CODEWITHSUNDEEP
f#
Overly Excessive
Overly Excessive

Reputation: 2125

Project Euler #8 in F#

I disgress, I am stuck and I really can't wrap my head around what's wrong. The problem reads.

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

Now I store the digit as a string, I only fetch the numbers, I loop through and grab each 13-digit "substring" as an array, multiply them together and compare them. Now I have verified that I am only getting a 1000-digit char array, I have verified that I get exactly 75 char arrays of equal size. Yet I am not getting the right answer.

Here is the code

let problem8() = 
    let str = @"731671765313306..."
                |> Seq.filter (Char.IsDigit)
                |> Seq.toArray

    (* We only need to go to 987 because 1000 isn't divisble by 13 and if we were to take the last 11 digits from 987 
       we would end up with 0 anyhow. *)
    seq { for i in 0.. 13 ..987 -> str.[i..i + 12] } 
    |> Seq.map (Seq.fold (fun acc chr -> acc * int64 (Char.GetNumericValue(chr))) 1L)
    |> Seq.max

problem8() 
|> printfn "%d"

Upvotes: 0

Views: 218

Answers (1)

Tomas Petricek
Tomas Petricek

Reputation: 243096

If you are using 0 .. 13 .. 987 in the sequence expression, then you are partitioning the array like this (for simplicity, using 3-sized blocks in 10 digits):

[012][345][678]9

I suppose that the question wants you to look for all possible sub-strings, i.e.

[012][345][678]9
0[123][456][789]
01[234][567]89

So, you probably need to try all indices using 0 .. 987.

By the way, I'd suspect that converting char to int64 will be faster using int64 c - 48L.

Upvotes: 1

Related Questions