#### Problem

n! means n x (n – 1) x … x 3 x 2 x 1

Find the sum of the digits in the number 100!

#### Solution

let rec factorial (n:bigint) = if n = 1I then 1I else n * factorial(n-1I) let number = factorial 100I let digits = number.ToString().ToCharArray() |> Seq.map (fun c -> int32(c.ToString())) let sum = digits |> Seq.sum

This solution is pretty straight forward, though you might be curious as to what the rec keyword in the *factorial* function is for. In F# you need to explicitly specify that a function can be recursive and you do so with the *rec* keyword.

Note that the last three lines can be merged into one:

let answer = (factorial 100I).ToString().ToCharArray() |> Seq.sumBy (fun c -> int32(c.ToString()))

The Seq.sumBy function is basically *Seq.map* and *Seq.sum* functions rolled into one.

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