I am a bit stuck on 5. Reversed List on this page of advanced challenges
I can see the suggested answer and understand it. However, I came up with a different solution that seems to get the same results and I’m not sure why it isn’t being marked as correct.
The code I tried was:
#Write your function here
def reversed_list(lst1, lst2):
i = -1
for num in lst1:
if num != lst2[i]:
return False
elif i == (0 - len(lst1)) and num == lst2[i]:
return True
else:
i -= 1
#Uncomment the lines below when your function is done
print(reversed_list([1, 2, 3], [3, 2, 1]))
print(reversed_list([1, 5, 3], [3, 2, 1]))
It seems to get the correct answers, and the logic makes sense to me. However, when I try to check my work, I just get the message:
reversed_list([], []) should have returned True , and it returned None
I’m not sure what this error means because it did return True and False as far as I can tell. I don’t see it ever returning None? So any advice would be appreciated.
It does give you the answer. print(reversed_list([], [])) does return None.
The reason is, well, in another language you might get a compile or syntan error of “not all code paths return…”. Python will, helpfully(?!?), implicitly return a None if you don’t return anything. Let’s make that explicit so you can see what’s going on:
#Write your function here
def reversed_list(lst1, lst2):
i = -1
for num in lst1:
if num != lst2[i]:
return False
elif i == (0 - len(lst1)) and num == lst2[i]:
return True
else:
i -= 1
return None # if you don't return inside that loop, you land here
#Uncomment the lines below when your function is done
print(reversed_list([1, 2, 3], [3, 2, 1]))
print(reversed_list([1, 5, 3], [3, 2, 1]))
print(reversed_list([], []))
I’ll leave it to you to impliment a fix.
Forgive me, but this looked like fun. Another solution might be:
def reversed_list(xs, ys):
sz = len(xs)
if sz != len(ys): # no need to check
return False
if sz == 0: # no need to loop
return True
for i in range(sz):
if xs[i] != ys[sz - 1 - i]:
return False
return True