I'm confused why this recursion loop doesn't stop

Question

I'm a novice when it comes to recursion (and python, be kind haha), so I wanted to give it a go with a codewars problem (https://www.codewars.com/kata/541c8630095125aba6000c00/train/python)

I'm just super confused as to why the break gets ignored, and the recursion continues.

def digital_root(n):
    x = list(str(n))
    z = 0
    while True:
        for i in range(0, len(x)):
            x[i] = int(x[i])
        for i in x:
            z = i + z
        if z < 10:
             break
        elif z >= 10:
            digital_root(z)
    return z

print(digital_root(942))

In your example, the break is met when z<10, which means previous z + all values of x less than 10, i.e, previous z < -5, but that's impossible using your current code — lincr
– lincr, Commented Sep 15, 2020 at 3:02

Tom Karzes · Accepted Answer · 2020-09-15 02:53:12Z

2

After calling itself recursively, it's discarding the return value, so z and x remain unchanged. Change the recursive call to:

return digital_root(z)

That way the recursion will end. In fact, the while loop should never execute more than once, so you could just do:

def digital_root(n):
    x = list(str(n))
    z = 0
    for i in range(0, len(x)):
        x[i] = int(x[i])
    for i in x:
        z = i + z
    if z < 10:
        return z
    return digital_root(z)

Or, if you'd rather eliminate the recursion entirely, all you really need is the following (which includes some additional simplifications):

def digital_root(n):
    while n >= 10:
        n = sum(int(d) for d in str(n))
    return n

edited Sep 15, 2020 at 2:53

answered Sep 15, 2020 at 2:23

Tom Karzes

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1 Comment

Smallestnoob Over a year ago

Thank you, I appreciate it! I did end up solving it similar to the way you mentioned. I just wanted to force recursion for practice.

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