标签(空格分隔): pythoncook笔记
import heapq
nums = [1, 8, 2, 23, 7, -4, 18, 23, 42, 37, 2]
print(heapq.nlargest(3, nums)) # Prints [42, 37, 23]
print(heapq.nsmallest(3, nums)) # Prints [-4, 1, 2]
两个函数都能接受一个关键字参数,用于更复杂的数据结构中:
portfolio = [
{'name': 'IBM', 'shares': 100, 'price': 91.1},
{'name': 'AAPL', 'shares': 50, 'price': 543.22},
{'name': 'FB', 'shares': 200, 'price': 21.09},
{'name': 'HPQ', 'shares': 35, 'price': 31.75},
{'name': 'YHOO', 'shares': 45, 'price': 16.35},
{'name': 'ACME', 'shares': 75, 'price': 115.65}
]
cheap = heapq.nsmallest(3, portfolio, key=lambda s: s['price'])
expensive = heapq.nlargest(3, portfolio, key=lambda s: s['price'])
import heapq
class PriorityQueue:
def __init__(self):
self._queue = [] # 初始化一个列表来储存队列
self._index = 0 # 初始化信号
def push(self, item, priority):
heapq.heappush(self._queue, (-priority, self._index, item)) # 向队列里添加元素,并添加优先级
self._index += 1 # 使每个添加进队列的元素具有不同的信号予以区分
def pop(self):
# heapq.heappop会删除队列中最小的元素并返回,这正好照应了之前把优先级设为负数的构想
return heapq.heappop(self._queue)[-1]
index的作用是在两个元素具有相同的优先级的情况下,可以按照它们的插入顺序来将元素跳出队列
看一下代码你就知道为什么了
>>> a = (1, Item('foo'))
>>> b = (5, Item('bar'))
>>> a < b
True
>>> c = (1, Item('grok'))
>>> a < c
Traceback (most recent call last):
File "" , line 1, in
TypeError: unorderable types: Item() < Item()
>>>
当加上index变量组成三元组进行比较时,由于每个元素的index不一样,所以就能很好的避免上面的问题了
>>> a = (1, 0, Item('foo'))
>>> b = (5, 1, Item('bar'))
>>> c = (1, 2, Item('grok'))
>>> a < b
True
>>> a < c
True
>>>