Easiest way to Rotate a List in c#

Question

Lists say I have a list List {1,2,3,4,5}

Rotate means:

=> {2,3,4,5,1} => {3,4,5,1,2} => {4,5,1,2,3}

Answer 1

I've used the following extensions for this:

static class Extensions
{
    public static IEnumerable<T> RotateLeft<T>(this IEnumerable<T> e, int n) =>
        n >= 0 ? e.Skip(n).Concat(e.Take(n)) : e.RotateRight(-n);

    public static IEnumerable<T> RotateRight<T>(this IEnumerable<T> e, int n) =>
        e.Reverse().RotateLeft(n).Reverse();
}

They're certainly easy (OP title request), and they've got reasonable performance (OP write-up request). Here's a little demo I ran in LINQPad 5 on an above-average-powered laptop:

void Main()
{
    const int n = 1000000;
    const int r = n / 10;
    var a = Enumerable.Range(0, n);

    var t = Stopwatch.StartNew();

    Console.WriteLine(a.RotateLeft(r).ToArray().First());
    Console.WriteLine(a.RotateLeft(-r).ToArray().First());
    Console.WriteLine(a.RotateRight(r).ToArray().First());
    Console.WriteLine(a.RotateRight(-r).ToArray().First());

    Console.WriteLine(t.ElapsedMilliseconds); // e.g. 236
}

Answer 2

You can use below code for left Rotation.

List<int> backUpArray = array.ToList();

for (int i = 0; i < array.Length; i++)
{
    int newLocation = (i + (array.Length - rotationNumber)) % n;
    array[newLocation] = backUpArray[i];
}

Answer 3

You can play nice in .net framework.

I understand that what you want to do is more up to be an iteration behavior than a new collection type; so I would suggest you to try this extension method based on IEnumerable, which will work with Collections, Lists and so on...

class Program
{
    static void Main(string[] args)
    {
        int[] numbers = { 1, 2, 3, 4, 5, 6, 7 };

        IEnumerable<int> circularNumbers = numbers.AsCircular();

        IEnumerable<int> firstFourNumbers = circularNumbers
            .Take(4); // 1 2 3 4

        IEnumerable<int> nextSevenNumbersfromfourth = circularNumbers
            .Skip(4).Take(7); // 4 5 6 7 1 2 3 
    }
}

public static class CircularEnumerable
{
    public static IEnumerable<T> AsCircular<T>(this IEnumerable<T> source)
    {
        if (source == null)
            yield break; // be a gentleman

        IEnumerator<T> enumerator = source.GetEnumerator();

        iterateAllAndBackToStart:
        while (enumerator.MoveNext()) 
            yield return enumerator.Current;

        enumerator.Reset();
        if(!enumerator.MoveNext())
            yield break;
        else
            yield return enumerator.Current;
goto iterateAllAndBackToStart;
    }
}

• Reasonable performance
• Flexible

If you want go further, make a CircularList and hold the same enumerator to skip the Skip() when rotating like in your sample.

Answer 4

I use this one:

public static List<T> Rotate<T>(this List<T> list, int offset)
{
    return list.Skip(offset).Concat(list.Take(offset)).ToList();
}