Project Euler in F#: Problem 4

Find the largest palindrome made from the product of two 3-digit numbers

This “obvious” solution to this problem didn’t force me to use any new techniques, but at least it got me to familiarize myself with F# array and string methods.

module Problem4 =
    //we have to write (fun n -> System.String n) instead of just System.String because class constructors can't be used without arguments :-(
    let isPalindrome n = n.ToString() |> Array.ofSeq |> Array.rev |> (fun n -> System.String n) |> System.Convert.ToInt32 = n
    let min = 100
    let max = 999

    //ridiculously inefficient way of generating all pairs
    let brutePairs = seq{for i in min .. max do
                            for j in min .. max do
                                yield i * j}
    let ans = brutePairs |> Seq.filter isPalindrome |> Seq.max

I put obvious in quotes above, because Dustin Campbell had an even more obvious solution for reversing a number that didn’t occur to me. The use of strings to manipulate numbers is a sure sign that I spent too much time using Ruby at Centaur :-).

    let reverse n =
        let rec loop x acc =
            if x = 0 then acc
            else loop (x/10) (acc*10 + (x%10))
        loop n 0

    let isPalindrome n =
        n = reverse n
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Project Euler in F#: Problem 4

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