| Description | |
|
Use a HashTable when you have a large amount of key-value data pairs to store, where the key has a good variance. Otherwise, use a Dictionary for key-value pair storage.
Hashing refers to a long standing programming storage mechanism. A mathematical algorithm is applied to a key to generate a position in a block of storage. This algorithm ensures that only that key value equates with its block position and vice versa.
The benefit of this hashing concept is speed : access to the value for a key is direct via the algorithm, with no searching or indexing required. The down side is that a potentially large amount of storage may be required for small amounts of data. It is therefore best suited to large amounts of data.
This is the theory. In .Net, the HashTable class uses a hybrid approach, with the key-value pairs actually stored in a Dictionary. A hash table is used to speed up access to this dictionary.
Objects added to the HashTable must provide the GetHash method, unless an IHashCodeProvider is provided in the HashTable constructor.
Note that some of the constructors are quite complicated to use - you will only see basic usage of HashTable in Delphi Basics. |
|
| Syntax | |
| | Constructor Create ( |
); | | Constructor Create ( | InitialCapacity : Integer
; ); | | Constructor Create ( | InitialCapacity : Integer; LoadFactor : Single
); | | Constructor Create ( | InitialCapacity : Integer; HashCodeProvider : IHashCodeProvider; Comparer : IComparer
); | | Constructor Create ( | InitialCapacity : Integer; LoadFactor : Single; HashCodeProvider : IHashCodeProvider; Comparer : IComparer
); | | Constructor Create ( | Dictionary : IDictionary
; ); | | Constructor Create ( | Dictionary : IDictionary; LoadFactor : Single
); | | Constructor Create ( | Dictionary : IDictionary; HashCodeProvider : IHashCodeProvider; Comparer : IComparer
); | | Constructor Create ( | Dictionary : IDictionary; LoadFactor : Single; HashCodeProvider : IHashCodeProvider; Comparer : IComparer
); | | Constructor Create ( | HashCodeProvider : IHashCodeProvider; Comparer : IComparer
); | | Constructor Create ( | Serialization : SerializationInfo; Streaming : StreamingContext ); |
|
| Methods | |
| | Add | Add an entry to the current HashTable | | Clear | Clear (remove) all key-value data pairs from the current HashTable | | Clone | Make a new HashTable that is a clone of the current HashTable | | Contains | Returns true if the current HashTable contains the given Key | | ContainsKey | Returns true if the current HashTable contains the given Key | | ContainsValue | Returns true if the current HashTable contains the given Value | | CopyTo | Copies elements from the HashTable to a single dimension array | | GetEnumerator | Gets an enumerator to allow reading the elements of the current HashTable | | Remove | Remove a key-value entry from the current HashTable |
|
|
|
| Properties | |
| | Count | | Integer | | Gets the number of key-and-value pairs contained in the Hashtable.
| | IsFixedSize | | Boolean | | Gets a value indicating whether the Hashtable has a fixed size.
| | IsReadOnly | | Boolean | | Gets a value indicating whether the Hashtable is read-only.
| | IsSynchronized | | Boolean | | Gets a value indicating whether access to the Hashtable is synchronized (thread-safe).
| | Item | | Object | | Gets or sets the value associated with the specified key. This is the default property, specifiable using [] brackets.
| | Keys | | ICollection | | Gets an ICollection containing the keys in the Hashtable.
| | SyncRoot | | Object | | Gets an object that can be used to synchronize access to the Hashtable.
| | Values | | ICollection | | Gets an ICollection containing the values in the Hashtable. |
|
Delphi Basics links
| |
| |
Microsoft MSDN links
| |
|
|