Alternative to global variable

Question

I got a recursive function, which reverses an integer. The reversed Integer can not start with zero, e.g.: 12340 becomes 4321.

res = 0
base = 1


def reverse(n):

    global res
    global base

    if n > 0:
        reverse(n // 10)
        res = res + (n % 10) * base
        base = base * 10

    return res

this code works, but only once and hence I want to get rid of the global variables. I thought of a helper function, inside of the reverse(n) function, but I could not get it to work properly. I have tried for almost an hour and would love to see the solution eventually.

Answer 1

It's totally possible with a helper method:

def reverse_recursive(i: int) -> int:
    def helper(i: int, result_up_to_now: int) -> int:
        if i == 0:
            return result_up_to_now
        return helper(i // 10, result_up_to_now * 10 + (i % 10))
    return helper(i, 0)

assert reverse_recursive(123456789) == 987654321

And even without one:

def reverse_recursive_nohelper(i: int, result_up_to_now=0) -> int:
    if i == 0:
        return result_up_to_now
    return reverse_recursive_nohelper(i // 10, result_up_to_now * 10 + (i % 10))

assert reverse_recursive_nohelper(123456789) == 987654321

But the latter one could of course be misused/misunderstood by someone.

Just for comparison, the non-recursive version:

def reverse_straight(i: int) -> int:
    result = 0
    while i != 0:
        result = result*10 + i % 10
        i //= 10
    return result

assert reverse_straight(123456789) == 987654321

Answer 2

global only on the agg_res variable for the result.

Anyway agg_res is reinitialized at the end of the calculation (no influence on the second re-use)

agg_res=0

def reverse(n, res=0, base=1):
    global agg_res
    ls = len(str(n))
    if n > 0:
        reverse(n // 10, res, base)
        res = res + (n % 10)
        base = base * (10**(ls-1))
        agg_res += res*base
    else :
        agg_res=0

# First time
reverse(5786)
print(agg_res)
# 6875

# Second time
reverse(5786)
print(agg_res)
# 6875