I've tried to develop a generic class "MinHeap(T)" implementing IComparable. It works fine when generic is "int" or another class "Code". Going further and using for T a generic class "Node(Code)"
leads to the error mentioned below.
I'm probably too new and not understanding subtle differences between IComparable and IComparable(T). Does someone have an idea ?
Thanks in advance for your help,
LJ
public class MinHeap<T> where T : IComparable
{
...
}
public class Node<T> where T : IComparable
{
T data
...
public int CompareTo(object obj)
{
Node<T> otherNode = obj as Node<T>;
return this.data.CompareTo(otherNode.data);
}
...
}
public class Code : IComparable
{
public int freq;
...
public int CompareTo(object obj)
{
Code otherCode = obj as Code;
return this.freq.CompareTo(otherCode.freq);
}
}
static void Main(string[] args)
{
MinHeap<int> hInt = new MaxHeap<int>(heapSeed); // works fine
MinHeap<Code> hCode = new MinHeap<Code>(codeList); // works fine
...
Node<Code>[] nodeCodeList = new Node<Code>[freqList.Length]; // ok
MinHeap<Node<Code>> h = new MinHeap<Node<Code>>(nodeCodeList); // Error
...
}
Error message:
Error 2 The type 'Algorithms.Node(Algorithms.Code)' cannot be used as type parameter 'T' in the generic type or method 'Algorithms.MinHeap(T)'. There is no implicit reference conversion from 'Algorithms.Node(Algorithms.Code)' to 'System.IComparable'.
The class Node<T> does not implement IComparable. It just haves a constraint for the type of T.
It looks like you've tried to implement the decorator pattern. Implement the interface as well and then map the methods to the decorated object.
You're probably better off defining your generic class so that it doesn't require the type to implement IComparable<T>. That's how the .NET generic classes are implemented. They will use the default comparison for the type, or use the IComparer<T> that you pass in. For example, SortedList has these constructors (among others):
SortedList<TKey, TValue>(); // uses default comparer for TKey
SortedList<TKey, TValue>(IComparer<T> comparer); // uses supplied comparer
It's easy enough to implement:
public class MinHeap<T>
{
private IComparer<T> _comparer;
public MinHeap<T>(IComparer<T> comp)
{
_comparer = comp;
}
public MinHeap<T>()
: this(Comparer<T>.Default)
{
}
}
That way, your clients can use MinHeap<T> to hold instances of classes that do not implement IComparable<T>.
When you do your comparisons, you call _comparer.Compare(item1, item2)
On another note, there's no particular need for your heap to require a node. You can implement a binary heap in an array. See, for example, A Generic BinaryHeap Class.
You should implement IComparable interface within Node class, like this:
public class Node<T>: IComparable<Node<T>> // <- IComparable<Node<T>> implemented
where T: IComparable {
T data;
...
#region IComparable<Node<T>> Members
// interface implementation, not just a method
public int CompareTo(Node<T> other) {
// Let us be accurate with possible nulls:
if (Object.ReferenceEquals(null, other))
return 1; // <- or -1 if you think it's right
else if (Object.ReferenceEquals(data, other.data))
return 0;
else if (Object.ReferenceEquals(null, data))
return -1; // <- or 1 if you think it's right
return data.CompareTo(other.data);
}
#endregion IComparable<Node<T>> Members
}
depending on algorithm of min heap, you may have to implement IComparable> in MinHeap class as well
Related
So I'm rolling my own max heap and I'm doing something wrong with using a generic class extending an interface. Say I have a class like this:
class SuffixOverlap:IComparable<SuffixOverlap>
{
//other code for the class
public int CompareTo(SuffixOverlap other)
{
return SomeProperty.CompareTo(other.SomeProperty);
}
}
And then I create my heap class:
class LiteHeap<T> where T:IComparable
{
T[] HeapArray;
int HeapSize = 0;
public LiteHeap(List<T> vals)
{
HeapArray = new T[vals.Count()];
foreach(var val in vals)
{
insert(val);
}
}
//the usual max heap methods
}
But when I try to do this:
LiteHeap<SuffixOverlap> olHeap = new LiteHeap<SuffixOverlap>(listOfSuffixOverlaps);
I get the error:
The type SuffixOverlap cannot be used as a type parameter T in the generic type or method LiteHeap<T>. There is no implicit reference conversion from SuffixOverlap to System.IComparable.
How do I create LiteHeap as a class that uses generic class T implementing IComparable so I can write new LiteHeap<SomeClass> and it will work where SomeClass implements IComparable
IComparable and IComparable<T> are different, completely-unrelated interfaces.
You need to change that to where T : IComparable<T>, so that it actually matches your class.
I am implementing a binary tree using the following code:
class Binary<T>:Binary
where T: IComparable
{
But I'm also looking to use class objects in the tree:
Binary Bin = null;
Bin = new Binary<Product>();
This obviously doesnt work because Product is not IComparable. I tried doing this to make it work:
public class Product: IComparable<Product>
{
public int Id = 0;
public int CompareTo(Product p)
{
if (p==null)
{
return 1;
}
if (p!=null)
{
return this.Id.CompareTo(p.Id);
}
return 0;
}
Id is the value of Product intended to be used in all comparisons. But this doesn't work and I dont know what else to do. Is it possible to do what i want? Or is there another way I can use the class Product as an IComparable?
Your class Product implements generic IComparable<T>, where your generic class Binary<T> expects generic type that implements non-generic IComparable.
Update Binary<T> declaration to useIComparable<T> in the constraint:
class Binary<T>:Binary
where T: IComparable<T>
{
I have searched around and found one somewhat relevant answer but, for the life of me, I still cannot figure out where I'm going wrong! I am trying to implement some generically typed tree data structures, using code similar to that below, but I get compiler error CS0311.
error CS0311: The type 'Test.TestType' cannot be used as type parameter 'K' in the generic type or method 'Test.TreeIndex<K>'. There is no implicit reference conversion from 'Test.TestType' to 'Test.IIndexable<Test.TestType>'.
I just can't figure out why the compiler doesn't know how to deal with this so any clues would be much appreciated.
public interface IIndexable<K> where K : IComparable
{
byte[] ToBytes();
K FromBytes(byte[] bytes);
}
public class TestType : IIndexable<byte>, IComparable
{
public int CompareTo(object b)
{
return 1;
}
public byte[] ToBytes()
{
return new byte[1];
}
public byte FromBytes(byte[] bytes)
{
return 0;
}
}
public class TreeIndex<K> where K : IComparable, IIndexable<K>
{
public int SomeMethod(K a, K b)
{
return a.CompareTo(b);
}
}
class Program
{
static void Main()
{
TreeIndex<TestType> treeIndex = new TreeIndex<TestType>(); // CS0311 generated twice here
}
}
Your TreeIndex<K> class requires K implement IIndexable<K>, so TestType should implement IIndexable<TestType> instead of IIndexable<byte>
public class TestType : IIndexable<TestType>, IComparable
{
public int CompareTo(object b)
{
return 1;
}
public byte[] ToBytes()
{
return new byte[1];
}
public TestType FromBytes(byte[] bytes)
{
//...
}
}
You might also want to consider using the generic IComparable<T> constraint on your IIndexable<K> interface i.e.
public interface IIndexable<K> where K : IComparable<K>
{
byte[] ToBytes();
K FromBytes(byte[] bytes);
}
Since you have the following generic constraint
public class TreeIndex<K> where K : IComparable, IIndexable<K>
And you´re declaring
TreeIndex<TestType>
You're saying that TestType implements IIndexable<TestType>, which cannot be true.
From Compiler Error CS0311
When a constraint is applied to a generic type parameter, an implicit
identity or reference conversion must exist from the concrete argument
to the type of the constraint. To correct this error
Change the argument you are using to create the class.
Change the argument you are using to create the class.
If you own the class, you can remove the constraint or else do something to enable an implicit reference or identity conversion. For
example, you can make the second type inherit from the first
Based on that, TreeIndex<K> class have to implement IIndexable<TestType> not IIndexable<byte>.
When you write TreeIndex<TestType> compiler asks for IIndexable<K> which means IIndexable<TestType> on your case.
But your TestType class doesn't implement IIndexable<TestType>. It implements IIndexable<byte>.
You are telling the compiler to make a new TreeIndex<TestType>()
The definition for TreeIndex is TreeIndex<K> where K : IComparable, IIndexable<K> which means that the type you pass for the generic must be of IIndexable<K>.
So when you say: TreeIndex<TestType> then the compiler expects TestType to be of type IIndexable<K>, where K is TestType, so it expects TestType to implement IIndexable<TestType>, which it does not. It implements IIndexable<byte> instead.
It is hard to tell what the best solution would be for your case, but one way to solve it is to specify 2 generics:
// change the TreeIndex definition to take 2 generic types
public class TreeIndex<K, Y>
where K : IComparable, IIndexable<Y>
where Y : IComparable
...
// specify both generic types
TreeIndex<TestType, byte> treeIndex = new TreeIndex<TestType, byte>();
Consider the following:
public interface ITree<X>
{
...
ITree<X> Union(ITree<X> other);
...
}
The idea is that I'm going to implement several types of tree. However, the Union() method only works if you try to union two trees of the same type. The type signature above does not enforce this restriction, however.
So, my question is: How can I write a type signature for Union() such that the other argument must have the same type as this?
(Obviously I can do a dynamic run-time test and throw an exception if the types don't match. But I would much, much rather to check this at compile-time if it can be done...)
There isn't a particularly clean way of expressing this, this is a consequence of using interfaces, since there's no way to know the implementing type of ITree<X>. The best method is probably to create another class/interface which constrains the concrete tree type and does the operation(s) you require:
public interface ITreeUnion<T, X> where T : ITree<X>
{
T Union(T left, T right);
}
you'll then have to pass instances of this interface type to where you need to carry out the required operation.
If you really require Union to go on the interface you can use a recurring template:
public interface ITree<T, X> where T : ITree<T, X>
{
T Union(T other);
}
public class RedBlackTree<T> : ITree<RedBlackTree<T>, T>
{
public RedBlackTree<T> Union(RedBlackTree<T> other)
{
}
}
According to your requirment, you would need a generic declaration of Union().
interface
public partial interface ITree<X> {
T Union<T>(T other) where T: ITree<X>;
}
sample classes
public partial class TreeOfObject: ITree<object> {
public T Union<T>(T other) where T: ITree<object> {
return default(T); // sample only; shuold be implemented yourself
}
}
public partial class TreeOfInt: ITree<int> {
public T Union<T>(T other) where T: ITree<int> {
return default(T); // sample only; shuold be implemented yourself
}
}
test
public static partial class TestClass {
public static void TestMethod() {
var x=new TreeOfObject();
var y=new TreeOfInt();
var xx=x.Union(x);
var yy=y.Union(y);
var xy=x.Union(y); // won't compile
var yx=y.Union(x); // won't compile
}
}
Why do you need the interface then? Simply implement a Replace method on each implementation of a tree:
public class RedBlackTree<T> {
public RedBlackTree<T> Union(RedBlackTree<T> other) { ... }
}
public class SplayTree<T> {
public SplayTree<T> Union(SplayTree<T> other) { ... }
}
Since you're looking for compile-time safety when dealing with each implementation of ITree, I would argue you just need to deal with the concrete types. Of course, you could have an ITree<T> with other methods on it if you require.
Somehow, the following actually compiles:
public interface ITree<TSelf, TItem> where TSelf : ITree<TSelf, TItem>
{
TSelf Union(TSelf other);
// ...
}
public class AvlTree<TItem> : ITree<AvlTree<TItem>, TItem> {
public AvlTree<TItem> Union(AvlTree<TItem> other) {
return other;
}
}
Of course it's not particularly useful, since then you'd have to declare variables as ITree<AvlTree>, at which point you might as well not use the interface. With C# generics, the values of generic type parameters have to be known at some point to reify the generic type.
What I basically wish to do is design a generic interface that, when implemented, results in a class that can behave exactly like T, except that it has some additional functionality. Here is an example of what I'm talking about:
public interface ICoolInterface<T>
{
T Value { get; set; }
T DoSomethingCool();
}
public class CoolInt : ICoolInterface<int>
{
private int _value;
public CoolInt(int value)
{
_value = value;
}
public int Value
{
get { return _value; }
set { _value = value; }
}
public int DoSomethingCool()
{
return _value * _value;
// Ok, so that wasn't THAT cool
}
}
And this is all well and good, but in order to use CoolInt, I need to do something like this:
CoolInt myCoolInt = new CoolInt(5);
int myInt = myCoolInt.Value;
I'd much rather, in terms of assignment at least, that CoolInt works just like int. In other words:
CoolInt myCoolInt = 5;
int myInt = myCoolInt;
To achieve this, I added these two conversion operators to my CoolInt class:
public static implicit operator CoolInt(int val)
{
return new CoolInt(val);
}
public static implicit operator int(CoolInt obj)
{
return obj.Value;
}
Works awesomely. Now, I would prefer it if I could add these two overloads to the interface, so that implementers of the interface are forced to implement these operators. The problem is, the prototypes of these operators refer directly to CoolInt.
C# has a lot of "placeholder" names for things that are implicitly defined or have yet to be defined. The T that is conventionally used in generic programming is one example. I suppose the value keyword, used in Properties, is another. The "this" reference could be considered another. I am hoping that there's another symbol I can use in my interface to denote "the type of the class that is implementing this interface", e.g. "implementer".
public static implicit operator implementer(int val)
{
return new IntVal(val);
}
public static implicit operator int(implementer obj)
{
return obj.Value;
}
Is this possible?
Why don't you create an abstract class? This way you can build some "default" functionality into your class.
Sadly no :(
C# doesn't do well when it comes to operator overloading (This is one example, another is generic constraints on certain operator types).
Why not use extension methods instead? That lets you "add" methods to int without having to use a different type.
This is probably the closest you can get using an abstract base type, but sadly even this has an issue with one of the implicit operators and you have to do:-
CoolInt x = (CoolInt)5;
int j = x;
Close enough?
// Slightly sneaky, we pass both the wrapped class and the wrapping class as type parameters to the generic class
// allowing it to create instances of either as necessary.
public abstract class CoolClass<T, U>
where U : CoolClass<T, U>, new()
{
public T Value { get; private set; }
public abstract T DoSomethingCool();
// Non-public constructor
protected CoolClass()
{
}
public CoolClass(T value)
{
Value = value;
}
public static implicit operator CoolClass<T, U>(T val)
{
return new U() { Value = val};
}
public static implicit operator T(CoolClass<T, U> obj)
{
return obj.Value;
}
}
public class CoolInt : CoolClass<int, CoolInt>
{
public CoolInt()
{
}
public CoolInt(int val)
: base(val)
{
}
public override int DoSomethingCool()
{
return this.Value * this.Value; // Ok, so that wasn't THAT cool
}
}
It would be helpful if, at least for interfaces, one could declare that a class implements an interface in terms of an object; this would be especially cool if there was an "interface" generic type constraint. Then one could, for example, do something like (VB syntax)
Class Foo(Of T as Interface)
Implements T via Bar ' Declares variable 'bar' of type T
Sub DoSomething
' Does something
End Sub
End Class
and then cast a Foo(of T) to a T and have it behave like a T. Maybe someone from MS can stumble on the idea and pass it on?
I should note, btw, a nice pattern similar to your ICoolInterface:
public interface ISelf<T>
{
T Self { get;}
}
public interface IFoozle
{
... definitions for IFoozle
}
public interface IFoozle<T> : ISelf<T> , IFoozle;
{
/* Empty except for above declarations */
}
... similarly define IWoozle and IWoozle<T> etc.
Then one can declare a field which can implement IWoozle and IFoozle, and inherits from BoozleBase (which implements neither), via:
IWoozle<IFoozle<BoozleBase>> myField1;
or
IFoozle<IWoozle<BoozleBase>> myField2;
Note that the above two types can be cast to each other, or to types which also contain other interfaces. If one only needs to pass a variable meeting multiple constraints to a method, one can obtain such a thing more easily by using a generic method. Unfortunately, there's no way to store an object of unknown type in a field in such a way that it can be passed to a generic function with multiple constraints.