itertools in Python (4)

#python #itertools #iterator #function

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*Memos:

My post explains itertools about count(), cycle() and repeat().
My post explains itertools about accumulate(), batched(), chain() and chain.from_iterable().
My post explains itertools about compress(), filterfalse(), takewhile() and dropwhile().
My post explains itertools about pairwise(), starmap(), tee() and zip_longest().
My post explains itertools about product() and permutations().
My post explains itertools about combinations() and combinations_with_replacement().

itertools has the functions to create iterators.

*more-itertools has more functions by installing with pip install more-itertools.

groupby() can return the iterator which groups the elements of iterable by key one by one to return a set of the key and element one by one as shown below:

*Memos:

The 1st argument is iterable(Required-Type:iterable).
The 2nd argument is key(Optional-Default:None-Type:callable).

from itertools import groupby

v = groupby(iterable=['AB', 'CDE', 'FG', 'H', 'IJK'])
v = groupby(iterable=['AB', 'CDE', 'FG', 'H', 'IJK'], key=None)

print(v)
# <itertools.groupby object at 0x0000026906F60B20>

key, x = next(v)
print(key, next(x)) # AB AB

key, x = next(v)
print(key, next(x)) # CDE CDE

key, x = next(v)
print(key, next(x)) # FG FG

key, x = next(v)
print(key, next(x)) # H H

key, x = next(v)
print(key, next(x)) # IJK IJK

key, x = next(v) # StopIteration:

from itertools import groupby

for key, x in groupby(iterable=['AB', 'CDE', 'FG', 'H', 'IJK']):
    print(key, next(x))
# AB AB
# CDE CDE
# FG FG
# H H
# IJK IJK

from itertools import groupby

for key, x in groupby(iterable=['AB', 'CDE', 'FG', 'H', 'IJK'], key=len):
    print(key, next(x))
# 2 AB
# 3 CDE
# 2 FG
# 1 H
# 3 IJK

from itertools import groupby

d = {}

for key, x in groupby(iterable=['AB', 'CDE', 'FG', 'H', 'IJK'], key=len):
    if not key in d:
        d.update({key:[]})
    d[key].append(next(x))

print(d) # {2: ['AB', 'FG'], 3: ['CDE', 'IJK'], 1: ['H']}

islice() can return the iterator which returns the selected elements of iterable one by one as shown below:

*Memos:

The 1st argument is iterable(Required-Type:iterable).
The 2nd argument is start(Required-Type:int): *Memos:
- It must be 0 <= x.
- If it's None or not set, it's 0.
The 2nd(Required) or 3rd(Optional) argument is stop(Type:int): *Memos:
- It must be 0 <= x.
- If it's None or not set, iterable can be read to the end.
The 4th argument is step(Optional-Type:int): *Memos:
- It must be 1 <= x.
- If it's None or not set, it's 1.
Only if two arguments are set, the 2nd argument is stop.

from itertools import islice

v = islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 9)
v = islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 20)
v = islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], None)
v = islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 0, 9)
v = islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 0, 20)
v = islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], None, None)
v = islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 0, 9, 1)
v = islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 0, 20, 1)
v = islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], None, None, None)

print(v)
# <itertools.islice object at 0x0000026906E7BD30>

print(next(v)) # -4
print(next(v)) # -3
print(next(v)) # -2
print(next(v)) # -1
print(next(v)) # 0
print(next(v)) # 1
print(next(v)) # 2
print(next(v)) # 3
print(next(v)) # 4
print(next(v)) # StopIteration:

from itertools import islice

for x in islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 4):
# for x in islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 0, 4):
# for x in islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 0, 4, 1):
    print(x)
# -4
# -3
# -2
# -1

from itertools import islice

for x in islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 3, 7):
# for x in islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 3, 7, 1):
    print(x)
# -1
# 0
# 1
# 2

from itertools import islice

for x in islice([-4, -3, -2, -1, 0, 1, 2, 3, 4], 1, 8, 2):
    print(x)
# -3
# -1
# 1
# 3

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