How can I fix this Array?

Question

#!/bin/python3



b = []
def rotate(d,a):
    if d == 0:
        return a

    b = []

    for i in range(len(a)):
        b.insert(i-1,a[i])
    a = b

    rotate(d-1,b)

if __name__ == '__main__':

    d = 4

    a = ['1','2','3','4','5']
    print(rotate(d,a))

I don't know why but when i return 'a' it it prints none. Can anyone help me with this

What do you mean by "fix"? – Nico Haase Jun 18 '20 at 06:43 — Nico Haase, Jun 18 '20 at 06:43

score 0 · Answer 1 · answered Jun 18 '20 at 09:16

your issue is the missing return statement when rotate is called recursively:

def rotate(d,a):
    if d == 0:
        return a
    #....
    rotate(d-1,b)  # <---- here

if you call rotate(d, a) with d > 0, you enter the recursion rotate(d-1,b)...but you throw away the result and python instead returns None. this is because in python any method call that doesn't explicitly return a value, always returns None...and your initial call to rotate(d, a) with d > 0 never passed by a return statement (only the recursive call with d = 0 does, but you throw away that result)...so it returns None.

this should fix your problem:

def rotate(d,a):
    if d == 0:
        return a
    #....
    return rotate(d-1,b)  # propagate the calculated value back up

see also: Why does my recursive function return None?

How can I fix this Array?

1 Answers1