Advanced NET Programming I 2 nd Lecture http

Advanced. NET Programming I 2 nd Lecture http: //d 3 s. mff. cuni. cz/~jezek Pavel Ježek pavel. jezek@d 3 s. mff. cuni. cz CHARLES UNIVERSITY IN PRAGUE faculty of mathematics and physics Some of the slides are based on University of Linz. NET presentations. © University of Linz, Institute for System Software, 2004 published under the Microsoft Curriculum License (http: //www. msdnaa. net/curriculum/license_curriculum. aspx)

IEnumerable and IEnumerator following statement: foreach (Element. Type element in collection) statement; is translated into: IEnumerator enumerator = ((IEnumerable) collection). Get. Enumerator(); try { Element. Type element; while (enumerator. Move. Next()) { element = (Element. Type) enumerator. Current; statement; } } finally { IDisposable disposable = enumerator as IDisposable; if (disposable != null) disposable. Dispose(); }

IEnumerable and IEnumerator (optimized) following statement: foreach (Element. Type element in collection) statement; is translated into: var enumerator = collection. Get. Enumerator(); try { Element. Type element; while (enumerator. Move. Next()) { element = (Element. Type) enumerator. Current; statement; } } finally { IDisposable disposable = enumerator as IDisposable; if (disposable != null) disposable. Dispose(); }

Iterators so far foreach loop can be applied to objects of classes which implement IEnumerable class My. Class: IEnumerable<T> {. . . public IEnumerator<T> Get. Enumerator() { return new My. Enumerator(. . . ); } private class My. Enumerator: IEnumerator<string> { public string Current { get {. . . } } public bool Move. Next() {. . . } public void Reset() {. . . } } } My. Class x = new My. Class(); . . . foreach (string s in x). . . complicated to implement!! interface IEnumerable<T> { IEnumerator<T> Get. Enumerator(); }

Iterator Methods class My. Class { string first = "first"; string second = "second"; string third = "third"; . . . public IEnumerator<string> Get. Enumerator() { yield return first; yield return second; yield return third; } } My. Class x = new My. Class(); . . . foreach (string s in x) Console. Write(s + " "); // prints "first second third " Note • My. Class need not implement IEnumerable! Characteristics of an interator method 1. has the signature public IEnumerator<T> Get. Enumerator 2. statement body contains at least one yield statement How does an iterator method work? 1. returns a sequence of values 2. foreach loop traverses this sequence

What Happens Behind the Scene? returns an object of the following class public IEnumerator<int> Get. Enumerator() { try {. . . } finally {. . . } } class _Enumerator 1 : IEnumerator<int> { int Current { get {. . . } } bool Move. Next() {. . . } void Dispose() {. . . } } is translated into foreach (int x in list) Console. Write. Line(x); IEnumerator<int> _e = list. Get. Enumerator(); try { while (_e. Move. Next()) Console. Write. Line(_e. Current); } finally { if (_e != null) _e. Dispose(); } Move. Next runs to the next yield statement Dispose executes a possibly existing finally block in the iterator method

Implementation class Stack<T>: IEnumerable<T> { T[] items; int count; public void Push(T item) { } public T Pop() { } public IEnumerator<T> Get. Enumerator() { for (int i = count - 1; i >= 0; --i) { yield return items[i]; } } } class Stack<T>: IEnumerable<T> {. . . public IEnumerator<T> Get. Enumerator() { return new __Enumerator 1(this); } class __Enumerator 1: IEnumerator<T>, IEnumerator { int __state; T __current; Stack<T> __this; int i; . . . public bool Move. Next() { switch (__state) { case 1: goto __state 1; case 2: goto __state 2; } i = __this. count - 1; __loop: if (i < 0) goto __state 2; __current = __this. items[i]; __state = 1; return true; __state 1: --i; goto __loop; __state 2: __state = 2; return false; } } }

IEnumerable and IEnumerator (optimized) following statement: foreach (Element. Type element in collection) statement; is translated into: var enumerator = collection. Get. Enumerator(); try { Element. Type element; while (enumerator. Move. Next()) { element = (Element. Type) enumerator. Current; statement; } } finally { IDisposable disposable = enumerator as IDisposable; if (disposable != null) disposable. Dispose(); }

Iterating Over a Tree class Node public : IEnumerable<int> { Node left; Node right; int value; public Node(Node left, Node right, int value) { this. left = left; this. right = right; this. value = value; } public Node(int value) : this(null, value) { } public IEnumerator<int> Get. Enumerator() { if (left != null) { foreach (int x in left) { yield return x; } } yield return value; if (right != null) { foreach (int x in right) { yield return x; } } class Program { static void Main(string[] args) { Node root = new Node(2), new Node(7), 5 ), new Node(14), new Node(17), 15 ), new Node(25), 20 ), 10 ); foreach (int x in root) { Console. Write("{0} ", x); } Console. Write. Line(); } }

Iterating Over a Tree = “Counter-example” class Node public : IEnumerable<int> { Node left; Node right; int value; public Node(Node left, Node right, int value) { this. left = left; this. right = right; this. value = value; } public Node(int value) : this(null, value) { } public IEnumerator<int> Get. Enumerator() { if (left != null) { foreach (int x in left) { yield return x; } } yield return value; if (right != null) { foreach (int x in right) { yield return x; } } class Program { static void Main(string[] args) { Node root = new Node(2), new Node(7), 5 ), new Node(14), new Node(17), 15 ), new Node(25), 20 ), 10 ); foreach (int x in root) { Console. Write("{0} ", x); } Console. Write. Line(); } } Generates new enumerator for each node in the tree!