Selection sort code is not outputting the right values

Question

I am trying my hand at algorithms for the first time and tried to create a sequential sorting algorithm. I have come up with the code below.

def SelectionSort(my_list):
    prev_num = None
    counter = 0
    new_list = []
    index_num = 0
    new_list_counter = 0
    for i in my_list:
        if my_list.index(i) == 0:
            prev_num = i
            counter += 1
            new_list.append(i)
        else:
            if i > prev_num:
                prev_num = i
                counter += 1
                new_list.append(i)
            else:
                prev_num = i
                for n in new_list:
                    if i >= n:
                        new_list_counter += 1
                        pass
                    else:
                        new_list.insert(new_list_counter, i)
                        index_num = new_list.index(i)
                        new_list_counter = 0
                        my_list.remove(i)
                        my_list.insert(index_num, i)
                        break
                counter += 1
                prev_num = i
    return my_list


print(SelectionSort([1, 3, 4, 2, 6, 5]))
print(SelectionSort([3, 4, 2, 6]))
print(SelectionSort([3, 7, 4, 1]))
print(SelectionSort([3, 10, 2, 5]))

Whilst it seems to sort the first 3 lists just fine, when it comes to the last list it outputs [2, 3, 10, 5].

Can anyone possible lend a hand?

Asked By: perilousranch

||

Source

Answer 1

I am sure the logic of your code could be improved and simplified. But with a small change it’s working, at least with your 4 test cases.

My solution is to save "prev_num" with the higest existing number in the list using max(). And only save this value at the end of each loop.

def SelectionSort(my_list):
    prev_num = None
    counter = 0
    new_list = []
    index_num = 0
    new_list_counter = 0
    for i in my_list:
        if my_list.index(i) == 0:
            prev_num = i
            counter += 1
            new_list.append(i)
        else:
            if i > prev_num:
                counter += 1
                new_list.append(i)
            else:
                for n in new_list:
                    if i >= n:
                        new_list_counter += 1
                    else:
                        new_list.insert(new_list_counter, i)
                        index_num = new_list.index(i)
                        new_list_counter = 0
                        my_list.remove(i)
                        my_list.insert(index_num, i)
                        break
                counter += 1
                prev_num = i
        prev_num = max(new_list)  # <-- This is the key: Save at the end of each loop the highest existing number
    return my_list

Improvements:

The renaming of the "prev_num" variable to something like "highest_num".
"elif" could be used instead of "else – if"
Comment the code, not only for the reviewrs, it’s useful for yourself too.

Answered By: Asi

Selection sort code is not outputting the right values

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