Euler 8 in FSharp

19 May 2008

The problem:

Find the greatest product of five consecutive digits in the 1000-digit number.

73167176531330624919225119674426574742355349194934 96983520312774506326239578318016984801869478851843 85861560789112949495459501737958331952853208805511 12540698747158523863050715693290963295227443043557 66896648950445244523161731856403098711121722383113 62229893423380308135336276614282806444486645238749 30358907296290491560440772390713810515859307960866 70172427121883998797908792274921901699720888093776 65727333001053367881220235421809751254540594752243 52584907711670556013604839586446706324415722155397 53697817977846174064955149290862569321978468622482 83972241375657056057490261407972968652414535100474 82166370484403199890008895243450658541227588666881 16427171479924442928230863465674813919123162824586 17866458359124566529476545682848912883142607690042 24219022671055626321111109370544217506941658960408 07198403850962455444362981230987879927244284909188 84580156166097919133875499200524063689912560717606 05886116467109405077541002256983155200055935729725 71636269561882670428252483600823257530420752963450

Treating it as a number is probably not going to be too pleasant.

let prob8num = "7316 ... 52963450"
let chars = prob8num.ToCharArray() |> Array.map(fun x -> Char.code(x) - Char.code('0'))
let rec prob8 (rest : array) =
    if rest.Length     else
        let tl = Array.sub rest 1 (rest.Length - 1)
        let curProd = Array.sub rest 0 5 |> Array.reduce_left( * )
        Math.Max(prob8(tl), curProd)
prob8 chars

So, store it as a string, but then convert it to a character array, and map to the actual numerical values (assuming ASCII I guess).

From that, just a simple recursive solution that does way too much Array.sub’ing. It would make much more sense to use List’s instead of Array’s for all of those .subs, but, there was no sub extraction built into List so I just did it that way instead.

I wrestled a little with the syntax of F# again in this question. I tried putting the “let tl” as inline into the body of the call to Math.Max, but I couldn’t get it to work correctly. Not sure what the required syntax is, maybe I need some of the keywords that I don’t know from non-#light yet?

Also, my copy of Expert F# has arrived, so I’m just starting to peruse it now. Hopefully my code will become more idiomatic soon enough.

And man, do I hate blog software’s handling of code. You’d think that maybe if I typed a less-than sign they could figure out how to escape it moderately properly, but apparently that’s way too complicated. Idiotic.