Python Equivalent to Ruby's #each_cons?
Is there a Pythonic equivalent to Ruby's #each_cons
?
In Ruby you can do this:
array = [1,2,3,4]
array.each_cons(2).to_a
=> [[1,2],[2,3],[3,4]]
I don't think there is one, I looked through the built-in module itertools
, which is where I would expect it to be. You can simply create one though:
def each_cons(xs, n):
return [xs[i:i+n] for i in range(len(xs)-n+1)]
For such things, itertools
is the module you should be looking at:
from itertools import tee, izip
def pairwise(iterable):
"s -> (s0,s1), (s1,s2), (s2, s3), ..."
a, b = tee(iterable)
next(b, None)
return izip(a, b)
Then:
>>> list(pairwise([1, 2, 3, 4]))
[(1, 2), (2, 3), (3, 4)]
For an even more general solution, consider this:
def split_subsequences(iterable, length=2, overlap=0):
it = iter(iterable)
results = list(itertools.islice(it, length))
while len(results) == length:
yield results
results = results[length - overlap:]
results.extend(itertools.islice(it, length - overlap))
if results:
yield results
This allows arbitrary lengths of subsequences and arbitrary overlapping. Usage:
>> list(split_subsequences([1, 2, 3, 4], length=2))
[[1, 2], [3, 4]]
>> list(split_subsequences([1, 2, 3, 4], length=2, overlap=1))
[[1, 2], [2, 3], [3, 4], [4]]
My solution for lists (Python2):
import itertools
def each_cons(xs, n):
return itertools.izip(*(xs[i:] for i in xrange(n)))
Edit: With Python 3 itertools.izip
is no longer, so you use plain zip
:
def each_cons(xs, n):
return zip(*(xs[i:] for i in range(n)))
A quick one-liner:
a = [1, 2, 3, 4]
out = [a[i:i + 2] for i in range(len(a) - 1)]
Python can surely do this. If you don't want to do it so eagerly, use itertool's islice and izip. Also, its important to remember that normal slices will create a copy so if memory usage is important you should also consider the itertool equivalents.
each_cons = lambda l: zip(l[:-1], l[1:])
UPDATE: Nevermind my answer below, just use toolz.itertoolz.sliding_window()
-- it will do the right thing.
For a truly lazy implementation that preserves the behavior of Ruby's each_cons
when the sequence/generator has insufficient length:
import itertools
def each_cons(sequence, n):
return itertools.izip(*(itertools.islice(g, i, None)
for i, g in
enumerate(itertools.tee(sequence, n))))
Examples:
>>> print(list(each_cons(xrange(5), 2)))
[(0, 1), (1, 2), (2, 3), (3, 4)]
>>> print(list(each_cons(xrange(5), 5)))
[(0, 1, 2, 3, 4)]
>>> print(list(each_cons(xrange(5), 6)))
[]
>>> print(list(each_cons((a for a in xrange(5)), 2)))
[(0, 1), (1, 2), (2, 3), (3, 4)]
Note that the tuple unpacking used on the arguments for izip is applied to a tuple of size n
resulting of itertools.tee(xs, n)
(that is, the "window size"), and not the sequence we want to iterate.
Close to @Blender's solution but with a fix:
a = [1, 2, 3, 4]
n = 2
out = [a[i:i + n] for i in range(len(a) - n + 1)]
# => [[1, 2], [2, 3], [3, 4]]
Or
a = [1, 2, 3, 4]
n = 3
out = [a[i:i + n] for i in range(len(a) - n + 1)]
# => [[1, 2, 3], [2, 3, 4]]
Python 3 version of Elias's code
from itertools import islice, tee
def each_cons(sequence, n):
return zip(
*(
islice(g, i, None)
for i, g in
enumerate(tee(sequence, n))
)
)
$ ipython
...
In [2]: a_list = [1, 2, 3, 4, 5]
In [3]: list(each_cons(a_list, 2))
Out[3]: [(1, 2), (2, 3), (3, 4), (4, 5)]
In [4]: list(each_cons(a_list, 3))
Out[4]: [(1, 2, 3), (2, 3, 4), (3, 4, 5)]
In [5]: list(each_cons(a_list, 5))
Out[5]: [(1, 2, 3, 4, 5)]
In [6]: list(each_cons(a_list, 6))
Out[6]: []
Here's an implementation using collections.deque
. This supports arbitrary generators as well
from collections import deque
def each_cons(it, n):
# convert it to an iterator
it = iter(it)
# insert first n items to a list first
deq = deque()
for _ in range(n):
try:
deq.append(next(it))
except StopIteration:
for _ in range(n - len(deq)):
deq.append(None)
yield tuple(deq)
return
yield tuple(deq)
# main loop
while True:
try:
val = next(it)
except StopIteration:
return
deq.popleft()
deq.append(val)
yield tuple(deq)
Usage:
list(each_cons([1,2,3,4], 2))
# => [(1, 2), (2, 3), (3, 4)]
# This supports generators
list(each_cons(range(5), 2))
# => [(0, 1), (1, 2), (2, 3), (3, 4)]
list(each_cons([1,2,3,4], 10))
# => [(1, 2, 3, 4, None, None, None, None, None, None)]
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