Python: Print the output N-1 number of times showing the swapswhile doing a selection sort on a random list of numbers

Question

I was trying to write a function called modified_selection_sort that takes a list of int values as a parameter and sorts it increasing order. The function should begin by swapping the largest element into the
last position first, then the second largest into the second-to-last position, etc.
The program should use nested loops and output the list after each iteration of the outer loop,
thus outputting the list N-1 times (where N is the size of the list), and the function should have no return value.

The code that I have written so far does sort the list but I can’t get the output to be displayed in the way it should be.

Here’s what I tried:

def modified_selection_sort(A:list):
    for i in range(len(A)):
        min_idx = i
        max_idx=i
        for j in range(i+1, len(A)):
            if A[min_idx] > A[j]:
                min_idx = j
            if A[max_idx] < A[j]:
                max_idx = j               
            print(A)    #I tried to print the results here?   
        A[max_idx], A[min_idx] = A[min_idx], A[max_idx] #here's the swapping
             
numbers = [int(number) for number in input().split()]
modified_selection_sort(numbers)

Here’s what I expected:
say the input list was: [30 40 20 10]

[30, 10, 20, 40]
[20, 10, 30, 40]
[10, 20, 30, 40]

But what I get is:

[30, 40, 20, 10]
[30, 40, 20, 10]
[30, 40, 20, 10]
[30, 10, 20, 40]
[30, 10, 20, 40]
[30, 40, 20, 10]

Am I missing a step somewhere? Should I try making another loop inside the j loop that prints out the output (n-1) number of times?

Asked By: degeneratematter

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Source

Answer 1

You don’t need min_idx. You should start from the last index of the list instead and work backwards to keep comparing items in the unsorted portion of the list with the item at max_idx:

def modified_selection_sort(A):
    for i in range(len(A) - 1, 0, -1):
        max_idx = i
        for j in range(i):
            if A[j] > A[max_idx]:
                max_idx = j
        A[i], A[max_idx] = A[max_idx], A[i]
        print(A)

so that:

modified_selection_sort([30, 40, 20, 10])

outputs:

[30, 10, 20, 40]
[20, 10, 30, 40]
[10, 20, 30, 40]

Demo: https://replit.com/@blhsing/RequiredCompatibleAnalysts

Answered By: blhsing

Answer 2

If you only want to see the list after the swaps, then place the print directly below the line where the swapping happens.

A[max_idx], A[min_idx] = A[min_idx], A[max_idx] #here's the swapping
print(A, min_idx, max_idx)

Besides that, there is a problem with this code’s logic, you are allowing swaps to happen always, even when they aren’t beneficial. The prints are indicating that the program is finding 40 and 10 as the max and min, then swapping them, then its finding the 40 and 10 as the max and min again, and swapping again, then repeating the process with 10 and 20

Answered By: PedroTurik

Python: Print the output N-1 number of times showing the swapswhile doing a selection sort on a random list of numbers

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