Find the sum of the digits in the number 100!
Permalink: http://projecteuler.net/problem=20
First of all we need to calculate factorial. Without integer overflows, stack overflows and other caveats.
Previous article Fast Factorial has working solution that we might use.
(defn ! [n]
(reduce *' (range 1 (inc n))))
Now, calculate the sum of digits, the same function that we used in Clojure Euler: Problem 008 and Clojure Euler: Problem 016
(defn sum-of-digits [n]
(reduce + (map #(- (int %) 48) (seq (str n)))))
And the final line
(sum-of-digits (! 100))
Simple enough, huh?
P.S. The functions sum-of-digits, count-digits and similar used often
and look very ugly, so I decided to create a separate set of functions (library?)
which I can refer. It should be a set of common mathematic functions, including
different sequences, sums, converters, interesting numbers. You might use it too.
Welcome to numberto!