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### Re^3: Recursively-generated Iterators

by runrig (Abbot)
 on May 23, 2005 at 18:13 UTC ( #459655=note: print w/replies, xml ) Need Help??

in reply to Re^2: Recursively-generated Iterators

Could you describe what a restartable iterator is, and what problem it addresses?

Well, I made up the name (and so maybe it's a badly chosen name), but what I mean is, an iterator whose behaviour you can modify after it has already started (in this case, it's a simple iterator, but I can restart it at any point each time it is exhausted). In iterating over the result set, I noticed that in choosing M out of a set of N objects (in a normal nested for loop way), the inner most loop would iterate from M to N, then from M+1 to N, then M+2 to N, etc (down to N to N), while the next outer loop would iterate from M-1 to N-1, then from M to N-1, then M+1 to N-1, etc. The outer most loop just iterates from 1 to N-M+1 (assuming the array index starts at 1, though in above program it starts at zero).

That was the basic pattern at first, but it was more complicated eventually, because the first time that the innermost iterator got down to the "N to N" iteration, it would have to restart at M+1 at the next iteration. So I thought it might be easier to construct an iterator that could tell the next "inner" iterator where to start (the ending point is always the same for each iterator in this case). The iterators I've seen so far return a function that accepts no arguments, but in my "restartable_iter()", it returns a function that can accept a new starting point.

This, of course, is not a generic solution for all iteration problems, because one iterator must be aware of how to tell the next iterator in line what to do. But it was fun to come up with.

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