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| Itertools |
Itertools is a python module of functions that return generators, objects that only function when iterated over. Some examples of itertool functions include but not limited to: chain(), imap(), product(), and compress().
Iterators terminating on the shortest input sequence:
chain()
The chain() function takes several iterators as arguments and returns a single iterator that produces the contents of all of them as though they came from one sequence.
import itertools
list(itertools.chain([1, 2], [3, 4]))
# Output
# [1, 2, 3, 4]
islice()
The islice() function returns an iterator which returns selected items from the input iterator, by index. It takes the same arguments as the slice operator for lists: start, stop, and step. Start and stop are optional.
import itertools
list(itertools.islice(count(), 5))
# Output
# [0,1, 2, 3, 4]
izip()
izip() returns an iterator that combines the elements of several iterators into tuples. It works like the built-in function zip(), except that it returns an iterator instead of a list.
import itertools
list(izip([1, 2, 3], ['a', 'b', 'c']))
# Output
# [(1, 'a'),(2, 'b'),(3, 'c')]
Combinatoric iterators
product()
product() returns a Cartesian product, equivalent to a nested for-loop. In comparison, the usual zip() function, which returns the convolution.
from itertools import product
list(product([1,2,3],[3,4]))
# Output
# [(1, 3), (1, 4), (2, 3), (2, 4), (3, 3), (3, 4)]
A = [[1,2,3],[3,4,5]]
list(product(*A))
# Output
# [(1, 3), (1, 4), (1, 5), (2, 3), (2, 4), (2, 5), (3, 3), (3, 4), (3, 5)]
B = [[1,2,3],[3,4,5],[7,8]]
list(product(*B))
# Output
# [(1, 3, 7), (1, 3, 8), (1, 4, 7), (1, 4, 8), (1, 5, 7), (1, 5, 8), (2, 3, 7), (2, 3, 8), (2, 4, 7), (2, 4, 8), (2, 5, 7), (2, 5, 8), (3, 3, 7), (3, 3, 8), (3, 4, 7), (3, 4, 8), (3, 5, 7), (3, 5, 8)]
Source: https://docs.python.org/3/library/itertools.html https://www.hackerrank.com/challenges/itertools-product/problem