Solutions to the F# Koan for AboutTheStockExample

AboutTheStockExample.fs

namespace FSharpKoans
open FSharpKoans.Core

//---------------------------------------------------------------
// 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 variance between the opening and closing price.
//
// The following functions may be of use:
// 
// abs - takes the absolute value of an argument
// 
// System.Double.Parse - converts a string argument into a 
//                       numerical value.
//
// The following function will convert a comma separated string
// into an array of the column values.
//                       
// let splitCommas (x:string) =
//     x.Split([|','|])
//---------------------------------------------------------------
[<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"; ]
    
    // Feel free to add extra [<Koan>] members here to write
    // tests for yourself along the way. You can also try 
    // using the F# Interactive window to check your progress.

    [<Koan>]
    let YouGotTheAnswerCorrect() =

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

        let columnNames = splitCommas stockData.Head

        let columnIndex colName =
            columnNames
            |> Array.findIndex (fun x -> x = colName) 

        let idxDate = columnIndex "Date"
        let idxOpen = columnIndex "Open"
        let idxClose = columnIndex "Close"

        // initial solution used a tuple of the date and the price difference
        let datePriceDiff (dateData: string []) =
            dateData.[idxDate], abs (System.Double.Parse dateData.[idxClose]) - (System.Double.Parse dateData.[idxOpen]) 


        // First solution was via a Seq
        let GreatestChange (data: string List) =
            data.Tail
            |> Seq.ofList
            |> Seq.map splitCommas
            |> Seq.map datePriceDiff
            |> Seq.maxBy snd
            |> fst

        let result = GreatestChange stockData
        AssertEquality "2012-03-13" result


        // Second is as a List. Wasn't sure which is idiomatic, Seq or List
        // Found this... http://stackoverflow.com/questions/10814203/when-to-use-a-sequence-in-f-as-opposed-to-a-list
        let GreatestChange2 (data: string List) =
            data.Tail
            |> List.map splitCommas
            |> List.map datePriceDiff
            |> List.maxBy snd
            |> fst

        let result2 = GreatestChange stockData
        AssertEquality "2012-03-13" result2



        // Third is without the tuple, which gives one less step to do on each data row
        let GreatestChange3 (data: string List) =
            let diff (dateData: string []) =
                abs (System.Double.Parse dateData.[idxClose]) - (System.Double.Parse dateData.[idxOpen]) 

            let bestRow =
                data.Tail
                |> List.map splitCommas
                |> List.maxBy diff
            bestRow.[idxDate]    

        let result3 = GreatestChange stockData
        AssertEquality "2012-03-13" result3