Generation Operations

Generation operations spawn new sequences of values.

The query operator methods used to perform these operations follow: DefaultIfEmpty, Empty, Range, and Repeat.

The DefaultIfEmpty method replaces an empty collection with a singleton collection of default values. Review its syntax below: 

public static IEnumerable<TSource> DefaultIfEmpty<TSource>(
	this IEnumerable<TSource> source
)

Review an example of its use below: 

class Boot
{
	public string Name { get; set; }
	public int Quantity { get; set; }
}

public static void Example()
{
	List<Boot> boots =
		new List<Boot>{ new Boot { Name="Fashion", Quantity=15 },
		new Boot { Name="Work", Quantity=10 },
		new Boot { Name="Casual", Quantity=20 } };

	foreach (Boot boot in boots.DefaultIfEmpty())
	{
		Console.WriteLine(boot.Name);
	}
}

This example operates on an empty sequence: 

List<int> quantities = new List<int>();

foreach (int quantity in quantities.DefaultIfEmpty())
{
	Console.WriteLine(quantity);
}

The Empty method simply returns an empty collection. Review its syntax below: 

public static IEnumerable<TResult> Empty<TResult>()

Review an example of its use below: 

IEnumerable<decimal> novalues = Enumerable.Empty<decimal>();

Review another example which partners Empty with Aggregate: 

string[] team1 = { "Hart, Timmy" };
string[] team2 = { "Adrien, Teddy", "Anderson, Henry B.", "Heder, Kwame",
"Ikamoto, Fred" };
string[] team3 = { "Singh, Jakob", "Appiah, Michael", "Anderson, Henry B.", "Lame,
Skylar", "McWaffle, Brad" };
List<string[]> teamList = new List<string[]> { team1, team2, team3 };

// Only include arrays with 4 or more elements
IEnumerable<string> allTeams =
	teamList.Aggregate(Enumerable.Empty<string>(),
	(current, next) => next.Length > 3 ? current.Union(next) :
current);
				
foreach (string member in allTeams)
{
	Console.WriteLine(member);
}

The Range method spawns a collection containing a number sequence. Review its syntax below: 

public static IEnumerable<int> Range(
	int start,
	int count
)

Review an example of its use below: 

IEnumerable<int> squaredVal= Enumerable.Range(1, 15).Select(y => y * y);

foreach (int value in squaredVal)
{
	Console.WriteLine(value);
}

The Repeat method spawns a collection containing a single repeated value. Review its syntax below: 

public static IEnumerable<TResult> Repeat<TResult>(
	TResult element,
	int count
)

Review an example of its use below: 

IEnumerable<string> statements =
	Enumerable.Repeat("What silence looks like.", 21);
foreach (String strg in statements)
{
	Console.WriteLine(strg);
}
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