deque(), islice() & more

08 Nov 2018

It feels really cool to eventually kick off my career! Here I want to discuss two small pieces of code which provides an efficient solution to assembling a sliding window over a sequence.

The sliding window function with deque()

from collections import deque

def sliding_window(seq, W):
  # deque initialization
  it = iter(seq)
  window = deque((next(it, None) for _ in range(W)), maxlen=W)
  yield tuple(window)
  # loop
  for e in it:
    window.append(e)
    yield tuple(window)

This function provides a generator with yield function. The generator returns a tuple of elements in the window in each iteration.

Several notes should be kept for this function:

  • During initialization, next() is processed for W times to form a deque of W elements.
  • _ in the range iterator means the variable is not important by itself but serves for grammatical purpose.
  • Into the for loop, iterator it continues from where next() ends.
  • In the deque, due to the restriction of maxlen handler, each time a new element is appended to the deque, an element would be discarded from the opposite end.
  • The function returns an iterable in which the values are sequentially given by yield.

islice()

islice() enables picking up specific elements from the iterable.

from itertools import islice
# get first k items
it = iter('abracagabra')
list(islice(it, 3))
>>> Out[]: ['a', 'b', 'r']
# get the kth item (0-based)
it = iter('abracagabra')
next(islice(it, 3, 3))
>>> Out[]: 'a'

islice() could be useful in the round-robin scheduling. Another post will be dedicated to the algorithm.

References

 Programming