Tag Archives: F#

Project Euler in F#: Problem 8

I’ve been trying to teach myself F# using the Project Euler problems, and I’m starting to feel I’m getting somewhere with the language. The few Euler problems I’ve solved so far have had very straightforward and natural solutions.

Problem 8 is as follows: Find the subsequence of 5 consecutive digits that yield the greatest product when multiplied together, in the 1000-digit number:


I was able to come up with an F# solution that is one line, plus a helper line to convert the string into a sequence of digits:

let str = "731<...>"

let digits = Seq.map (fun x -> int (Char.GetNumericValue x)) str

let maxproduct num list =
 Seq.max (Seq.map (fun x -> Seq.reduce (*) x) (Seq.windowed num list))

The value digits is just a list of the digits in the string, converted into integers. The function maxproduct works the obvious way: take every subsequence of five digits (Seq.windowed), multiply them together (Seq.reduce, applied to each element of the sequence with Seq.map) and then find the maximum (Seq.max).

The only reason this needs quite so little work is the existence of Seq.windowed in the standard library, which does exactly the right thing in turning a 1000-element list into 996 arrays of subsequences of consecutive digits.

I’m not sure I like ramming all the functions into one line, and I’m sure there must be a way to combine map and reduce without the lambda, which adds a lot of clutter. If this was real code, it would need quite a lot of work to make it readable. However, the standard library is a big win, because the process of ‘windowing’ a sequence is nicely separated from the code. It’s also nice (for toy problems like this, at any rate) that the program is pretty much a definition of the problem, with little thought being necessary as to how to do the processing.

First impressions of F#

I’ve just read through Foundations of F# and written one or two trivial scripts in F#, so it’s too early to make a proper balanced assessment of it as a language. However, I wanted to record my immediate reactions to it so far while they are still fresh in my mind.

I don’t have a strong background in functional programming, but I’ve dabbled with Common Lisp, Haskell, Clojure and Scala. I’m excited by the new trend of functional languages targetting the JVM and .Net CLR, since this seems to solve the major problem with functional languages of the past in not having library code in sufficient quantity and quality.

Targetting either of the widely available virtual machines seems to be a double-edged sword, however. From what I can tell neither the JVM nor the CLR are well suited to functional languages. Clojure, Scala and F# all seem to have (and I’m being polite here) idiosyncracies forced upon them by the underlying runtime.

My immediate reaction to F# is that it seems to work hard to make it easy to interact with OO languages on the CLR, at the expense of being a great functional language in its own right. Haskell and Clojure both have strong (and different) concepts of lazy evaluation that permeate the language and let you program in a whole new way. If F# has this, it’s not been obvious to me in the first couple of hours using it.

Nor (as far as I can tell) does F# have particularly good support for parallelisation, which is one of the key advantages of functional programming for me. In my opinion Clojure has the strongest claim here, having been built from the ground up to be parallelised safely. Haskell apparently has very strong tool support in the form of ghc. What does F# offer in this direction? It’s not at all clear to me, but with the ability to create mutable fields with a single keyword, and no support for inferring immutability via the type system, it doesn’t fill me with confidence.