#### Project Euler 20 Problem 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.

The factorial function is imported from the Python math library.

`from math import factorial`

To solve this problem we calculate 100! and convert the result to a string so we can iterate over each character-digit.^{1}

`sum(map(int, `

**str(factorial(100))**))

Then, using the `map`

function we iterate over the string and convert every character-digit back to an integer using the `int`

function.

Finally, we add the digits together for a digit sum using the `sum`

function.

**sum(map(int, str(factorial(100))****))**

#### HackerRank version

HackerRank’s Project Euler 20 increases the factorial from 100 to 1,000 and runs 100 test cases. This solution was fast enough to handle these parameters without change.

#### Python 2.7 Source

#### Last Word

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 – SOLVED