I think I understand now. Still, for the more general case, I think it could be useful to be able to specify a max window size for the the dequeue (or indexable list-like structure); though defaulting to None and storing all values may be additionally useful.
On Monday, November 14, 2016, Danilo J. S. Bellini <danilo.bell...@gmail.com> wrote: > 2016-11-06 23:27 GMT-02:00 Wes Turner <wes.tur...@gmail.com > <javascript:_e(%7B%7D,'cvml','wes.tur...@gmail.com');>>: > >> - So, IIUC, for recursive list comprehensions >> - "prev" = x_(n-1) >> - there is a need to define an initial value >> - chain([1000], [...]) >> - sometimes, we actually need window function >> - __[0] = x_(n-1) >> - __[1] = x_(n-2) # this >> - __[-1] = x_(n-2) # or this >> - this can be accomplished with dequeue >> - __= dequeue([1000], maxlen) >> - for recursive list comprehensions, we'd want to bind e.g. __ to a >> dequeue >> >> [f(__[0], x) for x in y with __ = dequeue((1000,), 1)] >> > > If I understood correctly, that's an alternative to get a general > recursive list comprehension with a syntax like: > > [f(hist, x) for x in y with hist = deque([start_values], size)] > > You're trying to solve the output lag/window problem using a circular > queue with random/indexed access to its values (a collections.deque > instance). You're using "with" instead of "from" to distinguish it from > my first proposal. > > That's not a scan anymore, but something more general. Some information > can be removed from that syntax, for example the size can be defined to be > the starting iterable/memory/history data size, and the deque can be > something internal. Also, using the negative indices would be more explicit > as hist[-1] would be the previous iteration result, hist[-2] would be its > former result and so on. The syntax would be: > > [func(hist, target) for target in iterable with hist = start_iterable] > > i.e., this idea is about a new "with hist = start_iterable" at the end > (or "from" instead of "with"). The resulting list size would be > len(list(start_iterable)) > + len(list(iterable)). As a generator instead of a list, that can be > implemented as this "windowed scan" generator function: > > >>> import collections > >>> def wscan(func, iterable, start_iterable): > ... pre_hist = [] > ... for item in start_iterable: > ... yield item > ... pre_hist.append(item) > ... hist = collections.deque(pre_hist, len(pre_hist)) > ... for target in iterable: > ... item = func(hist, target) > ... yield item > ... hist.append(item) > > The Fibonacci example would be written as: > > >>> list(wscan(lambda fibs, unused: fibs[-1] + fibs[-2], range(10), [0, > 1])) > [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89] > > With the "windowed scan" syntax proposal, it would become: > > >>> [fibs[-1] + fibs[-2] for unused in range(10) with fibs = [0, 1]] > [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89] > > Or: > > >>> [fibs[-1] + fibs[-2] for unused in range(10) from fibs = [0, 1]] > [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89] > > -- > Danilo J. S. Bellini > --------------- > "*It is not our business to set up prohibitions, but to arrive at > conventions.*" (R. Carnap) >
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