexer( SourceArray );
reindexed.Reindex( 1, 1 );
Console.WriteLine( "Shifted array: {0}", reindexed.ToString() );

Will produce the output:

Base array: [ 0, 1, 2, 3, 4, 5, 6 ]
Shifted array: [ 0, 2, 3, 4, 5, 6, null ]

I'm willing to bet that there will be a reason that such a solution won't work for you, but I believe this does match your initial stated requirements. 8 )

It's often helpful to think about all the different kinds of solutions to a problem before implementing a specific one, and perhaps that might be the most important thing that this example can demonstrate.

Hope this helps!