Recursive function seems to be fine, but won't return False

Question

So the function should check if a list is symmetric; if the list is empty or has one integer, it is symmetric.

Otherwise the function should check if the first and last integers are the same and go on recursively. If not, return False.

The code seems right, but I can't figure out what's wrong and why it won't return False.

def symm(lst):

    t = len(lst)
    if t == 0 or 1:
        return True

    if t>1:
        if lst[0] == lst[-1]:
            return symm(lst[1:-2])
        else:
            return False

print symm([6,6,6,6,7])

Something that is really useful for troubleshooting is the [`assert`](https://docs.python.org/2/reference/simple_stmts.html#the-assert-statement) statement. It lets you test little bits of your code at once, and see if something that 'seems' to be working is 'actually' working. — NotAnAmbiTurner, Nov 16 '15 at 19:59

score 5 · Answer 1 · answered Nov 16 '15 at 19:55

5

if t == 0 or 1: is always true since this is read as if (t == 0) or (1): and 1 is always going to be true.

Change to: if t in [0,1]: or simply if len(lst) < 2:

answered Nov 16 '15 at 19:55

R Nar

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2

or this: `if t == 0 or t == 1:` – NotAnAmbiTurner Nov 16 '15 at 19:56
1

@NotAnAmbiTurner, yes that works too but its not as nice looking lol – R Nar Nov 16 '15 at 19:57
2

@R Nar true, but more intuitive for beginners – NotAnAmbiTurner Nov 16 '15 at 19:59
But which one is faster? – michen00 Nov 16 '15 at 20:12
1

with something this simple, the speed difference is negligible. technically, getting rid of the assignment of `t` is faster, but, again, quite negligible. – R Nar Nov 16 '15 at 20:15
1

I wouldn't spend much time worrying about which one is faster as it's extremely unlikely either one of those would be a bottleneck. – kylieCatt Nov 16 '15 at 20:17

Recursive function seems to be fine, but won't return False

1 Answers1