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Python Heapq

From WikiOD

Largest and smallest items in a collection[edit | edit source]

To find the largest items in a collection, heapq module has a function called nlargest, we pass it two arguments, the first one is the number of items that we want to retrieve, the second one is the collection name:

import heapq

numbers = [1, 4, 2, 100, 20, 50, 32, 200, 150, 8]
print(heapq.nlargest(4, numbers))  # [200, 150, 100, 50]

Similarly, to find the smallest items in a collection, we use nsmallest function:

print(heapq.nsmallest(4, numbers))  # [1, 2, 4, 8]

Both nlargest and nsmallest functions take an optional argument (key parameter) for complicated data structures. The following example shows the use of age property to retrieve the oldest and the youngest people from people dictionary:

people = [
    {'firstname': 'John', 'lastname': 'Doe', 'age': 30},
    {'firstname': 'Jane', 'lastname': 'Doe', 'age': 25},
    {'firstname': 'Janie', 'lastname': 'Doe', 'age': 10},
    {'firstname': 'Jane', 'lastname': 'Roe', 'age': 22},
    {'firstname': 'Johnny', 'lastname': 'Doe', 'age': 12},
    {'firstname': 'John', 'lastname': 'Roe', 'age': 45}

oldest = heapq.nlargest(2, people, key=lambda s: s['age'])
# Output: [{'firstname': 'John', 'age': 45, 'lastname': 'Roe'}, {'firstname': 'John', 'age': 30, 'lastname': 'Doe'}]

youngest = heapq.nsmallest(2, people, key=lambda s: s['age'])
# Output: [{'firstname': 'Janie', 'age': 10, 'lastname': 'Doe'}, {'firstname': 'Johnny', 'age': 12, 'lastname': 'Doe'}]

Smallest item in a collection[edit | edit source]

The most interesting property of a heap is that its smallest element is always the first element: heap[0]

import heapq

numbers = [10, 4, 2, 100, 20, 50, 32, 200, 150, 8]

# Output: [2, 4, 10, 100, 8, 50, 32, 200, 150, 20]

heapq.heappop(numbers)  # 2
# Output: [4, 8, 10, 100, 20, 50, 32, 200, 150]

heapq.heappop(numbers)  # 4
# Output:  [8, 20, 10, 100, 150, 50, 32, 200]