How do I resolve the bug in this selection sort logic

Question

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The following code block runs for only 1 iteration.
Giving the output : [1,9,8,7,6,5,4,3,2,10]
Kindly help in resolving the logical bug.

What I have tried:

Python

'''
Program to implement Selection Sort
The time complexity of above algorithm is O(n^2)
'''
def find_min (list0):
    min = 0
    for i in range(1,len(list0)):
        if list0[i] < list0[min]:
            min = i
    return min

def selection_sort(list0):
    i = 0
    while i < len(list0):
        pos = find_min(list0[i:])
        if pos != 0:
            list0[i], list0[pos] = list0[pos], list0[i]
        i+=1
    return list0

list0 = [10,9,8,7,6,5,4,3,2,1]
sorted_list0 = selection_sort(list0)
print(sorted_list0)

Posted 15-Sep-20 1:01am

5atyam5rivastava

Updated 15-Sep-20 1:43am

v3

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2 solutions

Solution 2

Quote:
How do I resolve the bug in this selection sort logic

By investigating what is going on with a debugger, it is a wanted skill.
Check find_min, it is returning the position of minimum value in list, on every call.
Try

Python

def selection_sort(list0):
    i = 1
    while i < len(list0):
        pos = find_min(list0[i:])
        print(pos)
        if pos != 0:
            list0[i], list0[pos] = list0[pos], list0[i]
        i+=1
    return list0

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

27.3. pdb — The Python Debugger — Python 3.6.1 documentation[^]
Debugging in Python | Python Conquers The Universe[^]
pdb – Interactive Debugger - Python Module of the Week[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

Posted 15-Sep-20 1:44am

Patrice T

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CPallini · Accepted Answer · 2020-09-15T01:28:00

Solution 1

Try

Python

'''
Program to implement Selection Sort
The time complexity of above algorithm is O(n^2)
'''
def find_min (list0):
    min = 0
    for i in range(1,len(list0)):
        if list0[i] < list0[min]:
            min = i
    return min

def selection_sort(list0):
    i = 0
    while i < len(list0):
        pos = find_min(list0[i:])
        if pos != 0:
            list0[i], list0[pos+i] = list0[pos+i], list0[i]
        i+=1
    return list0

list0 = [10,9,8,7,6,5,4,3,2,1]
sorted_list0 = selection_sort(list0)
print(sorted_list0)

Posted 15-Sep-20 1:28am

CPallini

Comments

5atyam5rivastava 15-Sep-20 7:36am

Thank you so much. I got the logic error. I am referencing to pos but everytime the list being passed to find_min() is different and the min returned from the the function is relative to the array passed, not to the original array, hence I need to do (pos+i),
Thank You.

Patrice T 15-Sep-20 7:45am

my 5

CPallini 15-Sep-20 7:51am

Thank you.