标签：style blog http strong 2014 for
If you use a dictionary in a for statement, it traverses the keys of the dictionary. For example, print_hist prints each key and the corresponding value:
Print dictionary in alphabetical order:
Reverse lookup
Given a dictionary d and a key k, it is easy to find the corresponding value v = d[k]. This operation is called lookup. But what if you have v and you want to find k? You have two problems: first, there might be more than one key that maps to the value v. Depending on the application, you might be able to pick one, or you might have to make a list that contains all of them. Second, there is no simple syntax to do a reverse lookup; you have to search.
A reverse lookup is much slower than a forward lookup; if you have to do it often, or if the dictionary gets big, the performance of your program will suffer.
from Thinking in Python
Looping and dictionaries,布布扣,bubuko.com
标签：style blog http strong 2014 for
原文地址：http://www.cnblogs.com/ryansunyu/p/3844602.html
