Can someone share a simple example of using the foreach keyword with custom objects?
Given the tags, I assume you mean in .NET - and I'll choose to talk about C#, as that's what I know about.
The foreach statement (usually) uses IEnumerable and IEnumerator or their generic cousins. A statement of the form:
foreach (Foo element in source)
{
// Body
}
where source implements IEnumerable<Foo> is roughly equivalent to:
using (IEnumerator<Foo> iterator = source.GetEnumerator())
{
Foo element;
while (iterator.MoveNext())
{
element = iterator.Current;
// Body
}
}
Note that the IEnumerator<Foo> is disposed at the end, however the statement exits. This is important for iterator blocks.
To implement IEnumerable<T> or IEnumerator<T> yourself, the easiest way is to use an iterator block. Rather than write all the details here, it's probably best to just refer you to chapter 6 of C# in Depth, which is a free download. The whole of chapter 6 is on iterators. I have another couple of articles on my C# in Depth site, too:
Iterators, iterator blocks and data pipelines
Iterator block implementation details
As a quick example though:
public IEnumerable<int> EvenNumbers0To10()
{
for (int i=0; i <= 10; i += 2)
{
yield return i;
}
}
// Later
foreach (int x in EvenNumbers0To10())
{
Console.WriteLine(x); // 0, 2, 4, 6, 8, 10
}
To implement IEnumerable<T> for a type, you can do something like:
public class Foo : IEnumerable<string>
{
public IEnumerator<string> GetEnumerator()
{
yield return "x";
yield return "y";
}
// Explicit interface implementation for nongeneric interface
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator(); // Just return the generic version
}
}
(I assume C# here)
If you have a list of custom objects you can just use the foreach in the same way as you do with any other object:
List<MyObject> myObjects = // something
foreach(MyObject myObject in myObjects)
{
// Do something nifty here
}
If you want to create your own container you can use the yield keyword (from .Net 2.0 and upwards I believe) together with the IEnumerable interface.
class MyContainer : IEnumerable<int>
{
private int max = 0;
public MyContainer(int max)
{
this.max = max;
}
public IEnumerator<int> GetEnumerator()
{
for(int i = 0; i < max; ++i)
yield return i;
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
And then use it with foreach:
MyContainer myContainer = new MyContainer(10);
foreach(int i in myContainer)
Console.WriteLine(i);
From MSDN Reference:
The foreach statement is not limited to IEnumerable types and can be applied to an instance of any type that satisfies the following conditions:
has the public parameterless GetEnumerator method whose return type is either class, struct, or interface type,
the return type of the GetEnumerator method has the public Current property and the public parameterless MoveNext method whose return type is Boolean.
If you declare those methods, you can use foreach keyword without IEnumerable overhead. To verify this, take this code snipped and see that it produces no compile-time error:
class Item
{
public Item Current { get; set; }
public bool MoveNext()
{
return false;
}
}
class Foreachable
{
Item[] items;
int index;
public Item GetEnumerator()
{
return items[index];
}
}
Foreachable foreachable = new Foreachable();
foreach (Item item in foreachable)
{
}
Related
I'm not used to write C# code, only Java and Python.
Now I found some code example for an algorithm which is only available in C#.
There is one structure I don't understand, it is the Enumerator.
HashSet<Node>.Enumerator enumerator = hashSet.GetEnumerator();
enumerator.MoveNext();
Item next = enumerator.Current;
So Item is the data type stored in the HashSet hashSet. Is this equal to a for-loop iterating over a HashSet or how else can that be translated into python or java?
GetEnumerator() methods are presented in some data structures in C# such as List, Set, etc. It enables doing iteration through. Actually, foreach internally makes use of it.
foreach statement is to iterate through the elements of certain data structures. A foreach can be used when all of the following conditions hold:
The data structure implements either IEnumerable(which is to
satisfy legacy codes before generics) or IEnumerable<T> for some
type T.
You do not need to know the locations in the data structure of the
individual elements.
For example, the string class implements both IEnumerable and IEnumerable<Char>.
The IEnumerable<T> interface implies the data structure requires two methods:
public IEnumerator<T> GetEnumerator()
IEnumerator IEnumerable.GetEnumerator()
The latter method is required only because IEnumerable<T> is a subtype of IEnumerable, and that interface requires a GetEnumerator method that returns a non-generic IEnumerator. Both of these methods should return the same object; hence, because IEnumerator<T> is also a subtype of IEnumerator, this method can simply call the first method:
System.Collections.IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
As you can see, the IEnumerable.GetEnumerator() method returns a reference to another interface named System.Collections.IEnumerator. This interface provides the infrastructure to allow the caller to traverse the internal objects contained by the IEnumerable-compatible container:
public interface IEnumerator
{
bool MoveNext (); // Advance the internal position of the cursor.
object Current { get;} // Get the current item (read-only property).
void Reset (); // Reset the cursor before the first member.
}
Let's exemplify it.
public class PowersOfThree : IEnumerable<int>
{
public IEnumerator<int> GetEnumerator()
{
return new PowersOfThreeEnumerator();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
internal class PowersOfThreeEnumerator : IEnumerator<int>
{
private int index = 0;
public int Current
{
get { return (int)System.Math.Pow(3, index); }
}
object System.Collections.IEnumerator.Current
{
get { return Current; }
}
public bool MoveNext()
{
index++;
if (index > 10)
return false;
else
return true;
}
public void Reset()
{
index = 0;
}
public void Dispose()
{
}
}
public class Test
{
public static void Main(string[] str)
{
var p2 = new PowersOfThree();
foreach (int p in p2)
{
System.Console.WriteLine(p);
}
}
}
Current method returns the same element until MoveNext method is called. The initial index is 0 each MoveNext method increments the index from 1 to 10, inclusively, then it returns false. When the enumerator is at this position, subsequent calls to MoveNext also return false.
Do you see that what happened to Current when MoveNext returned false? Can you set Current to the first element of the collection again?
If you don't reinstantiate new enumerator, no.
I just find there's a nested struct named Enumerator within the List<> class.
// Returns an enumerator for this list with the given
// permission for removal of elements. If modifications made to the list
// while an enumeration is in progress, the MoveNext and
// GetObject methods of the enumerator will throw an exception.
//
public Enumerator GetEnumerator() {
return new Enumerator(this);
}
[Serializable]
public struct Enumerator : IEnumerator<T>, System.Collections.IEnumerator
{
private List<T> list;
private int index;
private int version;
private T current;
internal Enumerator(List<T> list) {
this.list = list;
index = 0;
version = list._version;
current = default(T);
}
//Omit other code
}
As I know the yield keyword will be translated to above nested struct in IL level, so why Microsoft not use yield directly?
I have to create my own circular list, I using the generic one.
first i create the Node<D> class which represents the data and the next of the element
private class Node<D> {
public D info;
public Node<D> next;
public Node() {
}
public Node(D p) {
info = p;
}
}
To create the circular list, I create the circularList<T> class. this class using Node<> as item of the element.
Here is the CircularList<T> class
class CircularList<T> : IEnumerable<T> {
public Node<T> start;
public Node<T> rear;
public int count = 0;
public CircularList(T firstItem) {
start = new Node<T>(firstItem);
rear = start;
rear.next = start;
}
public void Insert(T newItem) {
//Inserting code here
}
public void Update(T oldItem, T newItem) {
//Updating code is here
}
public void Delete(T theItem) {
//deleting code is here
}
}
when I start to loop using foreach
foreach(string item in CircularList<string>){
}
I got an error says that the circularlist class needs GetEnumerator().
Actually I can loop all of my circular list, but I am using do-while and I need Node<T> to start the loop. but I don't want using Node and do-while.
How do I create the GetEnumerator()?
Any help appreciated. :)
Thank you
Note: I really-really don't understand about IEnumerable and those things, please go easy with the example and explanation.
You need to implement GetEnumerator() method from IEnumerable for the foreach to work.
Use something like following:
public class CircularList<T> : IEnumerable<T>
{
private List<T> mylist = new List<T>();
public T this[int index]
{
get
{
return this.mylist[index];
}
set
{
this.mylist.Insert(index, value);
}
}
public IEnumerator<T> GetEnumerator()
{
return this.mylist.GetEnumerator();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return this.GetEnumerator();
}
}
mylist maintains the list of type T which you would insert, update and delete.
EDIT
myList is just the backing storage of your "custom" list. As long as you are implementing your underlying functionality correctly, myList could be array, arraylist and so on.
For simplicity, I've used List<T> here which already implements the interface IEnumerable<T> and IEnumerator<T>( notice IEnumera**tor** not IEnumerable).
See this answer for more details on the difference between IEnumerator and IEnumerable
Moreover, The foreach statement hides the complexity of the enumerators.
See IEnumerator for more details.
The compiler turns the foreach into something like this:
CircularList<string> testList=new CircularList<string>();
IEnumerator testEnumerator= testList.GetEnumerator();
while (testEnumerator.MoveNext())
{
string yourForEachIteratorVariable = (string)testEnumerator.Current
...
}
Please note that the codes here are only for the illustration purposes. You could/should modify to make it more flexible and "performant" according to your need/requirement.
And for linked list, you don't need a backing List<T>, you could simply add/insert on your head node and implement GetEnumerator() something like below:
public IEnumerator<T> GetEnumerator()
{
var node = start;
while(node != null)
{
yield return node.info;
node = node.next;
}
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
I have a class that stores a string list, I would like to make this class usable in a foreach statement, so I found these two interfaces and I tried to implement them.
public class GroupCollection : IEnumerable, IEnumerator
{
public List<string> Groups { get; set; }
public int Count { get { return Groups.Count; } }
int position = -1;
}
public IEnumerator GetEnumerator()
{
return (IEnumerator)this;
}
public object Current
{
get
{
try
{
return new Group(Groups[position]);
}
catch (IndexOutOfRangeException)
{
throw new InvalidOperationException();
}
}
}
public bool MoveNext()
{
position++;
return position < Groups.Count;
}
public void Reset()
{
position = 0;
}
I'm iterating through a GroupCollection variable twice:
foreach (GroupCollection.Group in groups) // where groups is a GroupCollection
{
}
foreach (GroupCollection.Group in groups)
{
}
// where Group is a nested class in GroupCollection.
When it is at the first foreach it works well (count is 1 at this time). I don't modify anything, and when it goes to the second foreach it doesn't go into the loop. I went through the code line by line in debugging mode and found out that the reset is not called after the first foreach. So should I manually call reset after the foreach? Isn't there a nicer way to do this?
I don't modify anything
Yes you do - your MoveNext() modifies the state of the class. This is why you shouldn't implement both IEnumerable and IEnumerator in the same class. (The C# compiler does for iterator blocks, but that's a special case.) You should be able to call GetEnumerator() twice and get two entirely independent iterators. For example:
foreach (var x in collection)
{
foreach (var y in collection)
{
Console.WriteLine("{0}, {1}", x, y);
}
}
... should give you all possible pairs of items in a collection. But that only works when the iterators are independent.
I went through the code line by line in debugging mode and found out that the reset is not called after the first foreach.
Why would you expect it to? I don't believe the specification says anything about foreach calling Reset - and that's a good job, as many implementations don't really implement it (they throw an exception instead).
Basically, you should make your GetEnumerator() method return a new object which keeps the mutable state of the "cursor" over your data. Note that the simplest way of implementing an iterator in C# is usually to use an iterator block (yield return etc).
I'd also strongly encourage you to implement the generic interfaces rather than just the non-generic ones; that way your type can be used much more easily in LINQ code, the iterator variable in a foreach statement can be implicitly typed appropriately, etc.
Reset is not called at the end of a foreach loop - you could do that in the GetEnumerator call, or just return the enumerator for the List:
public IEnumerator GetEnumerator()
{
return Groups.GetEnumerator;
}
Note that with the yield keyword there is almost no need to implement IEnumerator or IEnumerable explicitly:
public IEnumerator<string> GetEnumerator()
{
foreach(string s in Groups)
yield return s;
}
This code is not compiling, and it's throwing the following error:
The type 'TestesInterfaces.MyCollection' already contains a definition for 'Current'
But when I delete the ambiguous method, it keeps giving other errors.
Can anyone help?
public class MyCollection<T> : IEnumerator<T>
{
private T[] vector = new T[1000];
private int actualIndex;
public void Add(T elemento)
{
this.vector[vector.Length] = elemento;
}
public bool MoveNext()
{
actualIndex++;
return (vector.Length > actualIndex);
}
public void Reset()
{
actualIndex = -1;
}
void IDisposable.Dispose() { }
public Object Current
{
get
{
return Current;
}
}
public T Current
{
get
{
try
{
T element = vector[actualIndex];
return element;
}
catch (IndexOutOfRangeException e)
{
throw new InvalidOperationException(e.Message);
}
}
}
}
I think you're misunderstanding IEnumerator<T>. Typically, collections implement IEnumerable<T>, not IEnumerator<T>. You can think of them like this:
When a class implements IEnumerable<T>, it is stating "I am a collection of things that can be enumerated."
When a class implements IEnumerator<T>, it is stating "I am a thing that enumerates over something."
It is rare (and probably wrong) for a collection to implement IEnumerator<T>. By doing so, you're limiting your collection to a single enumeration. If you try to loop through the collection within a segment of code that's already looping through the collection, or if you try to loop through the collection on multiple threads simultaneously, you won't be able to do it because your collection is itself storing the state of the enumeration operation. Typically, collections (implementing IEnumerable<T>) return a separate object (implementing IEnumerator<T>) and that separate object is responsible for storing the state of the enumeration operation. Therefore, you can have any number of concurrent or nested enumerations because each enumeration operation is represented by a distinct object.
Also, in order for the foreach statement to work, the object after the in keyword, must implement IEnumerable or IEnumerable<T>. It will not work if the object only implements IEnumerator or IEnumerator<T>.
I believe this is the code you're looking for:
public class MyCollection<T> : IEnumerable<T>
{
private T[] vector = new T[1000];
private int count;
public void Add(T elemento)
{
this.vector[count++] = elemento;
}
public IEnumerator<T> GetEnumerator()
{
return vector.Take(count).GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
You need to define the interface the current is implementing.
Object IEnumerator.Current
{
//
}
public T Current
{
//
}
This way your class has 2 Current properties. but you can access them both.
MyCollection<string> col = new MyCollection<string>();
var ienumeratort = col.Current; //Uses IEnumerator<T>
var ienumerator = (IEnumerator)col.Current; //uses IEnumerator
I think with C# 2.0 onwards, you have a very easy way of implementing iterator and compiler does a lot of heavy lifting behind the scene by creating state machine. It's worth looking into it. Having said that, in that case your implementation would look something below:
public class MyCollection<T>
{
private T[] vector = new T[1000];
private int actualIndex;
public void Add(T elemento)
{
this.vector[vector.Length] = elemento;
}
public IEnumerable<T> CreateEnumerable()
{
for (int index = 0; index < vector.Length; index++)
{
yield return vector[(index + actualIndex)];
}
}
}
I am not sure about the purpose of actualIndex though - but i hope you get the idea.
After proper initialization of MyCollection, below is snippet somewhat looks like from consumer perspective:
MyCollection<int> mycoll = new MyCollection<int>();
foreach (var num in mycoll.CreateEnumerable())
{
Console.WriteLine(num);
}