Selection Sort Recursive Python

Question

I have this Python code, is a recursive form of the Selection Sort Algorithm, but it doesn't work, can someone help me?

def selection(lista):
    return selection_aux(lista,0,len(lista))
def selection_aux(lista,i,n):
    if i == n or lista[0]==None:
        return lista
    else:
        num_menor = menor_f(lista,i,n,i)
        temporal = lista[i]
        lista[i] = num_menor
        print(num_menor)
        lista[num_menor] = temporal
        return selection_aux(lista,i+1,n)
def menor_f(lista,j,n,menor):
    if j == n:
        return menor
    if lista[j] < lista[menor]:
        menor = j
        return menor_f(lista,j+1,n,menor)

Test:

>>> selection([1,2,3])

Output

lista[num_menor] = temporal
TypeError: list indices must be integers, not NoneType

Can you please add more details as to how it is not working. Something like, what output you expected but what output you got, some logical error, or something like that. — Bhargav Rao
– Bhargav Rao, Commented May 30, 2015 at 21:42
Without reading your code carefully, isn't this a bit too long and complicated for something as simple as selection sort? — Shashank
– Shashank, Commented May 30, 2015 at 21:54
Yes, it is, but the idea is write that algorithm by using tail recursion !! — Kendall González
– Kendall González, Commented May 30, 2015 at 21:59

Padraic Cunningham · Accepted Answer · 2015-05-30 21:56:29Z

2

Your indentation was wrong, you did not return anything is if lista[j] < lista[menor] was False so you were setting num_menor to None as all python functions that don't return a value return None by default .

def menor_f(lista,j,n,menor):
    if j == n:
        return menor
    if lista[j] < lista[menor]:
        menor = j
    return menor_f(lista,j+1,n,menor) # dedent

Cannot follow you variable names as I don't speak what seems to be french, but making the change works for selection([4,3,2])

print(selection([4,3,2]))
[2, 3, 4]

Fails for selection([ 8,3,4,2,5,6]) -> [3, 3, 4, 4, 5, 8] so you need to do some debugging.

edited May 30, 2015 at 21:56

answered May 30, 2015 at 21:50

Padraic Cunningham

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7 Comments

Kendall González Over a year ago

Actually is Spanish! I'm from Costa Rica!, Thank you for help me!!

Bhargav Rao Over a year ago

Err.. What is dedent? A short for de-indent?

Padraic Cunningham Over a year ago

@BhargavRao, stick with me, I will lead you down the path of pyenlightenment ;)

Bhargav Rao Over a year ago

Yep. You have been doing that since we first met :D

Padraic Cunningham Over a year ago

@BhargavRao,ah back when 12k+ imaginary rep points was only a dream!

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