Comparison of programming languages (associative array): Difference between revisions

Revision as of 12:17, 5 September 2013

Revision as of 15:20, 6 September 2013

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===C#===

<source lang=CSharp>

Dictionary<intstring, string> dic = new Dictionary<intstring, string>();

Hashtable ht = new Hashtable();

htdic.Add("testKeySally Smart", "AssociatedData555-9999");

dic.Add(7"John Doe", "Bond555-1212");

MessageBox.Show((string) ht["testKey"]);

dic.Add("J. Random Hacker", "553-1337");

</source>

// access an phone number and display it in a message box

<source lang=CSharp>

MessageBox.Show((string) htdic["testKeySally Smart"]);

Dictionary<int, string> dic = new Dictionary<int, string>();

</source>

dic.Add(7, "Bond");

MessageBox.Show(dic[7]);

</source>

The dictionary can also be initialized with all entries during construction. This is called "object initialization".

In the above sample the <code>Hashtable</code> class is only capable of associating a <code>String</code> key with a value of <code>Object</code> type. Because in .NET all types (except [[Pointer (computer programming)|pointer]]s) ultimately derive from <code>Object</code>, anything can be put into a <code>Hashtable</code>, even data of different types. This could lead to errors if consuming code expects data to be of a singular type. In the above code casting is required to convert the Object variables back to their original type. Additionally, casting value-types (structures such as integers) to <code>Object</code> to put into the <code>Hashtable</code> and casting them back requires boxing/unboxing which incurs both a slight performance penalty and pollutes the heap with garbage. This changes in C# 2.0 with generic hashtables called dictionaries. There are significant performance and reliability gains to these strongly typed collections because they do not require boxing/unboxing or explicit type casts and introduce compile-time type checks. C# 3.0 adds a shorthand notation for the dictionary initialization:

<source lang=CSharp>

Dictionary<intstring, string> dic = new Dictionary<intstring, string> {

{

{"Sally Smart", "555-9999"},

{"John Doe", "555-1212"},

{"J. Random Hacker", "553-1337"}

{ 7, "Bond" };

</source>

A foreach loop can enumerate through the entire collection. There is no guarantee of order. If order matters the programmer could choose to use a SortedDictionary or use a .Sort linq extension method.

<source lang=CSharp>

// loop through the collection and display each entry

Dictionary<int, string> = new Dictionary<int, string> {

foreach(KeyValuePair<string,string> kvp in dic)

{ 7, "Bond" }

{

};

MessageBox.Show(String.Format("Phone number for {0} is {1}", kvp.Key, kvp.Value);

};

</source>