What is the difference between an Iterator and a Generator?

Question

What is the difference between an Iterator and a Generator?

Answer 1

There's too much Python here, and too many people saying generators are the only way to implement an infinite iterator. Here's the example I mentioned (squares of all natural numbers) implemented in C#. ExplicitSquares explicitly implements an iterator (called IEnumerator in C#). ImplicitSquares uses a generator to do the same thing. Both are infinite iterators and have no backing collection. The only difference is whether the state machine is spelled out, or alternatively a generator is used.

using System.Collections;
using System.Collections.Generic;
using System;

class ExplicitSquares : IEnumerable
{
    private class ExplicitSquaresEnumerator : IEnumerator
    {
        private int counter = 0;

        public void Reset()
        {
            counter = 0;
        }

        public int Current { get { return counter * counter; }}

        public bool MoveNext()
        {
            counter++;
            return true;
        }

        object IEnumerator.Current { get { return Current; } }

        public void Dispose(){}
    }

    public IEnumerator GetEnumerator()
    {
        return new ExplicitSquaresEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

class ImplicitSquares : IEnumerable
{
    public IEnumerator GetEnumerator()
    {
        int counter = 1;
        while(true)
        {
            int square = counter * counter;
            yield return square;
            counter++;
        }
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

public class AllSquares
{
    private static readonly int MAX = 10;

    public static void Main()
    {
        int i = 0;
        foreach(int square in new ExplicitSquares())
        {
            i++;
            if(i >= MAX)
                break;
            Console.WriteLine(square);
        }

        Console.WriteLine();

        int j = 0;
        foreach(int square in new ImplicitSquares())
        {
            j++;
            if(j >= MAX)
                break;
            Console.WriteLine(square);
        }
    }
}