Jason Rowe » Uncategorized

Interview Programming Question – find perfect numbers

Jason — Tue, 15 Sep 2009 05:37:17 +0000

Here is a problem I got once for a entry level programming job. I received lots of help from the person asking and it turned out to be a good experience. If you ever use this in an interview be nice because this really has little to do with real world programming and more about finding out about the person.

Question: Create a method to find perfect numbers. In mathematics, a perfect number is defined as a positive integer which is the sum of its proper positive divisors

Possible Answer:


class Program
{
static void Main()
{

for (int j = 1; j < 10000; j++)
{
if (j % 2 == 0)
{
if (PerfectNumber.IsPerfectNumber(j))
Console.WriteLine(j + "is perfect");
}
}
Console.ReadLine();
}
}

public static class PerfectNumber
{
public static bool IsPerfectNumber(int n)
{
var totalFactor = 0;
for (var i = 1; i < n; i++)
{
if(n % i == 0)
{
totalFactor = totalFactor + i;
}
}

return totalFactor == n;
}
}

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Turing’s Lambda Notation

Jason — Sat, 05 Sep 2009 06:50:26 +0000

I’m still working through Turing’s paper on Computability and the Turing Machine. I am at the end and reading the appendix on Alonzo Church’s equivalent approach using Lambda Calculus. His work is so familiar to most programmers since we use the concepts in C, C++, Java, and C#. I decided to use C# Lambda syntax to get familiar before reading through the rest of the paper.

I started simply and considered how I could make the following expression be represented by M.

Math.Pow(x, 2.0) + 5 * x + 7

Most commonly we see it in this form when programming.


static double F(double x)
{

return Math.Pow(x, 2.0) + 5 * x + 7;

}

So convert it to a lambda expression.


OneV firstExp = x => Math.Pow(x, 2.0) + 5 * x + 7;

where OneV is defined as (not suprisingly)


delegate double OneV(double x);

In C# the two variable syntax could be something like this:


TwoV Exp = (x, y) => Math.Pow(y, 2.0) + 5 * y + 18 * x + 2 * x * y + 7;

So thats how I got my head around the very basic syntax.

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Intro to Propositional Logic Using C#

Jason — Fri, 28 Aug 2009 03:18:20 +0000

I’ve been reading about propositional (or sentential) logic in the book “The Annotated Turing”. I put together a quick console app to write out truth tables. It helped me get the hang of the mathematical notation “v”, “&”, and “->”. In C# those would be ||, &&, and !x || y respectively. I hope someone else finds it useful.


using System;
using System.Collections.Generic;

namespace PropositionalLogic
{
    public class Program
    {
        public class truthvalue
        {
            public bool X { get; set; }
            public bool Y { get; set; }
        }

        public static bool XorY(bool x, bool y)
        {
            return (x || y);
        }

        public static bool XandY(bool x, bool y)
        {
            return (x && y);
        }

        public static bool Exclusive(bool x, bool y)
        {
            return (x && y) && !(x && y);// always false
        }

        public static bool Tautoloty(bool x, bool y)
        {
            return x || y || (!x && !y);// always true
        }

        public static bool Contradition(bool x, bool y)
        {
            return x || (y && !y);
        }

        public static bool MaterialImplication(bool x, bool y)
        {
            return !x || y; //this can also be expressed as the following conjunction !(x && !y);
        }

        public static bool BiConditional(bool x, bool y)
        {
            return MaterialImplication(x, y) && MaterialImplication(y, x); //"if and only if"
        }

        static void Main(string[] args)
        {
            var truthTable = new List
                                 {
                                     new truthvalue() {X = false, Y = false},
                                     new truthvalue() {X = false, Y = true},
                                     new truthvalue() {X = true, Y = false},
                                     new truthvalue() {X = true, Y = true}
                                 };

            var sentences = new Dictionary>
                                {
                                    {"X v Y", XorY},
                                    {"X & Y", XandY},
                                    {"(x v y)& -(x & y)", Exclusive},
                                    {"x v y v (-x & -y)", Tautoloty},
                                    {"x -> y", MaterialImplication},
                                    {"x ~ y", BiConditional}
                                };

            CalculateTruthTable(truthTable, sentences);

            Console.ReadLine();
        }

        private static void CalculateTruthTable(IEnumerable truthTable, Dictionary> sentences)
        {
            foreach (var sentence in sentences)
            {
                Console.WriteLine("");
                Console.WriteLine("|X      |Y      |{0}  |", sentence.Key);

                foreach (var value in truthTable)
                {
                    var x = value.X;
                    var y = value.Y;

                    var result = sentence.Value(x, y);

                    Console.WriteLine("|{0}  |{1}  |{2}  |", PrettyPrint(x), PrettyPrint(y), PrettyPrint(result));
                }
            }
        }

        private static string PrettyPrint(bool input)
        {
            return input ? input + " " : input.ToString();
        }
    }
}

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St. Paul Pipeworks – Plumber Contractor Review

Jason — Mon, 24 Mar 2008 01:43:37 +0000

I hired St. Paul Pipeworks to replace 5 cold water shut off valves. I would have done it myself but this was in a condo so I wanted to hire someone with insurance incase something went wrong. So everything went fine until about 5 days later when I opened the kitchen sink cabnit. Apparently they did not tighten the new valve and it had a slow leak that was not noticed.

UPDATE: Dave from St. Paul Pipeworks sent me a email and said they would pay me back for this valve replacement on Friday. So I feel a little better about having to do the tightening and cleanup on this. It’s almost getting dry…

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Simpsonizme – Me as a simpson! :)

Jason — Fri, 20 Jul 2007 20:07:00 +0000

See what you look like – http://www.simpsonizeme.com/

show me how it turns out.

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Mouse Events onmouseout issue event bubbling

Jason — Fri, 12 Jan 2007 23:23:00 +0000

Quick JavaScript example to handle the onmouseout event to prevent event bubbling in IE.

if(window.addEventListener)

{

// Mozilla, Netscape, Firefox

object.addEventListener(‘mouseover’,function() {this.className = “objectVisible”;false)

object.addEventListener(‘mouseout’,function() {this.className = “objectNotVisible”;return true;}, false)}

else { // IE

object.onmouseenter = function() {this.className = “objectVisible”;return false;};

object.onmouseleave = function() {this.className = “objectNotVisible”;return false;};

}

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head spin

Jason — Fri, 27 Oct 2006 02:19:00 +0000

P1000650
Originally uploaded by photoshootya.

I took this photo last night at a break dance contest. It was fun to watch and a great challenge to get good photos.

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