# Month:September 2008

Linq

## Project Euler Problem One

(From their website) Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve. Although mathematics will help you arrive at elegant and efficient methods, the use of a computer and programming skills will be required to solve most problems.

I decided to take a crack or two at these, even though a trillion people have done these before me (32,022 registered answers as of the time that I’m writing this). My goal is to try to do this in whatever way occurs to me at first, and then to try to solve the problem using some new technique. Over the course of solving these problems, I want to try to dig a little into F#, Ruby, C# 3.0, or whatever seems like it might be cool ðŸ™‚

Today, I attack Problem One. I’ve included the answer in the comments, so don’t read on if you don’t want to know.

```namespace ProjectEuler
{
/// <summary>
/// Problem 1
/// 05 October 2001
/// If we list all the natural numbers below 10
/// that are multiples of 3 or 5, we get 3, 5, 6
/// and 9. The sum of these multiples is 23.
///
/// Find the sum of all the multiples of 3 or 5 below 1000.
///
/// </summary>
public class Problem1
{
public int SolveProblem()
{

for (int i = 1; i < 1000; i++)
{
if (IsMultipleOfThreeOrFive(i))
{
}
}

}

public static bool IsMultipleOfThreeOrFive(int number)
{
return (((number % 3) == 0) || ((number % 5) == 0));
}
}
}
```

Okay, that was a snoozefest. I was reading in the Pro LINQ book (the one that gave me this post) and I found Enumerable.Range. Do the wonders of LINQ never cease? Enumerable.Range will generate a range of numbers for you so that you don’t have to do a loop. Then, you could use that range to then perform a lambda on (the divisible by 3 or 5 check) and then just sum the remaining list. It turns out that this kind of thing is *exactly* what LINQ will just knock out for you. Check the resulting code.

```using System.Linq;
namespace ProjectEuler
{
/// <summary>
/// Problem 1
/// 05 October 2001
/// If we list all the natural numbers below 10
/// that are multiples of 3 or 5, we get 3, 5, 6
/// and 9. The sum of these multiples is 23.
///
/// Find the sum of all the multiples of 3 or 5 below 1000.
///
/// </summary>
public class Problem1
{
public int SolveProblem()
{
var problemSet = from i in
Enumerable.Range(1, 999)
where (((i % 3).Equals(0)) || ((i % 5).Equals(0)))
select i;

return problemSet.Sum();
}
}
}
```

Same answer? Check. Learned and used something new in the process? Check. Looks like I win the nerd prize!

Code Tips

## Enumerable.Intersect, Enumerable.Except, and Enumerable.Union

I love when things come up just in time for me to need them for a project that I’m involved in. Currently, I need to take a bunch of results and find only the intersection of those results. I was contemplating doing some lambdas to compare the lists, but then I was reading Pro LINQ: Language Integrated Query in C# 2008 and I found the Intersect() method. (Note: Thanks to Jeff Meyer for loaning me the book in such a timely fashion.)

The code looks like this:

```using System;
using System.Collections.Generic;
using System.Linq;

namespace Linq
{
class Program
{
static void Main(string[] args)
{
var listOne = new List<int>() { 1, 2, 3, 4, 5};
var listTwo = new List<int>() { 3, 4, 5, 6, 7};

var intIntersect = listOne.Intersect(listTwo);

foreach (var i in intIntersect)
{
Console.WriteLine(i);
}
}
}
}
```

What is output is

```3
4
5
```

There are two important things to note. First of all, you can use any IEnumerable to perform an Intersect. Secondly, it is important to realize that you are comparing from the first list to the second list. This isn’t important when doing an Intersect(), but lets look at another example.

```using System;
using System.Collections.Generic;
using System.Linq;

namespace Linq
{
class Program
{
static void Main(string[] args)
{
var listThree = new string[] { "Pete", "On", "Software" };
var listFour = new string[] { "Joel", "On", "Software" };

var stringExcept = listThree.Except(listFour);

foreach (var s in stringExcept)
{
Console.WriteLine(s);
}
}
}
}
```

The output of this code is

```Pete
```

In this example, I used a string array instead of a generic List to show that other IEnumerables could be used. When calling the Except() method, I get the unique value(s) from the first IEnumerable. Intersect() would have returned

```On
Software
```

```var stringExcept = listFour.Except(listThree);
```

it would have returned

```Joel
```

so much more care is needed when using Except() to ensure exactly which group has the unique values that you want to keep. However, there is one more thing you can do. What if you want to find every distinct value between the two lists? You would do something like the following

```using System;
using System.Collections.Generic;
using System.Linq;

namespace Linq
{
class Program
{
static void Main(string[] args)
{
var listThree = new string[] { "Pete", "Pete", "On", "Software" };
var listFour = new string[] { "Joel", "On", "Software", "Software" };

var uniqueStrings = listFour.Union(listThree);

foreach (var s in uniqueStrings)
{
Console.WriteLine(s);
}
}
}
}
```

which returns

```Joel
On
Software
Pete
```

The above shows all of the unique values from the first group and any of the unique values that the second group brings to the party that the first group didn’t already have.

This is pretty powerful stuff if you have to process lists and should ensure that you are doing the most efficient operations possible. Linq is, of course, pretty exciting stuff and as I uncover more nuggets from the Pro LINQ: Language Integrated Query in C# 2008 book, I will share them here.