Project Euler Problem 20 Solution: Factorial digit sum

Project Euler Problem 20 Statement

n! means n × (n − 1) × … × 3 × 2 × 1

Find the digit sum in the number 100!

Solution

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))))

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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 sum function.

HackerRank version

Solves all test cases for Hackerrank Project Euler Problem 20

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))))))

Run Project Euler Problem 20 using Python on repl.it.

Last Word

Embrace the power of arbitrary-precision integers and ignore the call to solve it in some primitive way.