F# Koans - The Stock Example

14 Jul, 2020

I’m currently in the process of learning functional programming and it feels like being back in my teens and discovering programming for the first time.. again. :) I’ve been going through F# Koans and this was the first exercise that took me a little more than no time to complete. It felt good to do it so I thought I’d share my solution.

namespace FSharpKoans
open FSharpKoans.Core
open System

//---------------------------------------------------------------
// Apply Your Knowledge!
//
// Below is a list containing comma separated data about
// Microsoft's stock prices during March of 2012. Without
// modifying the list, programatically find the day with the
// greatest difference between the opening and closing prices.
//
//---------------------------------------------------------------
[<Koan(Sort = 15)>]
module ``about the stock example`` =

    let stockData =
        [ "Date,Open,High,Low,Close,Volume,Adj Close";
          "2012-03-30,32.40,32.41,32.04,32.26,31749400,32.26";
          "2012-03-29,32.06,32.19,31.81,32.12,37038500,32.12";
          "2012-03-28,32.52,32.70,32.04,32.19,41344800,32.19";
          "2012-03-27,32.65,32.70,32.40,32.52,36274900,32.52";
          "2012-03-26,32.19,32.61,32.15,32.59,36758300,32.59";
          "2012-03-23,32.10,32.11,31.72,32.01,35912200,32.01";
          "2012-03-22,31.81,32.09,31.79,32.00,31749500,32.00";
          "2012-03-21,31.96,32.15,31.82,31.91,37928600,31.91";
          "2012-03-20,32.10,32.15,31.74,31.99,41566800,31.99";
          "2012-03-19,32.54,32.61,32.15,32.20,44789200,32.20";
          "2012-03-16,32.91,32.95,32.50,32.60,65626400,32.60";
          "2012-03-15,32.79,32.94,32.58,32.85,49068300,32.85";
          "2012-03-14,32.53,32.88,32.49,32.77,41986900,32.77";
          "2012-03-13,32.24,32.69,32.15,32.67,48951700,32.67";
          "2012-03-12,31.97,32.20,31.82,32.04,34073600,32.04";
          "2012-03-09,32.10,32.16,31.92,31.99,34628400,31.99";
          "2012-03-08,32.04,32.21,31.90,32.01,36747400,32.01";
          "2012-03-07,31.67,31.92,31.53,31.84,34340400,31.84";
          "2012-03-06,31.54,31.98,31.49,31.56,51932900,31.56";
          "2012-03-05,32.01,32.05,31.62,31.80,45240000,31.80";
          "2012-03-02,32.31,32.44,32.00,32.08,47314200,32.08";
          "2012-03-01,31.93,32.39,31.85,32.29,77344100,32.29";
          "2012-02-29,31.89,32.00,31.61,31.74,59323600,31.74"; ]

    let splitCommas (x:string) =
        x.Split([|','|])

    let findColumnIndex data key =
        data
        |> List.head
        |> splitCommas
        |> Array.findIndex ((=) key)

    let parseDailyData data =
        data
        |> List.tail
        |> List.map splitCommas

    let getDailyDiffs data =
        let dateIndex = findColumnIndex data "Date"
        let openIndex = findColumnIndex data "Open"
        let closeIndex = findColumnIndex data "Close"
        let diff x y = abs(Double.Parse(x) - Double.Parse(y))

        data
        |> parseDailyData
        |> List.map (fun x -> (x.[dateIndex], diff x.[closeIndex] x.[openIndex]))

    let getMaxDailyDiff data =
        data
        |> getDailyDiffs
        |> List.maxBy (fun (_, x) -> x)

    [<Koan>]
    let YouGotTheAnswerCorrect() =
        let result, _ = getMaxDailyDiff stockData

        AssertEquality "2012-03-13" result