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selection sort in python

by maxguy71

A selection sort is an in-place comparison sorting algorithm. It has an O(n2) time complexity, which makes it inefficient on large lists, and generally performs worse than the similar insertion sort. Selection sort is noted for its simplicity and has performance advantages over more complicated algorithms in certain situations, particularly where auxiliary memory is limited.

The algorithm divides the input list into two parts: a sorted sublist of items which is built up from left to right at the front (left) of the list and a sublist of the remaining unsorted items that occupy the rest of the list. Initially, the sorted sublist is empty and the unsorted sublist is the entire input list.

The algorithm proceeds by finding the smallest (or largest, depending on sorting order) element in the unsorted sublist, exchanging (swapping) it with the leftmost unsorted element (putting it in sorted order), and moving the sublist boundaries one element to the right.

The time efficiency of selection sort is quadratic, so there are a number of sorting techniques which have better time complexity than selection sort.

One thing which distinguishes selection sort from other sorting algorithms is that it makes the minimum possible number of swaps, n − 1 in the worst case.

Example

Here is an example of this sort algorithm sorting five elements:

Sorted sublist Unsorted sublist Least element in unsorted list
() (11, 25, 12, 22, 64) 11
(11) (25, 12, 22, 64) 12
(11, 12) (25, 22, 64) 22
(11, 12, 22) (25, 64) 25
(11, 12, 22, 25) (64) 64
(11, 12, 22, 25, 64) ()

(Nothing appears changed on these last two lines because the last two numbers were already in order.)

Selection sort can also be used on list structures that make add and remove efficient, such as a linked list. In this case it is more common to remove the minimum element from the remainder of the list, and then insert it at the end of the values sorted so far. For example:

arr[] = 64 25 12 22 11

// Find the minimum element in arr[0...4]
// and place it at beginning
11 25 12 22 64

// Find the minimum element in arr[1...4]
// and place it at beginning of arr[1...4]
11 12 25 22 64

// Find the minimum element in arr[2...4]
// and place it at beginning of arr[2...4]
11 12 22 25 64

// Find the minimum element in arr[3...4]
// and place it at beginning of arr[3...4]
11 12 22 25 64

Code

here is the python example fo a selection sort

# Selection Sort
import sys
A = [64, 25, 12, 22, 11]
  
# Traverse through all array elements
for i in range(len(A)):
      
    # Find the minimum element in remaining unsorted array
    min_idx = i
    for j in range(i+1, len(A)):
        if A[min_idx] > A[j]:
            min_idx = j
              
    # Swap the found minimum element with the first element        
    A[i], A[min_idx] = A[min_idx], A[i]
  
# test
print ("Sorted array")
for i in range(len(A)):
    print("%d" %A[i]), 

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