a recursive function to calculate sum of a number digits

Question

i have to write a recursive function which calculates sum of a number digits,here's the code i tried :

def sum_digit(n):
   sum=0
   a = n % 10
   n //= 10
   sum += a
   while n > 0 :
      sum = sum + sum_digit(n)
   return sum


print(sum_digit(67154))

i don't know why i don't get the 23 as answer...my program doesn't come to an end

for example 23 number(please correct me if I'm wrong,I'm a newbie in python),the 3 goes to sum, and n become2,since its > 0 then it should go to while,so now it should calculate sum digit(2),the a become 2 and 2 goes to sum and n become 0 and sum digit(2) returns 2,then it sum with the 3 and i must get 5. i appreciate your help.

Answer 1

You have an infinite loop because n never changes within the loop. Note that assigning a new value to n in the scope of the called function will not change n in the outer scope.

Also, it seems you are mixing an iterative solution with a recursive one. If you do the recursive call, you do not need the loop, and vice versa.

You can either do it recursively:

def sum_digit(n):
    if n > 0:
        return sum_digit(n // 10) + n % 10
    else:
        return 0

Or in an iterative way:

def sum_digit(n):
    s = 0
    while n > 0:
        s += n % 10
        n //= 10
    return s

Or just using bultin functions (probably not what your teacher wants to see):

def sum_digit(n):
    return sum(map(int, str(n)))

Answer 2

我必须用if来更改时间，并且它的工作原理非常感谢您的评论，并很抱歉在此处发布这样的问题。

Answer 3

This will do the trick:

def sum_digit(n, current_sum=0):
    if n == 0:
        return current_sum
    else:
        digit = n % 10
        current_sum += digit
        n //= 10
        return sum_digit(n, current_sum)

The output:

print(sum_digit(67154))
> 23

You were mixing up an iterative method (the while loop) and the recursive method (the function call).

In a recursive function, must make sure you get these things right:

The end condition (in our case, we return the sum when the digit is 0)

and

The recursive call (in our case, going down one digit each time)

Answer 4

Change your code as following:

def sum_digit(n):
   sum=0
   a = n % 10
   n //= 10
   sum += a
   if n > 0 :
      sum = sum + sum_digit(n)
   return sum

The reason is n is assigned new reference inside function, but it's invisible outside. so while part is loop died. In fact, while part is executed at most once, so code was changed as above.