2.5 数据结构的循环

1.字典

在字典中循环时，用 items() 方法可同时取出键和对应的值：

>>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
>>> for k, v in knights.items():
...     print(k, v)
...
gallahad the pure
robin the brave

2.序列(字符串/列表/元组)

在序列中循环时，用enumerate() 函数可以同时取出位置索引和对应的值：

>>> for index, v in enumerate(['tic', 'tac', 'toe']):
...     print(index, v)
...
0 tic
1 tac
2 toe

同时循环两个或多个序列时，用 zip()函数可以将其内的元素一一匹配：

>>> questions = ['name', 'quest', 'favorite color']
>>> answers = ['lancelot', 'the holy grail', 'blue']
>>> for q, a in zip(questions, answers):
...     print('What is your {0}?  It is {1}.'.format(q, a))
...
What is your name?  It is lancelot.
What is your quest?  It is the holy grail.
What is your favorite color?  It is blue.

逆向循环序列时，先正向定位序列，然后调用reversed()函数：

>>> for i in reversed(range(1, 10, 2)):
...     print(i)
...
9
7
5
3
1

按指定顺序循环序列，可以用sorted()函数，在不改动原序列的基础上，返回一个重新的序列：

>>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
>>> for i in sorted(basket):
...     print(i)
...
apple
apple
banana
orange
orange
pear

使用set()去除序列中的重复元素。使用sorted()加 set()则按排序后的顺序，循环遍历序列中的唯一元素：

>>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
>>> for f in sorted(set(basket)):
...     print(f)
...
apple
banana
orange
pear