Yet another Project Euler post follows. This time I am looking at problem number eight:

Discover the largest product of five consecutive digits in the 1000-digit number.

The first thing to do was get the string of digits into a usable form. I copied them from the Project Euler web site, pasted them into a document and then did some manual labour to make them into a long string:

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

I know this is kind of inefficient (each + operation will create a new immutable string object containing the contents of the previous operation with the new part appended) but it gets the job done. Now, in F#, a string is automatically a sequence so we can just process it like this:

chars |> 
    Seq.map (fun c -> int c - int '0') |>
    Seq.windowed 5 |>
    Seq.map (fun a -> Array.fold (*) 1 a) |>
    Seq.max |>
    printfn "max product = %A"

The idea here is that we convert the sequence of characters into a sequence of digits and then create a new sequence from it which consists of each set of five consecutive digits. Then we compute the product of each of these five digit sets and find the maximum.

This was one solution that I reworked extensively on revisiting it. My first attempt was not very idiomatic. I think this one is much better.