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