2016-11-06 23:27 GMT-02:00 Wes Turner <wes.turner@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)