Jacob Carpenter’s Weblog

April 23, 2008

C# abuse of the day: SwitchOnType

Filed under: csharp, extension methods — Jacob @ 5:30 pm

Today I encountered a situation where I wanted to switch based on a type. Maybe I stayed up a little too late reading Foundations of F#, last night.

While this is certainly no pattern matching, it didn’t seem like terrible C#:

DefinitionBase definitionBase = /*...*/;

var targetProperty = definitionBase.SwitchOnType(
        (ColumnDefinition col) => ColumnDefinition.WidthProperty,
        (RowDefinition row) => RowDefinition.HeightProperty);

Note that the lambdas require type decoration (you really don’t want to explicitly declare the generic parameters on this method).

Here’s the implementation (taking two Func projections—feel free to overload to your heart’s content):

public static TResult SwitchOnType<T, T1, T2, TResult>(this T source,
    Func<T1, TResult> act1, Func<T2, TResult> act2)
    if (source is T1)
        return act1((T1) source);

    if (source is T2)
        return act2((T2) source);

    throw new InvalidOperationException("No matching delegate found");

As you can see from the implementation, the method returns the result of the first delegate for which source can be converted into a parameter.

For a default case, add a final delegate that takes object.


April 16, 2008

PC#1: A solution

Filed under: challenge, csharp, extension methods, LINQ — Jacob @ 12:21 pm

So, when I initially posed the programming challenge #1 I stated:

… since I intended to output HTML, ASP.NET seemed a logical choice. But I was amazed at the amount of code required for such a seemingly simple task (not to mention how ugly code containing <% and %> is!).

Well, it turns out, using plain old C# with a little LINQ to XML functional construction made my solution a lot nicer.


I created a few DateTimeExtensions to enhance readability, though I could have easily inlined the implementation of each of those methods without any LOC impact.

public static class DateTimeExtensions
    public static DateTime ToFirstDayOfMonth(this DateTime dt)
        return new DateTime(dt.Year, dt.Month, 1);
    public static DateTime ToLastDayOfMonth(this DateTime dt)
        return new DateTime(dt.Year, dt.Month, DateTime.DaysInMonth(dt.Year, dt.Month));
    public static DateTime ToFirstDayOfWeek(this DateTime dt)
        return dt.AddDays(-((int) dt.DayOfWeek));
    public static DateTime ToLastDayOfWeek(this DateTime dt)
        return dt.AddDays(6 - ((int) dt.DayOfWeek));

I also relied on the Slice extension method I’ve previously blogged about.


static void Main(string[] args)
    DateTime today = DateTime.Today;
    DateTime firstDayOfMonth = today.ToFirstDayOfMonth();
    DateTime startCalendar = firstDayOfMonth.ToFirstDayOfWeek();
    DateTime lastDayOfMonth = today.ToLastDayOfMonth();
    DateTime endCalendar = lastDayOfMonth.ToLastDayOfWeek();

    var calendarPrefix =
        from day in Enumerable.Range(startCalendar.Day, (firstDayOfMonth - startCalendar).Days)
        select new XElement("td", new XAttribute("class", "prevMonth"), day);
    var calendarMonth =
        from day in Enumerable.Range(1, lastDayOfMonth.Day)
        select new XElement("td", day == today.Day ? new XAttribute("class", "today") : null, day);
    var calendarSuffix =
        from day in Enumerable.Range(1, (endCalendar - lastDayOfMonth).Days)
        select new XElement("td", new XAttribute("class", "nextMonth"), day);

    var calendar = calendarPrefix.Concat(calendarMonth).Concat(calendarSuffix);

    var table = new XElement("table",
        new XElement("thead",
            new XElement("tr",
                from offset in Enumerable.Range(0, 7)
                select new XElement("th", startCalendar.AddDays(offset).ToString("ddd")))),
        new XElement("tbody",
            from week in calendar.Slice(7)
            select new XElement("tr", week)));


I’d love to see more ways to solve this. If you’ve got a simpler or more beautiful implementation in your favorite programming langauge/web application framework, let me know in the comments of the original post.

April 15, 2008

Programming Challenge #1: HTML calendar

Filed under: challenge, web — Jacob @ 1:44 pm

While playing around at lunchtime today, I decided to write a quick little HTML calendar generator.

I consider C# my native programming language, and since I intended to output HTML, ASP.NET seemed a logical choice. But I was amazed at the amount of code required for such a seemingly simple task (not to mention how ugly code containing <% and %> is!).

The challenge

Using your favorite programming language / web application framework output the following html fragment:

<table class="calendar">
<tr><td class="prevMonth">30</td><td class="prevMonth">31</td><td>1</td><td>2</td><td>3</td><td>4</td><td>5</td></tr>
<tr><td>6</td><td>7</td><td class="today">8</td><td>9</td><td>10</td><td>11</td><td>12</td></tr>
<tr><td>27</td><td>28</td><td>29</td><td>30</td><td class="nextMonth">1</td><td class="nextMonth">2</td><td class="nextMonth">3</td></tr>

Whitespace is insignificant; class="today" should be applied to the current day’s cell; favor simplicity and elegance over efficiency.

Blog your solution and leave a comment; I’ll link to it from this post.

Please don’t comment suggesting I use some pre-built control. This is about the fun of writing such a thing, not about some specific calendaring need.

To avoid influencing your initial attempt, I’ll post my solution after awhile.


Will Asrari contributed a submission at http://www.willasrari.com/blog/coding-for-fun—html-calendar-challenge/000298.aspx.

I posted my solution at https://jacobcarpenter.wordpress.com/2008/04/16/pc1-a-solution/.

kick it on DotNetKicks.com

April 4, 2008

Euler 14

Filed under: csharp, Euler, extension methods, LINQ, Ruby — Jacob @ 12:41 pm

When I read Dustin Campbell’s latest post, I couldn’t help but feel a bit like Steve Carrell in this clip from the Office. While his solution is an admirably close port of the original F# solution, it makes me feel a little bit yucky.

Of course, it’s completely hypocritical of me to say so, since I’ve abused C# to make it exhibit F#-like behavior in the past.

But Project Euler invites elegantly simple solutions (like the original F#). Different languages have different idioms, and a literal port typically doesn’t exhibit the same beauty as the original.

If I was solving project Euler 14 in C# (with “elegance and brevity in mind”), my code would look more like:

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

namespace Euler14
    class Program
        static void Main(string[] args)
            var iterativeSequences = from start in 1.To(1000000L)
                select new
                    Start = start,
                    Length = SequenceUtility.Generate(start,
                        n => n % 2 == 0 ? n / 2 : 3 * n + 1,
                        n => n == 1).Count()

            Stopwatch sw = Stopwatch.StartNew();

            var longestSequence = iterativeSequences.Aggregate(
                (longest, current) => current.Length > longest.Length ? current : longest


            Console.WriteLine("Longest sequence starts with {0:#,#} (found in {1:#,#.000} seconds)",
                longestSequence.Start, (float) sw.ElapsedTicks / (float) Stopwatch.Frequency);

    public static class SequenceUtility
        // Can also overload To by changing the end value's type;
        // example: "int excludedEnd" returns "IEnumerable<int>"
        public static IEnumerable<long> To(this int start, long excludedEnd)
            for (long i = start; i < excludedEnd; i++)
                yield return i;

        public static IEnumerable<T> Generate<T>(T first, Func<T, T> getNext, Func<T, bool> isLast)
            T value = first;
            yield return value;

            while (!isLast(value))
                value = getNext(value);
                yield return value;

Which runs in an acceptable ~5 seconds on my machine.

[Okay. You caught me: I stole the idea for that To extension method from Ruby’s upto. I’m a huge hypocrite and take back everything I said before.

Do invest time learning the idoms of other programming languages, and try applying them to your native language. You may discover something beautiful, after all.]

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