3.17. Exercise: Reduce Number

Write a function named reduce_number that takes an integer and reduces the number by taking the sum of the digits. If the summation has more than one digit, the reduction is applied again. This continues until the result is a single digit number. The function must return the reduced number as a one-digit integer.

For example, if we call the reduce_number function with 325565 as argument, then the number is reduced to 26 first. Since 26 is a two-digit number, then we reduce it again and we get 8 as the final result.

Hint: You can define a helper function to calculate the sum of a number’s digits.

Sample I/O:

Input:
>>> reduce_number(589564123)
7

>>> reduce_number(19999999999999)
1