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Collections, Enumerators, and Iterators
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Hashtable
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Hashtable creates a collection that uses a hash table for storage As most readers will know, a hash table stores information using a mechanism called hashing In hashing, the informational content of a key is used to determine a unique value, called its hash code The hash code is then used as the index at which the data associated with the key is stored in the table The transformation of the key into its hash code is performed automatically you never see the hash code itself The advantage of hashing is that it allows the execution time of lookup, retrieve, and set operations to remain near constant, even for large sets Hashtable implements the IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback, and ICloneable interfaces Hashtable defines many constructors, including these frequently used ones: public Hashtable( ) public Hashtable(IDictionary d) public Hashtable(int capacity) public Hashtable(int capacity, oat loadFactor) The first form constructs a default Hashtable The second form initializes the Hashtable by using the elements of d The third form initializes the capacity of the Hashtable to capacity The fourth form initializes both the capacity and fill ratio The fill ratio (also called the load factor) must be between 01 and 10, and it determines how full the hash table can be before it is resized upward Specifically, when the number of elements is greater than the capacity of the table multiplied by its fill ratio, the table is expanded For constructors that do not take a fill ratio, 10 is used In addition to the methods defined by the interfaces that it implements, Hashtable also defines several methods of its own Some commonly used ones are shown in Table 25-5 To determine if a Hashtable contains a key, call ContainsKey( ) To see if a specific value is stored, call ContainsValue( ) To enumerate the contents of a Hashtable, obtain an IDictionaryEnumerator by calling GetEnumerator( ) Recall that IDictionaryEnumerator is used to enumerate the contents of a collection that stores key/value pairs
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Method public virtual bool ContainsKey(object key) public virtual bool ContainsValue(object value) public virtual IDictionaryEnumerator GetEnumerator( ) public static Hashtable Synchronized(Hashtable table)
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TABLE 25-5
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Description Returns true if key is a key in the invoking Hashtable Returns false otherwise Returns true if value is a value in the invoking Hashtable Returns false otherwise Returns an IDictionaryEnumerator for the invoking Hashtable Returns a synchronized version of the Hashtable passed in table
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Several Commonly Used Methods De ned by Hashtable
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The public properties available in Hashtable are those defined by the interfaces that it implements Two especially important ones are Keys and Values because they let you obtain a collection of a Hashtable s keys or values They are specified by IDictionary and are shown here: public virtual ICollection Keys { get; } public virtual ICollection Values { get; } Because Hashtable does not maintain an ordered collection, there is no specific order to the collection of keys or values obtained Hashtable also has a protected property: EqualityComparer Two other properties called hcp and comparer are flagged as obsolete Hashtable stores key/value pairs in the form of a DictionaryEntry structure, but most of the time, you won t be aware of it directly because the properties and methods work with keys and values individually For example, when you add an element to a Hashtable, you call Add( ), which takes two arguments: the key and the value It is important to note that Hashtable does not guarantee the order of its elements This is because the process of hashing does not usually lend itself to the creation of sorted tables Here is an example that demonstrates Hashtable:
// Demonstrate Hashtable using System; using SystemCollections; class HashtableDemo { static void Main() { // Create a hash table Hashtable ht = new Hashtable(); // Add elements to the table htAdd("house", "Dwelling"); htAdd("car", "Means of transport"); htAdd("book", "Collection of printed words"); htAdd("apple", "Edible fruit"); // Can also add by using the indexer ht["tractor"] = "Farm implement"; // Get a collection of the keys ICollection c = htKeys; // Use the keys to obtain the values foreach(string str in c) ConsoleWriteLine(str + ": " + ht[str]); } }
The output from this program is shown here:
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