Jacob Carpenter’s Weblog

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 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.]

March 26, 2008

LINQ to prime numbers

Filed under: csharp, Euler, LINQ — Jacob @ 4:41 pm

Having last Friday off, and finding myself in want of something to do, I decided to learn F#. Once I installed F#, though, I learned that desire and motivation are different things.

So I started killing time by solving Project Euler problems. In LINQPad.

(Which you really should download, if you haven’t already.)

People more eloquent than me can explain how embracing constraints helps creativity flourish. I’m not going to try.

Instead, I’ll share a prime number generator inspired by Euler problem 10 and implemented with LINQ:

var odds =
	from n in Enumerable.Range(0, int.MaxValue)
	select 3 + (long) n * 2;

var primes = (new[] { 2L }).Concat(
	from p in odds
	where ! odds.TakeWhile(odd => odd * odd <= p).Any(odd => p % odd == 0)
	select p);

This certainly isn’t the most efficient prime number generator in the world. But the full query to solve the problem (left as a exercise to the reader) runs in a perfectly acceptable less than six seconds on my machine. And it uses no intermediate storage for the primes!

Now that you’ve downloaded LINQPad—you have downloaded LINQPad, haven’t you?—you can start solving Project Euler problems in a blissfully constrained environment, too!

I’ve got Problem 1 down to 54 characters. 🙂

March 13, 2008

Dictionary To Anonymous Type

Filed under: csharp, extension methods, LINQ — Jacob @ 5:34 pm

There’s some buzz about how cool it is to initialize a Dictionary from an anonymous type instance. Roy Osherove recently wrote about it, though he attributes the technique to the ASP.NET MVC framework. Alex Henderson (whose blog I just subscribed to) also came up with an inspiring use of Lambda expressions to initialize Dictionaries (don’t miss the related posts at the bottom).

But I haven’t seen anyone do the reverse: initialize an anonymous type instance from a Dictionary.

Until now.


public static class DictionaryUtility
    public static TValue GetValueOrDefault<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key)
        TValue result;
        dict.TryGetValue(key, out result);
        return result;

Show me the code!

public static class AnonymousTypeUtility
    public static T ToAnonymousType<T, TValue>(this IDictionary<string, TValue> dict, T anonymousPrototype)
        // get the sole constructor
        var ctor = anonymousPrototype.GetType().GetConstructors().Single();

        // conveniently named constructor parameters make this all possible...
        var args = from p in ctor.GetParameters()
            let val = dict.GetValueOrDefault(p.Name)
            select val != null && p.ParameterType.IsAssignableFrom(val.GetType()) ? (object) val : null;

        return (T) ctor.Invoke(args.ToArray());

Notice anonymousPrototype. This is a technique called casting by example, coined by Mads Torgerson (of the C# team) in the comments of this post.

Since you can’t ever explicitly refer to the type of an anonymous type, you have to provide an example instance. Using an undocumented feature of the default keyword, we can strongly type the properties of our prototype object without a bunch of null casting.

Here’s some sample code to get you going:

class Program
    static void Main(string[] args)
        var dict = new Dictionary<string, object> {
            { "Name", "Jacob" },
            { "Age", 26 },
            { "FavoriteColors", new[] { ConsoleColor.Blue, ConsoleColor.Green } },

        var person = dict.ToAnonymousType(
                Name = default(string),
                Age = default(int),
                FavoriteColors = default(IEnumerable<ConsoleColor>),
                Birthday = default(DateTime?),

        foreach (var color in person.FavoriteColors)

And thanks to anonymous types overriding ToString(), our program reasonably outputs:

{ Name = Jacob, Age = 26, FavoriteColors = System.ConsoleColor[], Birthday =  }

Notice that the types don’t even need to exactly match! The dictionary’s “FavoriteColors” value is a ConosleColor[]. But the anonymous type has an IEnumerable<ConsoleColor> property.


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