Implementing A Circular Iterator

Many years ago, I wrote an article entitled “Implement A Circular Iterator” for The VisualC++ Developer’s Journal. (Unfortunately, VCDJ is now out of business, and it’s successor, Visual Studio Magazine doesn’t maintain an online archive of articles from that magazine. Fortunately, I kept a copy of it)

The essence of the C++ code is that given a templated collection, it will give you an iterator to would loop through the collection over & over.

A bit more recently, some one wrote C# variant of this, and published it on CodeProject. However, that one was a CircularList, which would be derived from a standard List. In the comments, I created an IEnumerator class. It could be used with any class the implemented IEnumerable, like this

{
	 int[ ] a = new int[ ] {1,2,3,4,5,6,7};
 
	 foreach(int i in a)
		Console.WriteLine("{0}", i);
 
	 Console.WriteLine("--------------------------");
	 int cnt = 0;
	 foreach(int i in new CircularEnumerator(a))
	 {
		 Console.Write("{0}", i);
		 ++cnt;
		 if (cnt == 30)
			 break;
	 }
}

would print “12345671234567123456712”. Note the “break” in the above. It’s vitally important, because the foreach will never exit on it own.

But, that used the non-generic IEnumerator interface, and manually implement all the parts of the interface. I figured that using an C# iterator it would be easier. And, as it turns out, it was:

static class Util
{
 public static IEnumerable<T> Circular<T>(this IEnumerable<T> coll)
 {
	while(true)
	{
		foreach(T t in coll)
			yield return t;
	}
 }
}

This would be used exactly like the previous version, with a single line changed:

  foreach(int a in Util.Circular(ary))

Update: As I revisit these old posts, I tweak a bit. Here I’ve made it an extension method. Now it can be called like this:

  foreach(int a in ary.Circular())

This kinda makes the next paragraph irrelevant.

However, this bring up an important point. Here I’ve put in into a static class named “Util”. Now, it would be a natural to put it into the same class as my SkipFirst & SkipLast enumerators I wrote about recently. However, I put those into a static class named “Skip” which won’t be appropriate for this. However (part II), when I wrote utility functions for enums I put them in a static class called “Enumr”. Now, do you (that’s the collectively, blog-reading “You” – i.e. answer in the comments) think it would be OK, to put both function dealing with enums and functions dealing with IEnumerators in a class called “Enumr”?