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
1 >>> for i in "python" : #用i这个变量遍历这个字符串的每一个字符
2 ... print i #将遍历的字符打印出来
3 ...
4 p
5 y
6 t
7 h
8 o
9 n
10 >>> lst =["baidu","google","ali"]
11 >>> for i in lst: #用变量i遍历这个列表，将每个元素打印出来
12 ... print i
13 ...
14 baidu
15 google
16 ali
17 >>> t =tuple(lst)
18 >>> t
19 (‘baidu‘, ‘google‘, ‘ali‘)
20 >>> for i in t: #用变量i遍历元组，将每个元素打印出来
21 ... print i
22 ...
23 baidu
24 google
25 ali
26 >>> d =dict([("lang","python"),("website","baidu"),("city","beijing")])
27 >>> d
28 {‘lang‘: ‘python‘, ‘website‘: ‘baidu‘, ‘city‘: ‘beijing‘}
29 >>> for k in d: #用变量k遍历这个字典，将每个key打印出来
30 ... print k
31 ...
32 lang
33 website
34 city
35 >>> for k in d: #用变量k遍历字典d
36 ... print k,"-->",d[k] #将key值和value值打印出来
37 ...
38 lang --> python
39 website --> baidu
40 city --> beijing
41 >>> d.items() #以列表返回可遍历的(键, 值) 元组
42 [(‘lang‘, ‘python‘), (‘website‘, ‘baidu‘), (‘city‘, ‘beijing‘)]
43 >>> for k,v in d.items(): #用key value遍历d.items()的元组列表
44 ... print k,"-->",v #取得key ,value
45 ...
46 lang --> python
47 website --> baidu
48 city --> beijing
49 >>> for k,v in d.iteritems(): iteritems 返回的是迭代器 推荐使用这个
50 ... print k,v
51 ...
52 lang python
53 website baidu
54 city beijing
55 >>> d.itervalues() 返回的是迭代器
56 <dictionary-valueiterator object at 0x0000000002C17EA8>
57 >>>
判断对象是否可迭代
1 >>> import collections #引入标准库 2 >>> isinstance(321,collections.Iterable) #返回false,不可迭代 3 False 4 >>> isinstance([1,2.3],collections.Iterable) #返回true,可迭代 5 True
1 >>> l =[1,2,3,4,5,6,7,8,9] 2 >>> l[4:] 3 [5, 6, 7, 8, 9] 4 >>> for i in l[4:]: #遍历4以后的元素 5 ... print i 6 ... 7 5 8 6 9 7 10 8 11 9 12 >>> help(range) #函数可创建一个整数列表，一般用在 for 循环中 13 Help on built-in function range in module __builtin__: 14 15 range(...) 16 range(stop) -> list of integers 17 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 18 19 Return a list containing an arithmetic progression of integers. 20 range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0. 21 When step is given, it specifies the increment (or decrement). 22 For example, range(4) returns [0, 1, 2, 3]. The end point is omitted! 23 These are exactly the valid indices for a list of 4 elements. 24 25 >>> range(9) 26 [0, 1, 2, 3, 4, 5, 6, 7, 8] 27 >>> range(2,8) 28 [2, 3, 4, 5, 6, 7] 29 >>> range(1,9,3) 30 [1, 4, 7] 31 >>> l 32 [1, 2, 3, 4, 5, 6, 7, 8, 9] 33 >>> range(0,9,2) 34 [0, 2, 4, 6, 8] 35 >>> for i in range(0,9,2): 36 ... print i 37 ... 38 0 39 2 40 4 41 6 42 8 43 >>>
1 #! /usr/bin/env python 2 #coding:utf-8 3 4 aliquot =[] #创建一个空的列表 5 6 for n in range(1,100): #遍历1到100 的整数 7 if n %3==0: #如果被3整除 8 aliquot.append(n) #将n值添加到列表中 9 10 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
1 >>> a =[1,2,3,4,5] 2 >>> b =[9,8,7,6,5] 3 >>> c =[] 4 >>> for i in range(len(a)): 5 ... c.append(a[i]+b[i]) 6 >>> for i in range(len(a)): 7 ... c.append(a[i]+b[i]) 8 ... 9 >>> c 10 [10, 10, 10, 10, 10] 11 >>> help(zip) 12 Help on built-in function zip in module __builtin__: 13 14 zip(...) 15 zip(seq1 [, seq2 [...]]) -> [(seq1[0], seq2[0] ...), (...)] 16 17 Return a list of tuples, where each tuple contains the i-th element 18 from each of the argument sequences. The returned list is truncated 19 in length to the length of the shortest argument sequence. 20 21 >>> a 22 [1, 2, 3, 4, 5] 23 >>> b 24 [9, 8, 7, 6, 5] 25 >>> zip(a,b) 26 [(1, 9), (2, 8), (3, 7), (4, 6), (5, 5)] 27 >>> c =[1,2,3] 28 >>> zip(c,b) 29 [(1, 9), (2, 8), (3, 7)] 30 >>> zip(a,b,c) 31 [(1, 9, 1), (2, 8, 2), (3, 7, 3)] 32 >>> d=[] 33 >>> for x,y in zip(a,b): 34 ... d.append(x+y) 35 ... 36 >>> d 37 [10, 10, 10, 10, 10] 38 >>> r =[(1,2),(3,4),(5,6),(7,8)] 39 >>> zip(*r) 40 [(1, 3, 5, 7), (2, 4, 6, 8)] 41 >>>
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
1 >>> help(enumerate)
2 Help on class enumerate in module __builtin__:
3
4 class enumerate(object)
5 | enumerate(iterable[, start]) -> iterator for index, value of iterable
6 |
7 | Return an enumerate object. iterable must be another object that supports
8 | iteration. The enumerate object yields pairs containing a count (from
9 | start, which defaults to zero) and a value yielded by the iterable argument.
10 | enumerate is useful for obtaining an indexed list:
11 | (0, seq[0]), (1, seq[1]), (2, seq[2]), ...
12 |
13 | Methods defined here:
14 |
15 | __getattribute__(...)
16 | x.__getattribute__(‘name‘) <==> x.name
17 |
18 | __iter__(...)
19 | x.__iter__() <==> iter(x)
20 |
21 | next(...)
22 | x.next() -> the next value, or raise StopIteration
23 |
24 | ----------------------------------------------------------------------
25 | Data and other attributes defined here:
26 |
27 | __new__ = <built-in method __new__ of type object>
28 | T.__new__(S, ...) -> a new object with type S, a subtype of T
29
30 >>> weeks =["sun","mon","tue","web","tue","fri","sta"]
31 >>> for i,day in enumerate(weeks):
32 ... print str(i)+":"+day
33 ...
34 0:sun
35 1:mon
36 2:tue
37 3:web
38 4:tue
39 5:fri
40 6:sta
41 >>> for i in range(len(weeks)):
42 ... print str(i)+":"+weeks[i]
43 ...
44 0:sun
45 1:mon
46 2:tue
47 3:web
48 4:tue
49 5:fri
50 6:sta
51 >>> raw ="Do you love canglaoshi? canglaoshi is a good teacher."
52 >>> raw_lst =raw.split(" ")
53 >>> raw_lst
54 [‘Do‘, ‘you‘, ‘love‘, ‘canglaoshi?‘, ‘canglaoshi‘, ‘is‘, ‘a‘, ‘good‘, ‘teacher.‘]
55 >>> for i,w in enumerate(raw_lst):
56 ... if w =="canglaoshi":
57 ... raw_lst[i]="luolaoshi"
58 ...
59 >>> raw_lst
60 [‘Do‘, ‘you‘, ‘love‘, ‘canglaoshi?‘, ‘luolaoshi‘, ‘is‘, ‘a‘, ‘good‘, ‘teacher.‘]
61 >>> for i,w in enumerate(raw_lst):
62 ... if "canglaoshi" in w:
63 ... raw_lst[i]="luolaoshi"
64 ...
65 >>> raw_lst
66 [‘Do‘, ‘you‘, ‘love‘, ‘luolaoshi‘, ‘luolaoshi‘, ‘is‘, ‘a‘, ‘good‘, ‘teacher.‘]
67 >>> a =range(10)
68 >>> a
69 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
70 >>> s =[]
71 >>> for i in a:
72 ... s.append(i*i)
73 ...
74 >>> s
75 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
76 >>> b = [i*i for i in a] #列表解析
77 >>> b
78 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
79 >>> c = [i*i for i in a if i%3==0] #列表解析，加入限制条件
80 >>> c
81 [0, 9, 36, 81]
82 >>>
列表解析
原文地址：https://www.cnblogs.com/wangruihua-521/p/8560899.html
时间： 2024-10-11 17:53:52