Selection Sort
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"""
This is a pure Python implementation of the selection sort algorithm
For doctests run following command:
python -m doctest -v selection_sort.py
or
python3 -m doctest -v selection_sort.py
For manual testing run:
python selection_sort.py
"""
def selection_sort(collection):
"""Pure implementation of the selection sort algorithm in Python
:param collection: some mutable ordered collection with heterogeneous
comparable items inside
:return: the same collection ordered by ascending
Examples:
>>> selection_sort([0, 5, 3, 2, 2])
[0, 2, 2, 3, 5]
>>> selection_sort([])
[]
>>> selection_sort([-2, -5, -45])
[-45, -5, -2]
"""
length = len(collection)
for i in range(length - 1):
least = i
for k in range(i + 1, length):
if collection[k] < collection[least]:
least = k
if least != i:
collection[least], collection[i] = (collection[i], collection[least])
return collection
if __name__ == "__main__":
user_input = input("Enter numbers separated by a comma:\n").strip()
unsorted = [int(item) for item in user_input.split(",")]
print(selection_sort(unsorted))
About this Algorithm
Problem Statement
Given an unsorted array of n elements, write a function to sort the array
Approach
- select the smallest element from the array
- put it at the beginning of the array
- then select the smallest array from the remaining unsorted list
- append it to the sorted array at the beginning
- keep doing this for every element of the array
- repeat the above process n times
Time Complexity
O(n^2) Worst case performance
O(n^2) Best-case performance
O(n^2) Average performance
Space Complexity
O(1) Worst case
Example
arr[] = {80, 10, 40, 30}
Indexes: 0 1 2 3
1. Index = 0
Select the minimum number from the array (between index 0-3), ie, 10
2. Swap 10 and 80 (arr[0])
3. The array now is {10, 80, 40, 30}
4. Index = 1
Select the minimum number from the array (between index 1-3), ie, 30
5. Swap 30 and 80 (arr[1])
6. The array now is {10, 30, 40, 80}
7. Index = 2
Select the minimum number from the array (between index 2-3), ie, 40
8. Swap 40 and 40 (arr[2])
9. The array now is {10, 30, 40, 80}
The array is now sorted.
Code Implementation Links
Video Explanation
A video explaining the Selection Sort Algorithm
