## Project Euler & HackerRank Problem 20 Solution

##### Factorial digit sum

by {BetaProjects} | MAY 18, 2009 | Project Euler & HackerRank### 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.

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.

`sum(map(int,`

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

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.

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

#### HackerRank version

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

### 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