Iterator with memory? - iterator

Question

I'm working on an application which use a markov chain.
An example on this code follows:
chain = MarkovChain(order=1)
train_seq = ["","hello","this","is","a","beautiful","world"]
for i, word in enum(train_seq):
chain.train(previous_state=train_seq[i-1],next_state=word)
What I am looking for is to iterate over train_seq, but to keep the N last elements.
for states in unknown(train_seq,order=1):
# states should be a list of states, with states[-1] the newest word,
# and states[:-1] should be the previous occurrences of the iteration.
chain.train(*states)
Hope the description of my problem is clear enough for

Answer 1

window will give you n items from iterable at a time.
from collections import deque
def window(iterable, n=3):
it = iter(iterable)
d = deque(maxlen = n)
for elem in it:
d.append(elem)
yield tuple(d)
print [x for x in window([1, 2, 3, 4, 5])]
# [(1,), (1, 2), (1, 2, 3), (2, 3, 4), (3, 4, 5)]
If you want the same number of items even the first few times,
from collections import deque
from itertools import islice
def window(iterable, n=3):
it = iter(iterable)
d = deque((next(it) for Null in range(n-1)), n)
for elem in it:
d.append(elem)
yield tuple(d)
print [x for x in window([1, 2, 3, 4, 5])]
will do that.

Answer 2

seq = [1,2,3,4,5,6,7]
for w in zip(seq, seq[1:]):
print w
You can also do the following to create an arbitrarily-sized pairs:
tuple_size = 2
for w in zip(*(seq[i:] for i in range(tuple_size)))
print w
edit: But it's probably better using the iterative zip:
from itertools import izip
tuple_size = 4
for w in izip(*(seq[i:] for i in range(tuple_size)))
print w
I tried this on my system with seq being 10,000,000 integers and the results were fairly instant.

Answer 3

Improving upon yan's answer to avoid copies:
from itertools import *
def staggered_iterators(sequence, count):
iterator = iter(sequence)
for i in xrange(count):
result, iterator = tee(iterator)
yield result
next(iterator)
tuple_size = 4
for w in izip(*(i for i in takewhile(staggered_iterators(seq, order)))):
print w