From (Visitor) to LINQLINQ (this is where we ends in this presentation):

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var query = from a in accounts.TestData where a.Balance > 1e6 select a;

foreach (Account a in query) Console.WriteLine(a.Balance);

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Operations on a container without Visitor

//Doubling evensfor (int i = 0; i < con1.N; i++) { if (con1.Get(i) % 2 == 0)

con1.Update(i, con1.Get(i) * 2);}//Doubling allfor (int i = 0; i < con1.N; i++){ con1.Update(i, con1.Get(i) * 2);}//Increasing small ones by onefor (int i = 0; i < con1.N; i++){ if (con1.Get(i) < 10) con1.Update(i, con1.Get(i) + 1);}

Visitor Pattern

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Visitor in POJO(Plain Old Java Objects)

interface Visitor {public int visit(MyInteger number);

}interface Visitable {

public int accept(Visitor visitor);}class MyInteger implements Visitable {

private int value;MyInteger(int i) {

this.value = i;}public int accept(Visitor visitor) {

return visitor.visit(this);}public int getValue() {

return value;}

}

Can implement selection criteria here

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class SubtractVisitor implements Visitor {int value;public SubtractVisitor(int value) {

this.value = value;}public int visit(MyInteger i) {

System.out.println("Subtract integer");return (i.getValue() - value);

}}

Visitor implemented with delegates

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public class Accounts { private List<Account> TestData { get; set; } public Accounts(){ TestData = new List<Account>(); TestData.Add(new Account { Balance = 2222 }); TestData.Add(new Account { Balance = 22222 }) …. More test data} public void ModifyAll(ModifyElement f) {//Applies f to all elements in the container foreach (Account a in TestData) f(a); } public void ModifySome(ModifyElement f, CheckElement p) { //Applies f only to elements in the containerthat satisfy p foreach (Account a in TestData) if (p(a)) f(a);}}

ModifyElement and CheckElement are delegates

The delegates

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public delegate bool CheckElement(Account e); public delegate Account ModifyElement(Account e);

Simple implementation

• Just a class with operations with an appropriate signature

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public class AccountOperations { public Account PrintBalance(Account a) { Console.WriteLine(a.Balance); return null; }

public Account DoubleBalance(Account a) { a.Balance *= 2; return a; }

public Account ClearBalance(Account a) { a.Balance = 0f; return a; }

public bool IsBigAccount(Account a) { return a.Balance > 1e6 && a.Balance <= 1e7; }….. }

Driver

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Accounts accounts = new Accounts(); AccountOperations oper = new AccountOperations();

Console.WriteLine("TestData: "); ModifyElement print = oper.PrintBalance; accounts.ModifyAll(print);

Console.WriteLine("Write big accounts"); CheckElement bigAccount = oper.IsBigAccount; accounts.ModifySome(print, bigAccount);

Console.WriteLine("Clear small accounts"); CheckElement small = oper.IsSmallAccount; ModifyElement clear = oper.ClearBalance; accounts.ModifySome(clear, small); accounts.ModifyAll(print);

Use anonymous delegates

• Given following delegate and method:‒ delegate int MyDelegate(int n);‒ void MyMethod(MyDelegate f){...}

• Example on syntax for implementing delegate method:‒ MyDelegate doubleUp=delegate(int n){return 2*n;};‒ MyMethod(doubleUp);

• or shorter:‒ MyMethod(delegate(int n){return 2*n;});‒ This is called inline implementation

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Exercise

• Rewrite delegate methods (Even, Double, LessThan10, etc.) in visitor example to use anonymous delegates.

• Consider where it is (not) a good idea to use inline implementation

Driver with Anonymous delegate

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Console.WriteLine("TestData: "); ModifyElement print = delegate(Account a) { Console.WriteLine(a.Balance); return null; }; accounts.ModifyAll(print);

Console.WriteLine("Write big accounts"); accounts.ModifySome(print, delegate(Account a) { return a.Balance > 1e6 && a.Balance <= 1e7; } );

The Print delegate is not inline as it is used multiple times

Lamda expression

• Another way of defining anonymous methods

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// finds x’s bigger than 5Predicate myPredicate = x=>x>5myList.FindAll(myPredicate);//ormyList.FindAll (x=>x>5);

How to pronounce ”=>”:Anders Hejlsberg: I usually read the => operator as "becomes" or "for which". For example,Func f = x => x * 2;Func test = c => c.City == "London";reads as "x becomes x * 2" and "c for which c.City equals London"

Lamda expression

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ModifyElement print = a=> { Console.WriteLine(a.Balance); return null; }; accounts.ModifyAll(print);

Console.WriteLine("Write big accounts"); accounts.ModifySome(print,

a=>a.Balance > 1e6 && a.Balance <= 1e7;);

Extention methods

• It is possible to extend methods to a class without using inheriance

• In fact, you might even extend a sealed class• This is done by using extention methods.• An extension method is a static method in a static class• By adding ‘this classname’ to the parameterlist, it becomes an

extention method to classname

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Example

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static class ExtensionEx { public static void WriteToConsole(this string s) { Console.WriteLine(s); } }

Use: String s = "Hello"; s.WriteToConsole(); Is this really OO?

Yes, it is just another way to write ExtentionEx.WriteToConsole(s);

Extension methods are the base of linq

• Sample implementation of Where

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public static IEnumerable < TSource > Where < TSource > (this IEnumerable < TSource > source,Func < TSource, bool > predicate){

foreach (TSource item in source)if (predicate(item))

yield return item;}

yield: Start to return elements in a collection before all elements are known

Formula 1, ex1

• Using the delegates, anonymious methods and predicates

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IEnumerable < Racer > brazilChampions =Formula1.GetChampions().Where(r = > r.Country == “Brazil”).OrderByDescending(r = > r.Wins).Select(r = > r);

foreach (Racer r in brazilChampions){

Console.WriteLine(“{0:A}”, r);}

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Exercise

• Implement ‘Where’ and ‘Select’ in visitor example.• ”Where” has the same role as ” CheckElement” • ”Select” has the same role as ” ModifyElement”

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Extention Methods (Where and Select)

static class Extentions { public static IEnumerable<int> Where(this Container source,

Func<int, bool> predicate) { for (int i = 0; i < source.N; i++) { int item =source.Get(i); if (predicate(item)) yield return item; } }

public static IEnumerable<int> Select(this IEnumerable<int> source, Func<int, int>

predicate) { foreach(int item in source) yield return predicate(item); }}

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Double all evens

foreach (int i in con1.Where(x => x % 2 == 0).Select(r => r * 2)

{ Console.WriteLine(i);

}

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Anonymous types

• Partly from MSDN:• Anonymous types provide a convenient way to encapsulate a set

of read-only properties into a single object without having to explicitly define a type first.

• The type name is generated by the compiler and is not available at the source code level. The type of each property is inferred by the compiler.

• You create anonymous types by using the new operator together with an object initializer

• The var type is telling that it is an anonymous type

var v = new { Amount = 108, Message = "Hello" }; Console.WriteLine(v.Amount + v.Message);

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Anonymous types

• Partly from MSDN:• Anonymous types contain one or more public read-only

properties. • That means that if you change a property value, you will get a new

type.• No other kinds of class members, such as methods or events, are

valid. • The expression that is used to initialize a property cannot be null,

an anonymous function, or a pointer type. • The most common scenario is to initialize an anonymous type with

properties from another type. • If you do not specify member names in the anonymous type, the

compiler gives the anonymous type members the same name as the property being used to initialize them.

• You must provide a name for a property that is being initialized with an expression, as shown in the previous slide.

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Anonymous types

• Partly from MSDN:• The most common scenario is to initialize an anonymous type with

properties from another type. • In the following example, assume that a class exists that is named

Product. • Class Product includes Color and Price properties, together with

other properties that you are not interested in. • Variable products is a collection of Product objects. • The anonymous type declaration starts with the new keyword. The

declaration initializes a new type that uses only two properties from Product.

• This causes a smaller amount of data to be returned in the query.• The names of the properties of the anonymous type are Color and

Price.

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Example (using LINQ)

• From MSDN:

var productQuery = from prod in products select new { prod.Color, prod.Price };

foreach (var v in productQuery) { Console.WriteLine("Color={0}, Price={1}", v.Color, v.Price);

}

Formula 1, ex2

• LINQ:

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var query = from r in Formula1.GetChampions()where r.Country == “Brazil”orderby r.Wins descendingselect r;

foreach (Racer r in query){

Console.WriteLine(“{0:A}”, r);}

Compare this to Formula 1, ex1

Another example of simple LinqMore in next presentation

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var query = from a in accounts.TestData where a.Balance > 1e6 select a;

foreach (Account a in query) Console.WriteLine(a.Balance);

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Summary