# The Standard Output

One man's standard output is another man's standard input!

## List Comprehensions in C#

A List Comprehension is a type of set notation in which the programmer can describe the properties that the members of a set must meet. It is usually used to create a set based on other, already existing, set or sets by applying some type of combination, transform or reduction function to the existing set(s).

Consider the following problem: You have a sequence of 10 numbers from 0 to 9 and you need to extract all the even numbers from that sequence. In a language such a C# version 1.1, you were pretty much confined to the following code to solve this problem:

```    ArrayList evens = new ArrayList();
ArrayList numbers = Range(10);
int size = numbers.Count;
int i = 0;

while (i < size) {
if (i % 2 == 0) {
}
i++;
}
```

The code above does not show the implementation of the `Range` function, which is available in the full code listing below. With the advent of C# 3.0 and the .NET Framework 3.5, a List Comprehension notation based on Linq is now available to C# programmers. The above C# 1.1 code can be ported to C# 3.0 like so:

```    IEnumerable numbers = Enumerable.Range(0, 10);
var evens = from num in numbers where num % 2 == 0 select num;
```

And technically speaking, the C# 3.0 code above could be written as a one-liner by moving the call to `Enumarable.Range` into the Linq expression that generates the `evens` sequence. In the C# List Comprehension I am reducing the set `numbers` by applying a function (the modulo 2) to that sequence. This produces the `evens` sequences in a much more concise manner and avoid the use of loop syntax. Now, you may ask yourself: Is this purely syntax sugar? I don't know, but I will definitelly investigate, and maybe even ask the question myself at [StackOverflow][lnk1]. I suspect that this is not just syntax sugar and that there are some true optimizations that can be done by utilizing the underlying Monads.

I also posted this as [an answer to a question][lnk2] on [StackOverflow.com][lnk1]. The full code listing is available below:

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

namespace CSharpListComprehensions {
public class Program {

public static void RunSnippet() {
Run11Version();
Run35Version();
}

public static void Run11Version() {
ArrayList evens = new ArrayList();
ArrayList numbers = Range(10);
int size = numbers.Count;
int i = 0;

WL(""); WL("Numbers:");
foreach (int x in numbers) {
WL(x.ToString());
}

while (i < size) {
if (i % 2 == 0) {
}
i++;
}

WL(""); WL("Evens:");
foreach (int y in evens) {
WL(y.ToString());
}
}

public static void Run35Version() {
IEnumerable numbers = Enumerable.Range(0, 10);

WL(""); WL("Numbers:");
foreach (int x in numbers) {
WL(x.ToString());
}

var evens = from num in numbers where num % 2 == 0 select num;

WL(""); WL("Evens:");
foreach (int y in evens) {
WL(y.ToString());
}
}

#region Range Function
public static ArrayList Range(int end) {
return Range(0, end);
}

public static ArrayList Range(int start, int end) {
ArrayList al = new ArrayList();
for (int i = start; i < end; i++) {
}

return al;
}
#endregion

#region Helper methods

public static void Main() {
try {
RunSnippet();
} catch (Exception e) {
string error = string.Format("---\nThe following error occurred while executing the snippet:\n{0}\n---", e.ToString());
Console.WriteLine(error);
} finally {
Console.Write("Press any key to continue...");
}
}

private static void WL(object text, params object[] args) {
Console.WriteLine(text.ToString(), args);
}

private static void RL() {
}

private static void Break() {
System.Diagnostics.Debugger.Break();
}

#endregion
}
}
```

Written by Jonas Gorauskas

June 4th, 2009 at 6:35 pm

Posted in Programming