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How would you extract items 3..6 efficiently, elegantly and pythonically from the following deque without altering it:
from collections import deque q = deque('',maxlen=10) for i in range(10,20): q.append(i) the slice notation doesn't seem to work with deque...
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makes sense to implement slicing in the deque class. The deque iterator has the same internal access to the linked list. You can do it just as efficiently by rotating the deque (unless you need to use the deque concurrently).
Basic Usage of Slices The full slice syntax is: start:stop:step. start refers to the index of the element which is used as a start of our slice. stop refers to the index of the element we should stop just before to finish our slice. step allows you to take each nth-element within a start:stop range.
Deque is preferred over a list in the cases where we need quicker append and pop operations from both the ends of the container, as deque provides an O(1) time complexity for append and pop operations as compared to list which provides O(n) time complexity.
Advertisements. A double-ended queue, or deque, has the feature of adding and removing elements from either end. The Deque module is a part of collections library. It has the methods for adding and removing elements which can be invoked directly with arguments.
import itertools output = list(itertools.islice(q, 3, 7)) For example:
>>> import collections, itertools >>> q = collections.deque(xrange(10, 20)) >>> q deque([10, 11, 12, 13, 14, 15, 16, 17, 18, 19]) >>> list(itertools.islice(q, 3, 7)) [13, 14, 15, 16] This should be more efficient the the other solutions posted so far. Proof?
[me@home]$ SETUP="import itertools,collections; q=collections.deque(xrange(1000000))" [me@home]$ python -m timeit "$SETUP" "list(itertools.islice(q, 10000, 20000))" 10 loops, best of 3: 68 msec per loop [me@home]$ python -m timeit "$SETUP" "[q[i] for i in xrange(10000, 20000)]" 10 loops, best of 3: 98.4 msec per loop [me@home]$ python -m timeit "$SETUP" "list(q)[10000:20000]" 10 loops, best of 3: 107 msec per loop I would prefer this, it's shorter so easier to read:
output = list(q)[3:6+1] If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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