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Python heapq.nsmallest() Method - GeeksforGeeks

Last Updated : 17 Mar, 2025

The heapq.nsmallest() method in Python is part of the heapq module and is used to retrieve the n smallest elements from an iterable, such as a list, tuple or other iterable objects. This method is highly useful when you need to efficiently find the smallest elements in a dataset, without sorting the entire collection. It uses the heap data structure to efficiently return the smallest elements.

Example: Finding the n Smallest Elements

import heapq

# A list of numbers
a = [1, 3, 5, 7, 9, 2]

# Get the 3 smallest numbers
smallest = heapq.nsmallest(3, a)

print("3 Smallest numbers:", smallest)

3 Smallest numbers: [1, 2, 3]

Explanation: The function returns the 3 smallest numbers from the list, which are [1, 2, 3], sorted in ascending order.

heapq.nsmallest(n, iterable, key=None)

• n: The number of smallest elements to retrieve.
• iterable: The iterable (list, tuple, etc.) from which you want to extract the smallest elements.
• key (optional): A function to specify the criteria for comparing elements. If not provided, the elements are compared directly.

The heapq.nsmallest() method returns a list of the n smallest elements from the iterable, sorted in ascending order. The comparison is done using the key function (if provided) and the elements are selected based on their value.

• The heapq.nsmallest() method uses a heap-based approach to find the smallest elements, which is more efficient than sorting the entire iterable.
• It operates with a time complexity of O(n log k), where n is the total number of elements in the iterable and k is the number of smallest elements requested. This makes it more efficient than sorting the entire dataset when you only need a few smallest elements.

You can use the key parameter to customize how the smallest elements are selected. For example, finding the smallest elements based on their absolute values.

import heapq

# A list of numbers, including negative values
a = [-10, 3, -5, 8, -2]

# Get the 3 smallest numbers by absolute value
smallest = heapq.nsmallest(3, a, key=abs)

print(smallest)

Explanation: The key=abs argument makes the function consider the absolute value of each number. So, -2 is chosen because it has the smallest absolute value, followed by 3 and -5.

heapq.nsmallest() can also be used with more complex structures like tuples. You can specify which element of the tuple to use for comparison.

import heapq

# List of tuples (priority, task)
a = [(2, "Task A"), (1, "Task B"), (3, "Task C")]

# Get the 2 tasks with the lowest priority (lowest priority value)
b = heapq.nsmallest(2, a, key=lambda x: x[0])

print("2 Smallest tasks by priority:", b)

2 Smallest tasks by priority: [(1, 'Task B'), (2, 'Task A')]

Explanation: The key=lambda x: x[0] specifies that the smallest elements should be selected based on the first item in each tuple, which represents the priority. The function returns the 2 tasks with the lowest priority.

You should use heapq.nsmallest() when you need to efficiently retrieve the n smallest elements from an iterable, especially when the list is large and you don't want to sort the entire dataset. Some common use cases include:

• Finding the smallest N elements: For example, finding the smallest salaries in a company or the least expensive items in a store.
• Priority Queues: When working with priority queues, where you need to retrieve the smallest (or highest priority) elements.
• Efficient Sorting: If you need to get a few smallest elements, but don't need to sort the entire collection.
• Task Scheduling: When tasks with deadlines or priorities are processed in order of urgency.

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