内置函数_枚举与迭代

枚举与迭代

• enumerate()函数用来枚举可迭代对象中的元素，返回可迭代的enumerate对象，其中每个元素都是包含索引和值的元组，支持start参数，用来指定枚举时的索引起始值

  >>> list(enumerate('abc'))
  [(0, 'a'), (1, 'b'), (2, 'c')]
  >>> list(enumerate(['Python','Java']))
  [(0, 'Python'), (1, 'Java')]
  >>> list(enumerate({'a':97,'b':98,'c':99}.items()))
  [(0, ('a', 97)), (1, ('b', 98)), (2, ('c', 99))]
  >>> for index,value in enumerate(range(10,15)):
  ...   print((index,value),end=' ')
  ...
  (0, 10) (1, 11) (2, 12) (3, 13) (4, 14) >>>
  >>> for item in enumerate(range(5), 6):     # 指定枚举时的索引起始值
  ...   print(item,end=',')
  ...
  (6, 0),(7, 1),(8, 2),(9, 3),(10, 4),>>>

• iter()函数用来返回指定对象的迭代器，有两种用法

  • iter(iterable)

    要求参数必须为序列或者有自己的迭代器

  • iter(callable，sentinel)

    持续调用参数callable直至其返回sentinel

• next()函数用来返回可迭代对象中的下一个元素，适用于生成器对象以及zip、enumerate、reversed、map、filter、iter等对象，等价于这些对象的next()方法

  >>> x = [1, 2, 3]
  >>> next(x)                           
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: 'list' object is not an iterator
  >>> y = iter(x)                   # 根据列表创建迭代对象
  >>> next(y)
  1
  >>> next(y)
  2
  >>> next(y)
  3
  >>> next(y)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  StopIteration
  >>> x = range(1,100,3) 
  >>> next(x)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: 'range' object is not an iterator
  >>> y = iter(x)
  >>> next(y)
  1
  >>>
  >>> next(y)
  4
  >>> next(y)
  7
  >>> next(y)
  10
  >>> next(y)
  13
  >>> next(y)
  16
  >>> x = {1, 2, 3}
  >>> y = iter(x)
  >>> next(y)
  1
  >>> next(y)
  2
  >>> class T:
  ...   def _ _init_ _(self,seq):
    File "<stdin>", line 2
      def _ _init_ _(self,seq):               ^SyntaxError: invalid syntax>>> class T:...   def __init__(self,seq):...     self.__data=list(seq)...   def __iter__(self):...     return iter(self.__data)...>>> t = T(range(3))>>> next(t)Traceback (most recent call last):  File "<stdin>", line 1, in <module>TypeError: 'T' object is not an iterator>>> ti = iter(t)>>> next(ti)0>>> next(ti)1>>> from queue import Queue   >>> q = Queue()             # 创建队列对象>>> for i in range(5):...   q.put(i)...>>> q.put('END')>>> def test():...   return q.get()...>>> for item in iter(test,'END'):...   print(item,end=' ')...0 1 2 3 4 >>>>>> x = map(int,'1234')     # map支持next()函数>>> next(x)1>>> next(x)2>>> x.__next__()3>>> x = reversed('12345678') # reversed支持next()函数>>> for i in range(4):...   print(next(x),end=' ')...8 7 6 5 >>>>>> x = enumerate({'a':97, 'b':98, 'c':99}.items()) # enumerate支持next()函数>>> next(x)(0, ('a', 97))