Python 迭代器

哎哎哎:# t0]https://www . geeksforgeeks . org/python-etrtools/

Python 的 Itertool 是一个模块，它提供了各种在迭代器上工作的函数来产生复杂的迭代器。该模块作为一种快速、高效的内存工具，可以单独使用，也可以组合使用，形成迭代器代数。 例如，让我们假设有两个列表，您想要增加它们的元素。有几种方法可以实现这一点。可以使用简单的方法，即同时遍历列表中的两个元素，并将它们相乘。另一种方法是使用映射函数，即将 mul 运算符作为第一个参数传递给映射函数，将 Lists 作为第二个和第三个参数传递给该函数。让我们看看每种方法花费的时间。

Python 3

# Python program to demonstrate
# iterator module

import operator
import time

# Defining lists
L1 = [1, 2, 3]
L2 = [2, 3, 4]

# Starting time before map
# function
t1 = time.time()

# Calculating result
a, b, c = map(operator.mul, L1, L2)

# Ending time after map
# function
t2 = time.time()

# Time taken by map function
print("Result:", a, b, c)
print("Time taken by map function: %.6f" %(t2 - t1))

# Starting time before naive
# method
t1 = time.time()

# Calculating result using for loop
print("Result:", end = " ")
for i in range(3):
    print(L1[i] * L2[i], end = " ")

# Ending time after naive
# method
t2 = time.time()
print("\nTime taken by for loop: %.6f" %(t2 - t1))

输出:

Result: 2 6 12
Time taken by map function: 0.000005
Result: 2 6 12 
Time taken by for loop: 0.000014

在上面的例子中，可以看到 map 函数花费的时间大约是 for 循环花费的时间的一半。这表明 itertools 是快速、内存高效的工具。

该模块提供的不同类型的迭代器有:

• 无限迭代器
• 组合迭代器
• 终止迭代器

无限迭代器

Python 中的迭代器是可以与“for in loop”一起使用的任何 Python 类型。Python 列表、元组、字典和集合都是内置迭代器的例子。但是迭代器对象不必用尽，有时它可以是无限的。这种类型的迭代器被称为无限迭代器。

Python 提供三种类型的无限迭代器:

• 计数(开始，步骤):这个迭代器从“开始”号开始打印，无限打印。如果提到步骤，则跳过这些数字，否则默认情况下步骤为 1。请参见下面的示例，了解它与 for in 循环的用法。 例:

Python 3

# Python program to demonstrate
# infinite iterators

import itertools

# for in loop
for i in itertools.count(5, 5):
    if i == 35:
        break
    else:
        print(i, end =" ")

输出:

5 10 15 20 25 30

• 循环(可迭代):这个迭代器从传递的容器中按顺序打印所有的值。当所有元素以循环方式打印时，它会重新开始打印。 例 1:****

Python 3

# Python program to demonstrate
# infinite iterators

import itertools

count = 0

# for in loop
for i in itertools.cycle('AB'):
    if count > 7:
        break
    else:
        print(i, end = " ")
        count += 1

输出:

A B A B A B A B 

示例 2: 使用下一个函数。

Python 3

# Python program to demonstrate
# infinite iterators

import itertools

l = ['Geeks', 'for', 'Geeks']

# defining iterator
iterators = itertools.cycle(l)

# for in loop
for i in range(6):

    # Using next function
    print(next(iterators), end = " ")

组合迭代器 T1】输出:

Geeks for Geeks Geeks for Geeks 

• 重复(val，num): 这个迭代器重复打印传递的值无数次。如果提到可选关键字 num，那么它会重复打印 num 的次数。 T3】例:

Python 3

# Python code to demonstrate the working of  
# repeat() 

# importing "itertools" for iterator operations 
import itertools 

# using repeat() to repeatedly print number 
print ("Printing the numbers repeatedly : ") 
print (list(itertools.repeat(25, 4)))

输出:

Printing the numbers repeatedly : 
[25, 25, 25, 25]

组合迭代器

用于简化组合结构(如排列、组合和笛卡尔乘积)的递归生成器称为组合迭代器。 Python 中有 4 个组合迭代器:

• 乘积():该工具计算输入项的笛卡尔乘积。为了计算一个可重复项与其自身的乘积，我们使用可选的 repeat 关键字参数来指定重复次数。这个函数的输出是按排序顺序排列的元组。 例:

Python 3

# import the product function from itertools module
from itertools import product

print("The cartesian product using repeat:")
print(list(product([1, 2], repeat = 2)))
print()

print("The cartesian product of the containers:")
print(list(product(['geeks', 'for', 'geeks'], '2')))
print()

print("The cartesian product of the containers:")
print(list(product('AB', [3, 4])))

输出:

The cartesian product using repeat:
[(1, 1), (1, 2), (2, 1), (2, 2)]

The cartesian product of the containers:
[('geeks', '2'), ('for', '2'), ('geeks', '2')]

The cartesian product of the containers:
[('A', 3), ('A', 4), ('B', 3), ('B', 4)]

• 置换():置换()顾名思义就是用来生成可迭代的所有可能的置换。所有元素都是基于它们的位置而不是它们的价值被视为唯一的。此函数采用可迭代和 group_size，如果 group_size 的值未指定或等于 None，那么 group_size 的值将成为可迭代的长度。 例:

Python 3

# import the product function from itertools module
from itertools import permutations

print ("All the permutations of the given list is:") 
print (list(permutations([1, 'geeks'], 2)))
print()
  Terminating iterators
print ("All the permutations of the given string is:") 
print (list(permutations('AB')))
print()

print ("All the permutations of the given container is:") 
print(list(permutations(range(3), 2)))

输出:

All the permutations of the given list is:
[(1, 'geeks'), ('geeks', 1)]

All the permutations of the given string is:
[('A', 'B'), ('B', 'A')]

All the permutations of the given container is:
[(0, 1), (0, 2), (1, 0), (1, 2), (2, 0), (2, 1)]

• 组合():这个迭代器按照排序顺序打印在指定组大小的参数中传递的容器的所有可能组合(没有替换)。 例:

Python 3

# import combinations from itertools module

from itertools import combinations

print ("All the combination of list in sorted order(without replacement) is:") 
print(list(combinations(['A', 2], 2)))
print()

print ("All the combination of string in sorted order(without replacement) is:")
print(list(combinations('AB', 2)))
print()

print ("All the combination of list in sorted order(without replacement) is:")
print(list(combinations(range(2), 1)))

输出:

All the combination of list in sorted order(without replacement) is:
[('A', 2)]

All the combination of string in sorted order(without replacement) is:
[('A', 'B')]

All the combination of list in sorted order(without replacement) is:
[(0, ), (1, )]

• Combinations _ with _ replacement():这个函数从 iterable 的元素中返回一个长度为 n 的子序列，其中 n 是这个函数用来确定这个函数生成的子序列的长度的参数。单个元素可以在具有替换功能的组合中重复出现。 例:

Python 3

# import combinations from itertools module

from itertools import combinations_with_replacement

print ("All the combination of string in sorted order(with replacement) is:")
print(list(combinations_with_replacement("AB", 2)))
print()

print ("All the combination of list in sorted order(with replacement) is:")
print(list(combinations_with_replacement([1, 2], 2)))
print()

print ("All the combination of container in sorted order(with replacement) is:")
print(list(combinations_with_replacement(range(2), 1)))

输出:

All the combination of string in sorted order(with replacement) is:
[('A', 'A'), ('A', 'B'), ('B', 'B')]

All the combination of list in sorted order(with replacement) is:
[(1, 1), (1, 2), (2, 2)]

All the combination of container in sorted order(with replacement) is:
[(0, ), (1, )]Terminating iterators

终止迭代器

终止迭代器用于处理短输入序列，并根据所用方法的功能产生输出。

不同类型的终止迭代器有:

• 累加(iter，func): 这个迭代器接受两个参数，ITER target 和 target 中值每次迭代时要遵循的函数。如果没有传递函数，默认情况下会发生加法。如果输入表为空，输出表也将为空。 T3】例:

Python 3

# Python code to demonstrate the working of 
# accumulate()

import itertools
import operator

# initializing list 1
li1 = [1, 4, 5, 7]

# using accumulate()
# prints the successive summation of elements
print ("The sum after each iteration is : ", end ="")
print (list(itertools.accumulate(li1)))

# using accumulate()
# prints the successive multiplication of elements
print ("The product after each iteration is : ", end ="")
print (list(itertools.accumulate(li1, operator.mul)))

# using accumulate()
# prints the successive summation of elements
print ("The sum after each iteration is : ", end ="")
print (list(itertools.accumulate(li1)))

# using accumulate()
# prints the successive multiplication of elements
print ("The product after each iteration is : ", end ="")
print (list(itertools.accumulate(li1, operator.mul)))

输出:

The sum after each iteration is : [1, 5, 10, 17]
The product after each iteration is : [1, 4, 20, 140]
The sum after each iteration is : [1, 5, 10, 17]
The product after each iteration is : [1, 4, 20, 140]

• 链(iter1，iter2..):此函数用于打印其参数中提到的可迭代目标中的所有值。 T3】例:

Python 3

# Python code to demonstrate the working of 
# and chain()

import itertools

# initializing list 1
li1 = [1, 4, 5, 7]

# initializing list 2
li2 = [1, 6, 5, 9]

# initializing list 3
li3 = [8, 10, 5, 4]

# using chain() to print all elements of lists
print ("All values in mentioned chain are : ", end ="")
print (list(itertools.chain(li1, li2, li3)))

输出:

All values in mentioned chain are : [1, 4, 5, 7, 1, 6, 5, 9, 8, 10, 5, 4]

• chain.from_iterable(): 这个函数类似于 chain()实现，但这里的参数是列表列表或任何其他可 iterable 容器。 T3】例:

Python 3

# Python code to demonstrate the working of 
# chain.from_iterable()

import itertools

# initializing list 1
li1 = [1, 4, 5, 7]

# initializing list 2
li2 = [1, 6, 5, 9]

# initializing list 3
li3 = [8, 10, 5, 4]

# initializing list of list
li4 = [li1, li2, li3]

# using chain.from_iterable() to print all elements of lists
print ("All values in mentioned chain are : ", end ="")
print (list(itertools.chain.from_iterable(li4)))

输出:

All values in mentioned chain are : [1, 4, 5, 7, 1, 6, 5, 9, 8, 10, 5, 4]

• compress(iter，selector): 这个迭代器根据作为其他参数传递的布尔列表值，有选择地从传递的容器中挑选要打印的值。打印对应于布尔真的参数，否则跳过所有参数。 T3】例:

Python 3

# Python code to demonstrate the working of 
# and compress()

import itertools

# using compress() selectively print data values
print ("The compressed values in string are : ", end ="")
print (list(itertools.compress('GEEKSFORGEEKS', [1, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0])))

输出:

The compressed values in string are : ['G', 'F', 'G']

• dropwhile(func，seq): 这个迭代器只在 func 之后开始打印字符。in 参数第一次返回 false。 T3】例:

Python 3

# Python code to demonstrate the working of 
# dropwhile()

import itertools

# initializing list 
li = [2, 4, 5, 7, 8]

# using dropwhile() to start displaying after condition is false
print ("The values after condition returns false : ", end ="")
print (list(itertools.dropwhile(lambda x : x % 2 == 0, li)))

输出:

The values after condition returns false : [5, 7, 8]

• filterfalse(func，seq): 顾名思义，这个迭代器只打印为传递的函数返回 false 的值。 T3】例:

Python 3

# Python code to demonstrate the working of 
# filterfalse()

import itertools

# initializing list 
li = [2, 4, 5, 7, 8]

# using filterfalse() to print false values
print ("The values that return false to function are : ", end ="")
print (list(itertools.filterfalse(lambda x : x % 2 == 0, li)))

输出:

The values that return false to function are : [5, 7]

• islice(iterable，start，stop，step): 这个迭代器有选择地打印作为参数传递的 iterable 容器中提到的值。这个迭代器接受 4 个参数，可迭代容器，起始位置。，结束位置和步骤。 T3】例:

Python 3

# Python code to demonstrate the working of 
# islice()

import itertools

# initializing list 
li = [2, 4, 5, 7, 8, 10, 20]

# using islice() to slice the list acc. to need
# starts printing from 2nd index till 6th skipping 2
print ("The sliced list values are : ", end ="")
print (list(itertools.islice(li, 1, 6, 2)))

输出:

The sliced list values are : [4, 7, 10]

• 星图(func。，元组列表):这个迭代器以一个函数和元组列表作为参数，从列表的每个元组中根据函数返回值。 T3】例:

Python 3

# Python code to demonstrate the working of 
# starmap()

import itertools

# initializing tuple list
li = [ (1, 10, 5), (8, 4, 1), (5, 4, 9), (11, 10, 1) ]

# using starmap() for selection value acc. to function
# selects min of all tuple values
print ("The values acc. to function are : ", end ="")
print (list(itertools.starmap(min, li)))

输出:

The values acc. to function are : [1, 1, 4, 1]

• takewhile(func，iterable): 这个迭代器与 dropwhile()相反，它打印值，直到函数第一次返回 false。 T3】例:

Python 3

# Python code to demonstrate the working of 
# takewhile()

import itertools

# initializing list 
li = [2, 4, 6, 7, 8, 10, 20]

# using takewhile() to print values till condition is false.
print ("The list values till 1st false value are : ", end ="")
print (list(itertools.takewhile(lambda x : x % 2 == 0, li )))

输出:

The list values till 1st false value are : [2, 4, 6]

• tee(迭代器，计数):- 这个迭代器将容器拆分成参数中提到的多个迭代器。 T3】例:

Python 3

# Python code to demonstrate the working of 
# tee()

import itertools

# initializing list 
li = [2, 4, 6, 7, 8, 10, 20]

# storing list in iterator
iti = iter(li) 

# using tee() to make a list of iterators
# makes list of 3 iterators having same values.
it = itertools.tee(iti, 3)

# printing the values of iterators
print ("The iterators are : ")
for i in range (0, 3):
    print (list(it[i]))

输出:

The iterators are : 
[2, 4, 6, 7, 8, 10, 20]
[2, 4, 6, 7, 8, 10, 20]
[2, 4, 6, 7, 8, 10, 20]

• zip _ long(iterable 1，iterable2，fillval): 这个迭代器按顺序交替打印 iterable 的值。如果其中一个 iterables 已完全打印，剩余的值将由分配给 fillvalue 的值填充。 T3】例:

Python 3

# Python code to demonstrate the working of 
# zip_longest()

import itertools

# using zip_longest() to combine two iterables.
print ("The combined values of iterables is  : ")
print (*(itertools.zip_longest('GesoGes', 'ekfrek', fillvalue ='_' )))

输出:

The combined values of iterables is  : 
('G', 'e') ('e', 'k') ('s', 'f') ('o', 'r') ('G', 'e') ('e', 'k') ('s', '_')

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