Dictionaries

Definition

A dictionary is a general-purpose data structure for storing a group of objects. A dictionary has a set of keys and each key has a single associated value. When presented with a key, the dictionary will return the associated value. Dictionaries are special lists, whereas every value in the list has a key which is also a variable. A good example for a dictionary is a phone book.

Declaring & Adding to the Dictionary

Dictionary<string, long> phonebook = new Dictionary<string, long>();
phonebook.Add("Alex", 4154346543);
phonebook["Jessica"] = 4159484588;

Notice that when defining a dictionary, we need to provide a generic definition with two types - the type of the key and the type of the value. In this case, the key is a string whereas the value is an integer.

There are also two ways of adding a single value to the dictionary, either using the brackets operator or using the Add method.

Checking for a key

To check whether a dictionary has a certain key in it, we can use the ContainsKey method:

Dictionary<string, long> phonebook = new Dictionary<string, long>();
phonebook.Add("Alex", 415434543);
phonebook["Jessica"] = 415984588;

if (phonebook.ContainsKey("Alex"))
{
    Console.WriteLine("Alex's number is " + phonebook["Alex"]);
}

Removing a key/value pair

To remove an item from a dictionary, we can use the Remove method. Removing an item from a dictionary by its key is fast and very efficient. When removing an item from a List using its value, the process is slow and inefficient, unlike the dictionary Remove function.

Dictionary<string, long> phonebook = new Dictionary<string, long>();
phonebook.Add("Alex", 415434543);
phonebook["Jessica"] = 415984588;

phonebook.Remove("Jessica");
Console.WriteLine(phonebook.Count);