Here are two different ways of rewriting that mapc with iter:

`(iter top (repeat 1) (iter (for x in '(1 2 3)) (in top (collect x))))`

`(iter (repeat 1) (appending '(1 2 3)))`

`(iter (repeat 1) (appending (iter (for x in '(1 2 3)) (collect x))))`

If I am using iter, I try hard to *only* use iter, as one of the reasons for this library (for me) is to have a single syntax for iteration.

That said, I wrote the `collectors` library to handle arbitrary collection tasks that were not easily expressed as a straight iteration. The collectors library binds local functions, so you can `(mapc #'collect ...)` etc.


Russ Tyndall

On 03/13/2018 11:52 AM, Luís Oliveira wrote:
Just a curiosity: you could compute the average incrementally: <>, but doing it the usual way is probably more efficient and with smaller numeric error when using floats.

Slightly off-topic question: oftentimes I have to use map-like iterators. Is there a good way to use such iterators in conjunction with iter? An obvious way would be something like:

    (iter (repeat 1) (mapc (lambda (x) (collect x)) '(1 2 3)))

The (iter (repeat 1) ...) bit could be hidden under a macro, I suppose. But it'd be nicer if collect were a function, so I could do (mapc #'collect '(1 2 3)). Perhaps said macro could rewrite #'clause to (lambda (x) (expansion-of-clause x)) or something similar. Looks like a fun idea to explore so I'm wondering if someone's played with something like this before.

(It's even more fun to turn map-like iterators into generators using continuations, but that's fully off-topic. :-))


On Tue, Mar 13, 2018 at 3:08 PM 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)
            (with ,sample)
            (with ,sample-size = (or ,size 100))
            (with ,buffer = (make-array ,sample-size :initial-element
            (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)
             ;; 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)))))))))

    Russ Tyndall

    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?


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