Nesting C#'s yield

re: Nesting C#'s yield

ben — Fri, 12 Aug 2005 18:19:43 GMT

could you explain what's going on in your "pseudo tail-recursion" code? i having trouble following the logic.

thanks.

re: Nesting C#'s yield

jmstall — Fri, 12 Aug 2005 18:58:34 GMT

Ben - I've updated it with more explanation.

re: Nesting C#'s yield

ROn — Fri, 12 Aug 2005 20:29:31 GMT

typically how can yield be applied in the real world? For example I don't often need to calculate the fibonacci sequence.

It seems attractive to me for parsing. Anything else out there?

re: Nesting C#'s yield

jmstall — Fri, 12 Aug 2005 20:58:34 GMT

Yield is awesome for actually *writing* enumerators. For example, how would you write an enumerator that traverses a tree?
Think of it as the other half of for-each.
Some examples off yield:
http://blogs.msdn.com/brada/archive/2004/3/4.aspx
http://blogs.msdn.com/jmstall/archive/2005/08/08/textreader_yield.aspx
http://blogs.msdn.com/toub/archive/2004/10/29.aspx

re: Nesting C#'s yield

ben — Fri, 12 Aug 2005 21:10:53 GMT

ok, so the "yield return x" is never executed?

explaining in more detail

jmstall — Fri, 12 Aug 2005 22:50:33 GMT

The "yield return x" is executed, but all it's doing is just propogating the results from GetValues(top-1) back to GetValue(top)
foreach (int x in GetValues(top-1))
{
yield return x;
}

Conceptually, it's like:
yield GetValues(top-1)); // this is not actually legal in C#2.0

Or think of it this way: in order for GetValues(5) to yield the set {5,4,3,2,1}, it has to call "yield 5","yield 4" ... "yield 1".
So in this code:
static IEnumerable<int> GetValues(int top) // <-- assuing top=5
{
if (top == 0) yield break; // done
yield return top; <--- this will "yield 5"

foreach (int x in GetValues(top-1))
{
yield return x; <--- this will loop around and call "yield 4", "yield 3" ... "yield 1".
}
}

You can always play with it under a debugger to get a real feeling for it.