Radix Sort in Python

Background

  1. Non comparison integer sorting by grouping numbers based on individual digits or radix (base)
  2. Perform iteratively from least significant digit (LSD) to most significant digit (MSD) or recusively from MSD to LSD.
  3. At each iteration, sorting of target digit is based usually on Counting sort as subroutine.
  4. Complexity: O(d*n+b)) where b is the base for representing numbers eg 10. d is the number of digits. Close to Linear time if d is constant amount

Counting Sort as subroutine

  • Recap on the counting sort. See Counting Sort in Python for more info
  • Taking “get_sortkey ” function that generate the keys based on objects characteristics.
  • Modified the get_sortkey function to perform radix sort.
import random, math

def get_sortkey(n):
    """ Define the method to retrieve the key """
    return n

def counting_sort(tlist, k, get_sortkey):
    """ Counting sort algo with sort in place.
        Args:
            tlist: target list to sort
            k: max value assume known before hand
            get_sortkey: function to retrieve the key that is apply to elements of tlist to be used in the count list index.
            map info to index of the count list.
        Adv:
            The count (after cum sum) will hold the actual position of the element in sorted order
            Using the above, 

    """

    # Create a count list and using the index to map to the integer in tlist.
    count_list = [0]*(k)

    # iterate the tgt_list to put into count list
    for n in tlist:
        count_list[get_sortkey(n)] = count_list[get_sortkey(n)] + 1  

    # Modify count list such that each index of count list is the combined sum of the previous counts
    # each index indicate the actual position (or sequence) in the output sequence.
    for i in range(k):
        if i ==0:
            count_list[i] = count_list[i]
        else:
            count_list[i] += count_list[i-1]

    output = [None]*len(tlist)
    for i in range(len(tlist)-1, -1, -1):
        sortkey = get_sortkey(tlist[i])
        output[count_list[sortkey]-1] = tlist[i]
        count_list[sortkey] -=1

    return output

Radix sort with up to 3-digits numbers

  • Replace the get_sortkey with the get_sortkey2 which extract the integer based on the digit place and uses the counting sort at each iteration
# radix sort
from functools import partial

def get_sortkey2(n, digit_place=2):
    """ Define the method to retrieve the key
        return the key based on the digit place. Current set base to 10
    """
    return (n//10**digit_place)%10

## Create random list for demo counting sort.
random.seed(1)
tgt_list = [random.randint(20,400) for n in range(10)]
print("Unsorted List")
print(tgt_list)

## Perform the counting sort.
print("\nSorted list using counting sort")

output = tgt_list
for n in range(3):
    output = counting_sort(output, 30, partial(get_sortkey2, digit_place=n))
    print(output)

## output
# Unsorted List
# [88, 311, 52, 150, 80, 273, 250, 261, 353, 214]

# Sorted list using counting sort
# [150, 80, 250, 311, 261, 52, 273, 353, 214, 88]
# [311, 214, 150, 250, 52, 353, 261, 273, 80, 88]
# [52, 80, 88, 150, 214, 250, 261, 273, 311, 353]

See also:

Resources:

  1. Getting To The Root Of Sorting With Radix Sort
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