Python3 from zero--{initial consciousness: 004~ iterators and generators}

One, reverse iteration: Reversed ()

>>> a[1, 2, 3, 4] for in  reversed (a): ... Print (x, end="

#反向迭代只有在待处理的对象具有确定的大小或者对象实现了__reversed () __ Special methods to work, otherwise you must first convert the object to a list (may consume a lot of memory)

 >>> with open ( " /etc/passwd  , "   RT   " ) as file: ...  for  x #   ... print   (x) ... Traceback (most recent): File    , Line 2, in  < Module>typeerror:argument to reversed () must is a sequence  

Second, iterator slicing: itertools.islice

Import Itertools def count (n): ...    while True:    ... yield N. ...    n + = 1
...  for  in Itertools.islice (count (0), 2, ten):    #相当于列表切片取 [2:10]   ... Print (x, end="

 >>>for  x in  Itertools.islice (count (0), 5 print  (x, End= "  ) ...  if  x >10: ...  break   ...  5 6 7 8 9 10>>> for  x in  Itertools.islice (count (0), 5 print  (x, End= "  ) ...  0 1 2 3 4 

#迭代器和生成器无法进行普通的切片操作 (whose length is indeterminate and does not implement an index), Islice produces a new iterator that consumes all the data from the initial iteration sequence

Iii. iterative sequence in the form of index-value pairs: Enumerate

>>> a[1, 2, 3, 4] for in Enumerate (a, 1):    # starting from 1 counts, Syntax: Enumerate (iterable[, start])   ... Print  1 12 23) 34 4

#enumerate的返回值是一个迭代器, the element is a tuple

Iv. simultaneous iteration of multiple sequences

Parallel paired iterations: Zip (), itertools.zip_longest ()

>>>a[1, 2, 3, 4]>>>b[1, 2, 3, 4, 8, 9]>>> forX, yinchZip (A, b): ...Print(x, y) ...1 4>>> forX, yinchItertools.zip_longest (A, B): ...Print(x, y) ...1 12 23) 34 4None8None9>>> forX, yinchItertools.zip_longest (A, B, fillvalue=0): ...Print(x, y) ...1 12 23) 34 40809

Serial Sequential iterations: Itertools.chain ()

 for inch Itertools.chain (A, B): ...   Print  1234123489

Serial Cross Iteration: Heapq.merge ()

Import HEAPQ  for inch Heapq.merge (A, B): ...   Print  1122334489

Python3 from zero--{initial consciousness: 004~ iterators and generators}