Python数据结构 – 利用headp模块寻找最大N个元素并实现优先队列
- 2019 年 10 月 5 日
- 笔记
用headp找到最大最小的N个值
import heapq nums = [1, 8, 2, 23, 7, -4, 18, 23, 42, 37, 2] print(heapq.nlargest(3, nums)) print(heapq.nsmallest(3, nums)) [42, 37, 23] [-4, 1, 2]数据结构复杂时
可以用key这个参数,传入一个lambda表达式
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']) print(cheap) print(expensive)[{'name': 'YHOO', 'shares': 45, 'price': 16.35}, {'name': 'FB', 'shares': 200, 'price': 21.09}, {'name': 'HPQ', 'shares': 35, 'price': 31.75}] [{'name': 'AAPL', 'shares': 50, 'price': 543.22}, {'name': 'ACME', 'shares': 75, 'price': 115.65}, {'name': 'IBM', 'shares': 100, 'price': 91.1}]## 当N的大小和元素差不多时,可以直接用sorted nums = [1, 8, 2, 23, 7, -4, 18, 23, 42, 37, 2] sorted(nums,reverse=False)[-3:] # 默认从小到大,改成True从小到大[23, 37, 42]也可以让一个列表堆化
nums = [1, 8, 2, 23, 7, -4, 18, 23, 42, 37, 2] heapq.heapify(nums) nums[-4, 2, 1, 23, 7, 2, 18, 23, 42, 37, 8]print(heapq.heappop(nums)) print(heapq.heappop(nums)) print(heapq.heappop(nums)) # 相对较大-4 1 2使用headp实现优先队列
## 优先级为负先弹出高的 class PriorityQueue: def __init__(self): self._queue = [] self._index = 0 def push(self, item, priority): heapq.heappush(self._queue, (-priority, self._index, item)) self._index += 1 def pop(self): return heapq.heappop(self._queue)[-1] # Example use class Item: def __init__(self, name): self.name = name def __repr__(self): return 'Item({!r})'.format(self.name)#tostring q = PriorityQueue() q.push(Item('foo'), 1) q.push(Item('bar'), 5) q.push(Item('spam'), 4) q.push(Item('grok'), 1) print("Should be bar:", q.pop()) print("Should be spam:", q.pop()) print("Should be foo:", q.pop()) print("Should be grok:", q.pop()) Should be bar: Item('bar') Should be spam: Item('spam') Should be foo: Item('foo') Should be grok: Item('grok')