Finding the Largest or Smallest N Items

Problem

• You want to make a list of the largest or smallest N items in a collection.

Solution

• The heapq module has two functions—nlargest() and nsmallest()

Example 1

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]

[42, 37, 23]
[-4, 1, 2]

Example 2

• Use with key parameter

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)

[{'price': 16.35, 'name': 'YHOO', 'shares': 45}, {'price': 21.09, 'name': 'FB', 'shares': 200}, {'price': 31.75, 'name': 'HPQ', 'shares': 35}]
[{'price': 543.22, 'name': 'AAPL', 'shares': 50}, {'price': 115.65, 'name': 'ACME', 'shares': 75}, {'price': 91.1, 'name': 'IBM', 'shares': 100}]

Understand heap

• heap converts data into an ordered list

nums = [1, 8, 2, 23, 7, -4, 18, 23, 42, 37, 2]
heap = list(nums)
heapq.heapify(heap)
heap

[-4, 2, 1, 23, 7, 2, 18, 23, 42, 37, 8]

• heap[0] is always the smallest item

print(heapq.heappop(heap))
print(heapq.heappop(heap))
print(heapq.heappop(heap))

-4
1
2

Sorted

• You can also use sorted() function

sorted(nums)[:3]

[-4, 1, 2]