Python Program for Counting Sort

Python Program for Counting Sort Using counter method.

The provided Python code demonstrates Counting Sort, a non-comparison-based sorting algorithm. Counting Sort works by determining each element’s count in the input sequence, then reconstructing the sorted array. The code comprises a single function countSort that sorts a given string arr in alphabetical order. It uses auxiliary arrays count and output to keep track of character frequencies and sorted positions. The algorithm counts characters’ occurrences, modifies the count array to hold their positions, and constructs the sorted output array. The driver code initializes a string, applies the countSort function, and prints the sorted character array. This algorithm’s efficiency relies on its linear time complexity, making it ideal for a range of values with a limited span.

Python3

# Python program for counting sort
 
# The main function that sort the given string arr[] in 
# alphabetical order
def countSort(arr):
 
    # The output character array that will have sorted arr
    output = [0 for i in range(256)]
 
    # Create a count array to store count of individual
    # characters and initialize count array as 0
    count = [0 for i in range(256)]
 
    # For storing the resulting answer since the 
    # string is immutable
    ans = ["" for _ in arr]
 
    # Store count of each character
    for i in arr:
        count[ord(i)] += 1
 
    # Change count[i] so that count[i] now contains actual
    # position of this character in output array
    for i in range(256):
        count[i] += count[i-1]
 
    # Build the output character array
    for i in range(len(arr)):
        output[count[ord(arr[i])]-1] = arr[i]
        count[ord(arr[i])] -= 1
 
    # Copy the output array to arr, so that arr now
    # contains sorted characters
    for i in range(len(arr)):
        ans[i] = output[i]
    return ans 
 
# Driver program to test above function
arr = "w3wiki"
ans = countSort(arr)
print ("Sorted character array is %s"  %("".join(ans)))
 
# This code is contributed by Nikhil Kumar Singh

Output:

Sorted character array is eeeefggkkorss

Time Complexity: O(n)

Space Complexity: O(n)

Please refer complete article on Counting Sort for more details!