Python 学习笔记（十一）Python语句（二）

For 循环语句

基础知识

for循环可以遍历任何序列的项目，如一个列表或者一个字符串。

语法：

for 循环规则：

　　do sth

 >>> for i in "python" : #用i这个变量遍历这个字符串的每一个字符
 ...     print i  #将遍历的字符打印出来
 ...
 p
 y
 t
 h
 o
 n
 >>> lst =["baidu","google","ali"]
 >>> for i in lst: #用变量i遍历这个列表，将每个元素打印出来
 ...     print i
 ...
 baidu
 google
 ali
 >>> t =tuple(lst)
 >>> t
 ('baidu', 'google', 'ali')
 >>> for i in t: #用变量i遍历元组，将每个元素打印出来
 ...     print i
 ...
 baidu
 google
 ali
 >>> d =dict([("lang","python"),("website","baidu"),("city","beijing")])
 >>> d
 {'lang': 'python', 'website': 'baidu', 'city': 'beijing'}
 >>> for k in d: #用变量k遍历这个字典，将每个key打印出来
 ...     print k
 ...
 lang
 website
 city
 >>> for k in d: #用变量k遍历字典d
 ...     print k,"-->",d[k]  #将key值和value值打印出来
 ...
 lang --> python
 website --> baidu
 city --> beijing
 >>> d.items() #以列表返回可遍历的(键, 值) 元组
 [('lang', 'python'), ('website', 'baidu'), ('city', 'beijing')]
 >>> for k,v in d.items(): #用key  value遍历d.items()的元组列表
 ...     print k,"-->",v   #取得key ,value
 ...
 lang --> python
 website --> baidu
 city --> beijing
 >>> for k,v in d.iteritems():  iteritems 返回的是迭代器  推荐使用这个
 ...     print k,v
 ...
 lang python
 website baidu
 city beijing
 >>> d.itervalues()  返回的是迭代器
 <dictionary-valueiterator object at 0x0000000002C17EA8>
 >>>

判断对象是否可迭代

 >>> import collections  #引入标准库
 >>> isinstance(321,collections.Iterable) #返回false,不可迭代
 False
 >>> isinstance([1,2.3],collections.Iterable) #返回true,可迭代
 True

 >>> l =[1,2,3,4,5,6,7,8,9]
 >>> l[4:]
 [5, 6, 7, 8, 9]
 >>> for i in l[4:]: #遍历4以后的元素
 ...     print i
 ...
 5
 6
 7
 8
 9
 >>> help(range) #函数可创建一个整数列表，一般用在 for 循环中
 Help on built-in function range in module __builtin__:
 
 range(...)
     range(stop) -> list of integers
     range(start, stop[, step]) -> list of integers  #计数从 start 开始,计数到 stop 结束，但不包括 stop,step：步长，默认为1
 
     Return a list containing an arithmetic progression of integers.
     range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0.
     When step is given, it specifies the increment (or decrement).
     For example, range(4) returns [0, 1, 2, 3].  The end point is omitted!
     These are exactly the valid indices for a list of 4 elements.
 
 >>> range(9)
 [0, 1, 2, 3, 4, 5, 6, 7, 8]
 >>> range(2,8)
 [2, 3, 4, 5, 6, 7]
 >>> range(1,9,3)
 [1, 4, 7]
 >>> l
 [1, 2, 3, 4, 5, 6, 7, 8, 9]
 >>> range(0,9,2)
 [0, 2, 4, 6, 8]
 >>> for i in range(0,9,2):
 ...    print i
 ...
 0
 2
 4
 6
 8
 >>>

 #! /usr/bin/env python
 #coding:utf-8
 
 aliquot =[] #创建一个空的列表
 
 for n in range(1,100):   #遍历1到100 的整数
     if n %3==0:          #如果被3整除
         aliquot.append(n) #将n值添加到列表中
 
 print aliquot

zip() 函数

函数用于将可迭代的对象作为参数，将对象中对应的元素打包成一个个元组，然后返回由这些元组组成的列表。

如果各个迭代器的元素个数不一致，则返回列表长度与最短的对象相同，利用 * 号操作符，可以将元组解压为列表。

返回一个列表，这列表是以元组为元素

 >>> a =[1,2,3,4,5]
 >>> b =[9,8,7,6,5]
 >>> c =[]
 >>> for i in range(len(a)):
 ...     c.append(a[i]+b[i])
 >>> for i in range(len(a)):
 ...     c.append(a[i]+b[i])
 ...
 >>> c
 [10, 10, 10, 10, 10]
 >>> help(zip)
 Help on built-in function zip in module __builtin__:
 
 zip(...)
     zip(seq1 [, seq2 [...]]) -> [(seq1[0], seq2[0] ...), (...)]
 
     Return a list of tuples, where each tuple contains the i-th element
     from each of the argument sequences.  The returned list is truncated
     in length to the length of the shortest argument sequence.
 
 >>> a
 [1, 2, 3, 4, 5]
 >>> b
 [9, 8, 7, 6, 5]
 >>> zip(a,b)
 [(1, 9), (2, 8), (3, 7), (4, 6), (5, 5)]
 >>> c =[1,2,3]
 >>> zip(c,b)
 [(1, 9), (2, 8), (3, 7)]
 >>> zip(a,b,c)
 [(1, 9, 1), (2, 8, 2), (3, 7, 3)]
 >>> d=[]
 >>> for x,y in zip(a,b):
 ...     d.append(x+y)
 ...
 >>> d
 [10, 10, 10, 10, 10]
 >>> r =[(1,2),(3,4),(5,6),(7,8)]
 >>> zip(*r)
 [(1, 3, 5, 7), (2, 4, 6, 8)]
 >>>

enumerate()函数

函数用于将一个可遍历的数据对象(如列表、元组或字符串)组合为一个索引序列，同时列出数据和数据下标，一般用在 for 循环当中。

语法：

enumerate(sequence, [start=0])

sequence -- 一个序列、迭代器或其他支持迭代对象

start -- 下标起始位置。

返回值： enumerate枚举对象

 >>> help(enumerate)
 Help on class enumerate in module __builtin__:
 
 class enumerate(object)
  |  enumerate(iterable[, start]) -> iterator for index, value of iterable
  |
  |  Return an enumerate object.  iterable must be another object that supports
  |  iteration.  The enumerate object yields pairs containing a count (from
  |  start, which defaults to zero) and a value yielded by the iterable argument.
  |  enumerate is useful for obtaining an indexed list:
  |      (0, seq[0]), (1, seq[1]), (2, seq[2]), ...
  |
  |  Methods defined here:
  |
  |  __getattribute__(...)
  |      x.__getattribute__('name') <==> x.name
  |
  |  __iter__(...)
  |      x.__iter__() <==> iter(x)
  |
  |  next(...)
  |      x.next() -> the next value, or raise StopIteration
  |
  |  ----------------------------------------------------------------------
  |  Data and other attributes defined here:
  |
  |  __new__ = <built-in method __new__ of type object>
  |      T.__new__(S, ...) -> a new object with type S, a subtype of T
 
 >>> weeks =["sun","mon","tue","web","tue","fri","sta"]
 >>> for i,day in enumerate(weeks):
 ...     print str(i)+":"+day
 ...
 0:sun
 1:mon
 2:tue
 3:web
 4:tue
 5:fri
 6:sta
 >>> for i in range(len(weeks)):
 ...     print str(i)+":"+weeks[i]
 ...
 0:sun
 1:mon
 2:tue
 3:web
 4:tue
 5:fri
 6:sta
 >>> raw ="Do you love canglaoshi? canglaoshi is a good teacher."
 >>> raw_lst =raw.split(" ")
 >>> raw_lst
 ['Do', 'you', 'love', 'canglaoshi?', 'canglaoshi', 'is', 'a', 'good', 'teacher.']
 >>> for i,w in enumerate(raw_lst):
 ...     if w =="canglaoshi":
 ...             raw_lst[i]="luolaoshi"
 ...
 >>> raw_lst
 ['Do', 'you', 'love', 'canglaoshi?', 'luolaoshi', 'is', 'a', 'good', 'teacher.']
 >>> for i,w in enumerate(raw_lst):
 ...     if  "canglaoshi" in w:
 ...             raw_lst[i]="luolaoshi"
 ...
 >>> raw_lst
 ['Do', 'you', 'love', 'luolaoshi', 'luolaoshi', 'is', 'a', 'good', 'teacher.']
 >>> a =range(10)
 >>> a
 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 >>> s =[]
 >>> for i in a:
 ...     s.append(i*i)
 ...
 >>> s
 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
 >>> b = [i*i for i in a] #列表解析
 >>> b
 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
 >>> c = [i*i for i in a if i%3==0] #列表解析，加入限制条件
 >>> c
 [0, 9, 36, 81]
 >>>

列表解析