The prime factors of 13195 are 5, 7, 13 and 29.
What is the largest prime factor of the number 600851475143 ?
open System let findFactorsOf(n:int64) = let upperBound = int64(Math.Sqrt(double(n))) [2L..upperBound] |> Seq.filter (fun x -> n % x = 0L) let isPrime(n:int64) = findFactorsOf(n) |> Seq.length = 0 let findMaxPrimeFactorOf(n:int64) = let upperBound = int64(Math.Sqrt(double(n))) [2L..upperBound] |> Seq.filter (fun x -> n % x = 0L) |> Seq.filter isPrime |> Seq.max let maxPrime = findMaxPrimeFactorOf(600851475143L)
As this solution requires a bit more work, I created two helper functions findFactorsOf and isPrime, and as you’ve probably noticed, I had explicitly declare that the value n should be a int64 type:
That’s because the number 600851475143 in the problem is outside of the range of the int32 type so all the functions must be able to work with a int64 type. This is one of few cases where you can’t just rely on the type inference in F#.
Another thing you might have noticed is that the syntax for casting in F# is type(value), e.g.
as opposed to the syntax in C#:
And did you notice that I used the Math.Sqrt method in my solution? That’s right, you can use any CLR types in F# just as you can in C#! In order to import a namespace in F#, you need to use the open keyword:
which is equivalent to the using directive in C#: