Heap queue (or heapq) in Python

Heap queue is a special tree structure in which each parent node is less than or equal to its child node. In python it is implemented using the heapq module. It is very useful is implementing priority queues where the queue item with higher weight is given more priority in processing.

Create a Heap

A heap queue is created by using python’s inbuilt library named heapq. This library has the relevant functions to carry out various operations on a heap data structure. Below is a list of these functions.

• heapify – This function converts a regular list to a heap. In the resulting heap the smallest element gets pushed to the index position 0. But rest of the data elements are not necessarily sorted.
• heappush – This function adds an element to the heap without altering the current heap.
• heappop – This function returns the smallest data element from the heap.
• heapreplace – This function replaces the smallest data element with a new value supplied in the function.

Creating a Heap

A heap is created by simply using a list of elements with the heapify function. In the below example we supply a list of elements and the heapify function rearranges the elements bringing the smallest element to the first position.

Example

import heapq
H = [21,1,45,78,3,5]
# Use heapify to rearrange the elements
heapq.heapify(H)
print(H)

Output

When the above code is executed, it produces the following result −

[1, 3, 5, 78, 21, 45]

Inserting into heap

Inserting a data element to a heap always adds the element at the last index. But you can apply heapify function again to bring the newly added element to the first index only if it smallest in value. In the below example we insert the number 8.

Example

import heapq
H = [21,1,45,78,3,5]
# Covert to a heap
heapq.heapify(H)
print(H)
# Add element
heapq.heappush(H,8)
print(H)

Output

When the above code is executed, it produces the following result −

[1, 3, 5, 78, 21, 45]
[1, 3, 5, 78, 21, 45, 8]

Removing from heap

You can remove the element at first index by using this function. In the below example the function will always remove the element at the index position 1.

Example

import heapq

H = [21,1,45,78,3,5]
# Create the heap

heapq.heapify(H)
print(H)

# Remove element from the heap
heapq.heappop(H)

print(H)

Output

When the above code is executed, it produces the following result −

[1, 3, 5, 78, 21, 45]
[3, 21, 5, 78, 45]

Replacing in a Heap

The heapreplace function always removes the smallest element of the heap and inserts the new incoming element at some place not fixed by any order.

Example

import heapq
H = [21,1,45,78,3,5]
# Create the heap
heapq.heapify(H)
print(H)
# Replace an element
heapq.heapreplace(H,6)
print(H)

Output

When the above code is executed, it produces the following result −

[1, 3, 5, 78, 21, 45]
[3, 6, 5, 78, 21, 45]