Wouldn't this technique have trouble with code movement issues? For example, if I have ``` (iter (for x in data-table) (averaging x into mean) (finally (return mean))) ``` I know that's a contrived example, but I think the point is clear: if I'm going to make `averaging` a collector, I'd like to be able to use its result, and if that result is computed in a `finally` block that's invisible and out of my control, I can't do it, can I? There's `finally-protected`, but I think that's the *opposite* of what I need. Best, r On 13 Mar 2018, at 10:08, Russ Tyndall wrote:
Here is an existing "sampling" clause to pull a random sample from a larger data set. The long and short is just use a finally clause, as you would when writing a normal iterate loop.
(iterate:defmacro-clause (sampling expr &optional into var size size) "resevoir sample the input" (let ((sample (or var iterate::*result-var*))) (alexandria:with-unique-names (i sample-size sigil buffer row) `(progn (with ,sample) (with ,sample-size = (or ,size 100)) (with ,buffer = (make-array ,sample-size :initial-element ',sigil)) (with ,i = 0) (if (< ,i ,sample-size) (setf (aref ,buffer ,i) ,expr) (let ((r (random ,i))) (when (< r ,sample-size) (setf (aref ,buffer r) ,expr)))) (incf ,i) (finally ;; convert our sample to a list, but only if we actually took the sample (when (plusp ,i) (setf ,sample (iter (for ,row in-vector ,buffer) (until (eq ,row ',sigil)) (collect ,row)))))))))
Cheers, Russ Tyndall Acceleration.net On 03/13/2018 10:49 AM, Robert Goldman wrote:
I was going to define an |AVERAGING| collector clause for iterate, but I'm not sure how to do it. The obvious thing, it seemed to me, would be to sum the values as I go along, and count them, and then divide the sum by the count when leaving the loop.
But the examples for |DEFMACRO-CLAUSE| in the manual do all of their work while iterating, and there doesn't seem to be an "at-end" hook. Is the kind of thing I would like feasible, and if so, how is it to be done?
thanks! r