Project Euler Problem 20 Statement
n! means n × (n − 1) × … × 3 × 2 × 1
Find the digit sum in the number 100!
Python natively supports arbitrary-precision integers and arithmetic with as many digits as necessary to perform a calculation. For example, it takes 158 digits to represent 100! and Python handles it easily.
sum(map(int,str(factorial(100))))Text with a dashed underscore will highlight the code section being talked about when moused over or tapped.
To solve this problem, we calculate 100! using the
factorial function from the math library and convert the result to a string so we can iterate over each character-digit. This conversion is required because integers are not iterable, but strings are.
Then, using the
map function, we convert every character-digit back to an integer. This allows us to break the individual digits apart and add them up for a digit sum using the
HackerRank’s Project Euler 20 increases the factorial from 100 to 0 ≤ N ≤ 1,000 and runs 100 test cases. This solution was fast enough to handle these parameters without change.
Python Source Code
from math import factorial as f n = int(input("Factorial? ")) print ("Sum of digits for %d! is %d" % (n, sum(map(int, str(f(n))))))
Embrace the power of arbitrary-precision integers and ignore the call to solve it in some primitive way.
See also, Project Euler 16: Digit sum for a large power of 2