F# Project Euler Problem 2

Solving second problem for Project Euler Problem 2 is an interesting one.

Each new term in the Fibonacci sequence is generated by
adding the previous two terms. By starting with 1 and 2, the first
10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, …
By considering the terms in the Fibonacci sequence whose
values do not exceed four million, find the sum of the even
valued terms.

Challenge here is to generate a fibonacci sequence, which is the fib(n-1) + fib(n-2) formula. This generates a evaluation tree during recursion and becomes quite large when generating a large N fibonacci value, and stackoverflow happens. To tackle this problem in a more efficient way, we shouldn’t generate the number for N re-cursing backwards, but generate the number from summation forwards.

Here’s my first F# solution I came up with.

let initiniteFib =
    let rec fib1 n1 n2 =
        seq {
            yield n1
            yield! fib1 n2 (n1+n2)
    fib1 0I 1I

let answer = initiniteFib 
                |> Seq.takeWhile (fun x -> x < 4000000I)
                |> Seq.filter (fun x -> x % 2I = 0I)
                |> Seq.sum

Another way to solve the same problem, is using Seq.unfold.

let answer = Seq.unfold (fun (first, second) -> Some(second, (second, first+second))) (0I, 1I)
                |> Seq.takeWhile (fun x-> x < 4000000I)
                |> Seq.filter (fun x -> x % 2I = 0I)
                |> Seq.sum

If you have more interesting ways to solve this, please share.


F# Project Euler Problem 1

Recently I have started getting more exposure about F# at work. Another developer and myself managed woto build a scheduling service purely written in F#, and it was a very humbling experience.

I’ve gotten much more confidence now writing in F#, and starting to actually enjoy it more than C#. It’s a very different mindset and paradigm to write functionally.

We’ve also started a small community at work learning, sharing and practicing in F#. One of the interesting exercises is solving mathematical problems in F# from Project Euler.

Here’s my solution in solving Problem 1.

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
Find the sum of all the multiples of 3 or 5 below 1000.

let answer = seq {1..999}
|> Seq.filter (fun n -> n % 3 = 0 || n % 5 = 0)
|> Seq.sum

Simple enough 🙂

Article: Great F# Overview

My colleague sent me a link to a great F# overview from Chris Smith, who presented at DevLink and is a member on the F# team at Microsoft. Everything you want to know about F# on a high level is covered in his post, in a very objective approach.

Check it out here.

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Moving to a functional state of mind with F#

In my previous post, I talked about my uneventful journey of learning a functional language and how it was revived again by discovering this great book called Functional Programming for the real world. What’s great about this book is that the authors prepare you mentally for the transition to a functional state of mind. If you are not interested in functional programming, understanding the key aspects of it is still beneficial because it opens up a different approach to solving problems, which is never a bad thing. For example, functional programmers aim to write elegant and succinct code, as well as aiming to be natural to read by making use of it’s declarative nature. Once you learn this skill, you can apply this even to static language, as we will soon see.

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Is there a functional programmer inside you?

I only just realized I have had a crush on Functional Programming for ages. When I first learned and used Generics and Anonymous Methods in .NET 2.0, I instantly fell in love. Next came C# 3.0, LINQ and XAML, and I was instantly drawn to it’s declarative nature and style. Having done programming for a relatively short time, I delved mostly in Imperative Programming in statically typed languages like C#, VB.NET and C++. C# 3.0 had a facelift by getting some aspects of functional programming (like lambda expressions) and that made it more powerful, fun and simpler to write/understand. Eventually I decided to venture into the world of Functional Programming, namely F#.

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