2012-06-23 Scan - 7 higher order functions

2012-06-23 scan-7-higher-order-functions

# Title: Scan - 7 higher order functions
# Author: Mikael Lundin
# Link: http://blog.mikaellundin.name/2012/06/23/scan-7-higher-order-functions.html

Balance.cs

// Data
var initialBalance = 1122.73;
var transactions = new[] { -100.00, 450.34, -62.34, -127.00, -13.50, -12.92 };
 
// Scan transaction history
var accountHistory = transactions.Scan((balance, transaction) => balance + transaction, initialBalance);
             
// Print
Console.WriteLine("Initial balance:\n{0:c}", initialBalance);
Console.WriteLine("{0,15} {1,15}", "Transactions", "Balance");
foreach (var balance in transactions.Zip(accountHistory, Tuple.Create))
{
    Console.WriteLine("{0,15:c} {1,15:c}", balance.Item1, balance.Item2);
}
 
// =>
// Initial balance:
// 1.122,73 kr
// Transactions         Balance
//   -100,00 kr     1.022,73 kr
//    450,34 kr     1.473,07 kr
//    -62,34 kr     1.410,73 kr
//   -127,00 kr     1.283,73 kr
//    -13,50 kr     1.270,23 kr
//    -12,92 kr     1.257,31 kr

Departures.cs

// The bus departs at 20 minutes / 10 minutes intervals
// [20, 10, 20, 10, 20, ..]
var departures = 0.Expand(minute => minute == 20 ? 10 : 20);
 
// First bus departs at 10:09
var firstDeparture = DateTime.Parse("2012-06-23 10:09");
 
// Get timetable for each departure
var timetable = departures.Scan((time, minute) => time.AddMinutes(minute), firstDeparture);
 
Console.WriteLine("Bus departures at");
foreach (var departure in timetable.Take(10))
{
    Console.WriteLine(departure.ToString("t"));
}
 
// Bus departures at
// 10:09
// 10:29
// 10:39
// 10:59
// 11:09
// 11:29
// 11:39
// 11:59
// 12:09
// 12:29

Scan.cs

public static IEnumerable<U> Scan<T, U>(Func<U, T, U> fn, U state, IEnumerable<T> list)
{
    if (list == null)
    {
        throw new ArgumentNullException("list", "Supplied list to Scan is not allowed to be null");
    }
 
    if (fn == null)
    {
        throw new ArgumentNullException("fn", "Supplied function to Scan is not allowed to be null");
    }
 
    var result = state;
    foreach (var item in list)
    {
        yield return result = fn(result, item);
    }
}
 
public static IEnumerable<U> Scan<T, U>(this IEnumerable<T> list, Func<U, T, U> fn, U state)
{
    return Scan(fn, state, list);
}