Project Euler: Problem 20

n! means n x (n - 1) x ... x 3 x 2 x 1

For example, 10! = 10 x 9 x ... x 3 x 2 x 1 = 3628800,
and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
Find the sum of the digits in the number 100!

The approach to this problem is quite similar to problem 16 which we can see here. We just need to deal with a really large number here. So, GMP again to the rescue.

library(gmp)

x <- factorialZ(100)

sum(as.numeric(unlist(strsplit(as.character(x), split=""))))

# as.character(x) coverts the big number into a string

# strsplit() extracts individual elements of the string. This can be ensured by using the split = "" argument, which splits the string one element at a time

# strsplit() returns a list. To access the elements of this list, use unlist()

# convert the individual characters to numeric using as.numeric()

# sum the elements

Ans: 648